Boccaletti, et al (2005) The vertical structure of ocean heat transport. Geophys. Res. Lett., 32, L10603, doi:10.1029/2005GL022474, May 17
“One of the most important contributions the ocean makes to Earth’s climate is through its poleward heat transport: about 1.5 PW or more than 30% of that accomplished by the ocean-atmosphere system (Trenberth and Caron, 2001). Recently, concern has arisen over whether global warming could affect this heat transport (Watson et al., 2001), for example, reducing high latitude convection and triggering a collapse of the deep overturning circulation (Rahmstorf, 1995). While the consequences of abrupt changes in oceanic circulation should be of concern, we argue that the attention devoted to deep circulations is disproportionate to their role in heat transport. For this purpose, we introduce a heat function which identifies the contribution to the heat transport by different components of the oceanic circulation. A new view of the ocean emerges in which a shallow surface intensified circulation dominates the poleward heat transport…..”
[Response: Your point being what exactly? This paper is an assessment of the current climate poleward heat transport. The post is discussing the possibilty of one aspect of that transport being disrupted. As stated above, the wind-driven component is unlikely to change much, and so while the shallow, wind-driven circulations may actually transport more heat (and of course the atmosphere transfers even more), the variability in the heat transport can still be dominated by the variability in the overturning. -gavin]
I’m agree when you speak about the wind-driven (firstly) and thermohaline (secondarly) forces for the GS.
But if I “understand” the thermohaline circulation , the wind-driven force is more difficult to unbderstand.
We have a cycle, by definition , with the thermohaline circulation ,but how is realized the return with the wind-driven force ?
I think that ,to plunge down the water of the GS (or North Atlantic current) must be ,it’s an evidence , more dense then the present water.
To be more dense ,this water must be more saline ,because its temperature cannot be colder than the present water.
In all the cases it’s a thermohaline circulation not entirely induced by the thermohaline force.
If this return does’nt exist we can get an accumulation of warm water ,then a decrease of the stream owing by hydrostatic force.
[Response:The wind-driven circulation is harder to understand, yes, but is nonetheless well understood. You just need to do some dynamics, whereas the THC bit seems more obvious. But if you want a very rough idea... (others will no doubt correct me) then the wind-driven circ ends up being a large gyre, which concentrates into a narrow current along the western boundary, with a much broader return in the rest of the basin - William]
This is a good overview of the Gulf Stream – thermohaline circulation (THC) story, but in addressing the question in this post I fear you have left out a discussion of the major factor that keeps northern Europe warm compared to similar latitudes in eastern North America. Atmospheric circulation patterns, specifically the predominant shape of the Jet Stream, controlled largely by orographic forcing of the Rocky Mountains, have more to do with keeping northern Europe warm than does the Gulf Stream or THC. The shape and location of the Rockies help to cause a persistent wave in the Jet Stream, with a trough in over eastern North America which brings cold continental air, via northwesterly winds, to northeastern North America, and a crest over the eastern north Atlantic and western Europe which brings warm maritime air, via southwesterly winds, to northern Europe. The ocean plays a role in that the seasonal storage and release of heat by the Atlantic Ocean mixed layer, coupled with wind advection, lend northern Europe a maritime climate with mild winters, but this is true whether the Gulf Stream operates or not.
This is argued very clearly by one of your colleagues from across the Hudson River at LDEO, Richard Seager.
See Seager et al., 2002. Is the Gulf Stream responsible for Europe’s mild winters?, Quarterly Journal of the Royal Meteorlogical Society, v. 128, 2563-2586.
Also see this news release from Columbia U: http://www.columbia.edu/cu/news/03/02/richardSeager_research.html
I understand that this post is meant to address variation in one side of the story (the Gulf Stream), but if we are to educate the public away from the Gulf Stream = warm Europe idea, we must give atmospheric circulation its due as the dominant forcing. I agree that THC largely determines the amount of ocean heat transport to the North Atlantic, but this affects both sides of the Atlantic, not just Europe. According to Seager et al. (2002) THC appears to account for the North America vs. Europe winter temperature difference only in the highest latitudes, north of about 60 degrees N, due to the fact that the heat transport limits sea ice cover there.
[Response:You are partly right. Two issues are sometimes confused:
(1) What keeps the eastern side of the Atlantic (say, Britain) warmer than the western side (say, Labrador)? That is largely atmospheric circulation and the general presence of an ocean upstream, i.e. maritime vs. continental air masses, as was affirmed in the paper by Seager et al. (2002). (But note they have a mistake in calculating heat storage vs. heat transport contributions, underestimating the latter, as explained by Rhines and Häkkinen (2003))
(2) What keeps the northern Atlantic and surroundings warmer than the northern Pacific and surroundings? That is indeed largely the ocean heat transport associated with the THC, in lay-persons terminology often referred to as the Gulf Stream.
Hence, traditionally the effect of the ocean heat transport is illustrated by comparing temperatures on the eastern side of both oceans: e.g. the classic review paper by Weaver and Hughes (1992) compares Bodö (Norway, 67°N) with average January temperature of -2°C to Nome (Alaska, 65°N) with average January temperature of -15°C. Another way to look at data is to plot the deviation of temperatures from the zonal average, as in Fig. 3 here.
A good way to estimate the effect of the thermohaline part of the heat transport is to shut it down by dumping a lot of freshwater into the north Atlantic in a climate model, which stops deep water formation there. Two examples are shown in the figures in Gavin's post; it leads to several degrees cooling over Europe but also on the western side of the Atlantic. Hence, the Gulf Stream does contribute a lot to Europe's warmth, and I see no reason why anybody would want to "educate the public away" from this idea. - Stefan]
It’s news like this that really catches my attention.
(While most of Europe has shivered through an unusually cold March, a snow festival in Arctic Greenland has been postponed indefinitely because of a “heat wave.” The 11th annual international Snow Sculpture Festival in Nuuk was scheduled from March 18 to 21, when the average temperature in Greenland’s capital would usually be well below freezing.
“The snow has been melting because of the mild weather and last week we had several days of rain,” Nuuk Tourism manager Flemming Nicolaisen said.)
In your opinion, how stable are the estimates of fresh water comming from Greenland and mixing with the North Atlantic Current?
[Response: How stable are the estimates, or how stable are the fluxes? The estimates are quite variable because of the difficulty in measuring these things in a difficult part of the world and the complexity of the processes (ice berg calving; under ice shelve melting, snow blowing, under glacier melt etc.). Thus assessing how variable these fluxes are is doubly difficult. The integrated measures from large-scale ocean salinity measurements are quite good, but it is difficult to work out exactly where that freshwater has come from. With respect to your first point, remember that weather is quite variable - especially around the North Atlantic, and I doubt that rain in March is unprecedented. - gavin]
[Response:Here's a simple back-of-envelope consideration for the future: if the Greenland ice sheet melts completely over the next ~1,000 years (Jim Hansen argues in the current Climatic Change that the time scale could be centuries), this would contribute an average flux of ~0.1 Sv of freshwater to the surrounding ocean. Modelling uncertainty currently is such that in some climate models, this amount of freshwater (without any other forcing) would shut down deep water formation, in some it wouldn't. The reasons for those model differences are not yet understood (we're working on it but it turns out to be a hard problem).
For global warming scenarios, additional forcing comes into play: surface warming and enhanced high-latitude precipitation, which will also reduce density of northern surface waters (an effect which alone has shut down deep water formation in some model experiments, e.g. Manabe and Stouffer 1993, 1994).
In most future global warming simulations with climate models no meltwater from Greenland is included so far. -Stefan]
The physical mechanism of the solar variability influence on electrical and climatic characteristics of the troposphere
G.A. Zherebtsov, V.A. Kovalenko and S.I. Molodykh,
Possible mechanisms of solar-climatic connections, which may be of importance over short and long time intervals, are discussed. The variations of energetic balance of Earth’s climatic system for the last 50 years are estimated. It is ascertained that the imbalance between the flux of solar energy that comes to the Earth and radiates to space is of 0.1% for the last ten years. The significance is analyzed for the possible influence of variations of solar constant upon the energetic balance of the atmosphere. The physical mechanism of the influence of solar activity on climatic characteristics and the atmospheric circulation is suggested and theoretically substantiated. The mechanism is based on the redistribution in lower-troposphere of condensation nuclei by the vertical electric field. This electric field is determined by the ionosphere-Earth electric potential, which in the Polar Regions is controlled not only by tropical thunderstorms and by the galactic cosmic-ray intensity but also by solar cosmic-ray fluxes. The height redistribution in the atmosphere of condensation nuclei with a change of the electric field of the atmosphere is accompanied by a change in total latent heat (phase transition of water vapor), by changes in radiation balance, and by subsequent changes of the thermobaric field of troposphere. The results of analysis of thermobaric field variations for the periods of invasion of abnormally powerful solar cosmic ray fluxes and magnetic storms confirm the reality of manifestation of heliogeophysical disturbances.
The THC is probably not shutting down anytime soon, as the post tries to point out. Somebody needs to say here that hysteria about abrupt shutdown of the “gulf stream” (sigh) is counterproductive with respect to critical thinking about the real ongoing effects of climate change like Tropical Glacier Retreat which few people seem interested in. This nonsense from CNN:
Without the influence of the Gulf Stream and its two northern branches, the North Atlantic Drift and the Canary Current, the weather in Britain could be more like that of Siberia, which shares the same latitude.
Oh, No! it’s the “The Day After Tomorrow”! Smallpox, dirty bombs, THC shutdown!
Abrupt changes happened in the late Quaternary and there’s a scary lesson in this, but no modeling studies have predicted, to my knowledge, any shutdown of the THC in this century. Here’s a good page Questions and Answers about Abrupt Climate Change from Lamont-Doherty.
[Response:I fully agree with your statement: "The THC is probably not shutting down anytime soon." And anybody will agree that hysteria is counter-productive. Nevertheless, the risk of triggering ocean circulation changes as a result of global warming cannot be ruled out at present (which is why you use the word "probably"), and it needs to be studied and discussed. This is an issue of risk assessment, as we discuss in detail in Rahmstorf and Zickfeld (2005). The issue is: what is the risk, and what risk are we willing to accept? Would you accept a 1% risk of shutting down the THC? Or a 5% risk? - Stefan]
I absolutely agree that the press is mistaken in conflating the Gulf Stream with the thermohaline circulation. On the other hand, it’s not quite right (Carl Wunsch notwithstanding) to say that the Gulf Stream is an entirely separate thing. The fact is that most of the mass flux (both North and South) in the thermohaline circulation takes place in boundary currents, and not in nice broad zonally uniform sheets like you might think of by looking at the streamfunctions. You’d get something like a Gulf Stream even if you just had buoyancy forcing, though it would be a good bit weaker than the present Gulf Stream. Rui Xin Huang has written about this. What Carl means, no doubt, is that you’d still have a Gulf Stream even without THC, since you’d still have the wind driven part.
Now, since the problem is nonlinear, the heat flux in the THC part of the “Gulf Stream” and the wind driven part don’t just superpose, so it’s not a priori clear to what extent one can think of them as separate entities. I’ve never really dug into this, but I’m not aware of the interaction issue having been treated.
[Response: Of course you are correct, one would see differences in the Gulf Stream transports in the case of a THC change (though no reversal). It should be easy enough to look at some of these model runs and see how much the wind-driven part changes though.... Maybe I'll look into it at the weekend! - gavin]
I’m apologized to insist ,but I repeat my question about the becoming of the wind-driven part of the oceanic currents towards the north-east Atlantic ocean.
Is there a plunge-down favoured by THC or another thing?
From another part I’m very surprised to see the comments and your introduction (gavin) in this topic .
Clearly there is a (voluntary?) minoration of the THC and of the role of the GS itself.
I don’t understand why.
I think that the heat transport towards the North Pole by the GS (generic term for atlantic currents towards the North) is a significant part of the total heat transport.
This calculation can be made in considering the flow and the delta temperatures between mid-latitudes and high-latitudes of the NAD and by comparing this delta to the theorical necessary energy to obtain the high-latitude recorded temperatures.
It’s not a very precise calculation but it allows, I think, an approach of the scales of sizes.
Many thanks for posting this, I was one of those “clamouring” for more info. ;)
You haven’t made any reference to Dr Bogi Hansen (et al) of the Faroese Fisheries Laboratory. I’Ve had difficulty tracking down references to any more recent work. But as an explanation of what they’re up to: “ICES 1999 Annual Science Conference CM 1999/L:19 Nordic Seas Exchanges The Deep Overflow through the Faroe Bank Channel” In this paper he doesn’t make any firm commitment to trends, but notes “a slight indication of a decreasing trend in ISOW transport” ; “as yet the ADCP measurements in the Faroe Bank Channel are of too short duration to allow any conclusions on this important question.”
I can’t find the source used to support this later story but on 21 June, 2001 Alex Kirby of BBC news reported “They (Hansen et al) estimated that the flow had fallen by 20% in the last half century, with most of the decrease in the last 30 years. And they say the rate of decline has accelerated within the last five years.” The same finding was also reported by Caty Weaver of Voice of America on 25 July 2001.
Due to intensive reading since January I’ve gone from sceptic, to being staggered at the pace of change. One thing I’ve noticed is that estimates “seem” to continually roll forward the issues raised by projection scenarios as our scientific understanding improves (this is just an impression from my reading I’ve yet to do a timeline to fully satisfy myself on this “apparent” trend). Whilst deferring to your expertise: I find myself in the curious position of having gone from an uninformed sceptic of Climate Change, to wondering if this is another change whose pace the experts are underestimating.
“All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.”
- Arthur Schopenhauer (1788-1860)
It’s a pity science just couldn’t jump to stage three… without the tiresome delays that individual scientists invent to promote their own predominant [and usually wrong] assumptions. In the nineties, scientists were predicting triple crop rotations per year as a result of enhanced weather from global warming. Then the warning was turbulence! Now the warning is fear of cold. Admit your intellectual fraility – virtually all judgements are transient and nearly all eminent scientists are eventually proven to be way off the mark. I for one, and probably much of public opinion, believe it is better to be roughly right than precisiely wrong!
In the margins between one era and another, uncertainty is king! There are no experts, “the best of the last era, becoming worst of the next!So especially true of science and scientists!! It is amazing to hear so many scientists definitely state exactitudes based on next to no absolute understanding of what is truly going on! This is what gives science a bad name.
When I was at school in the fifties, the oceanic warm and cold currents were clearly marked and learnt as fact. They are still there, the same today – just! Scientists – ask this of yourselves – precisely what would the map of oceanic currents have been during the last iceage? The differences might just explain why ‘The Times’ could be more right than you think! If you don’t know the differences… you have no right to pass judgements on so-called non-scientific thinking. If we are about to change eras, conventional wisdom will be the last to recognise it!
In defence of the ‘Times’ any publication aiming to tap into growing public interest must be good – as climate change accelerates up a public’s cause for concern ‘Richter scale’.
Judgements based solely on precision of the day generate the ‘ass’ fronting all assumption! Much public concern with science is based upon its arrogant intolerance for new insight, total reliance upon evidence and incestuous drive for self-publication. Open minds always beat closed ones!
Comment by Trevor McMinnis — 27 May 2005 @ 6:19 AM
thank’s for your response William (#2)
The entire circulation is not, undoubtedly, only salinity-driven.
I’m agree that there are wind-driven gyres and loops in the central Atlantic even in the North.
To illustrate what I said ,concerning the relations between the two circulation forces ,please look at this link : http://www.cosis.net/abstracts/EAE03/07512/EAE03-J-07512.pdf
No amount of risk of a major change in the conveyor is acceptable but it is too late for that. If the press would frame the issue as a risk analysis, as you do, then there would be no issue. I did not mean to imply (and did not, I think) that the issue should not be studied and assessed.
WRT #9 – Part of the effect of things being predicted by theory and then found is that after a theory predicts something, people go looking for it. The converse is also true, after something is found, people go looking for a model. A good (bad) example is the Antarctic spring ozone hole (cold fusion).
Thanks for the reply, Stefan (comment #3). I have downloaded and skimmed the Rhines and Hakkinen (2003) response to Seager et al. (2002), but it will take a more carfeul reading for me to digest its contents. On first glance, it does seem to present meaningful challenges to certain areas of Seager et al.’s approach. I’d be interested to hear if Seager has responded in kind – anybody know?
Anyway, to address a couple points in your response to my previous comment …
1) The data you cite from Weaver and Hughes (1992) merely supports my last statement (paraphrased from Seager et al., 2002), that THC (or more specifically, the North Atlantic Drift) keeps European latitudes north of 60 deg N warmer than other land areas at those same latitudes. So, while it is instructive to say that the North Atlantic Drift, which is indeed a remote branch of the Gulf Stream, has a dominant influence on keeping Scandinavia warm, I’m not convinced that the same can be said for it in terms of keeping Britain (between about 50-60 deg N) warm, as Gavin implies in the original post. Comparing temperatures in Britain to temperatures in British Columbia, Canada would be more meaningful.
[Response: That would be more meaningful, and it does give the same qualitative answer. Britain is around 5 degrees warmer than the mean for it's latitude, BC is not. Look at Stefan's excellent fact sheet, and as he points out above figure 3 in particular. -gavin]
[Further Response: It is true that the effect is strongest in high latitudes. Have a look at Fig. 10 of Seager et al. 2002, which shows the January cooling in their model after switching off ocean heat transport. This cooling exceeds 20 ºC in northern Norway, and it is "only" 3-6 ºC in Britain, Germany and more generally in central and southern Europe. Clearly much less than in the far north, but clearly not peanuts. Other models get similar results. So I think one should say: the Atlantic ocean currents keep the whole of Europe warmer than it would otherwise be by several degrees, with the effect getting stronger the further north you go. - Stefan]
2) In terms of “educating the public away” from misleading ideas, well, I think that is part of our job, of pursuing scientific truth and honestly reporting our findings about the natural environment. That said, however, I could have chosen my words more carefully in my previous comment. In more precise terms, I don’t think it is necessary to completely disavow the public of the notion that the Gulf Stream contributes to western Europe’s mild climate; rather I would argue that we need to emphasize “contributes” over “determines” and include the atmospheric side of the story, which is at least as important as the oceanic side.
[Response: Part of the education has to be differentiating the factors that contribute to current climate conditions (i.e. the 33 C greenhouse effect, the atmospheric poleward heat transport), and the things that are most likely to contribute to variations. For instance, there is no evidence that, with the current configuration, atmospheric heat transports have vastly different modes of behaviour - and so they are unlikely to suddenly flip into a new state. However, such changes have been modelled (and observed in the climate record) for the ocean transports. - gavin]
I actually caught the authors of the book on the radio the other day, discussing this very same topic/these very same articles. They thought it was worse than they ever could imagine, something they expected to take centuries but was taking place in just a period of several years. One of them discussed reading one the articles and then saying, “It hit me right between the eyes. Between…the…eyes!”
The last thing I heard was one of them basically implying that after the THC shuts-down, the world will have co-existing glaciers and desert with basically nothing in-between and no agriculture. The way they talked, I envisioned each one of them doubled-over in pain. I think one of them said it was the worst news he’d ever seen before.
Comment by Michael Jankowski — 27 May 2005 @ 11:27 AM
Any takers for the map of oceanic currents during the last ice age?
Comment by Trevor McMinnis — 27 May 2005 @ 11:50 AM
(Re#13…maybe it was the screenplay writers, not book authors…not sure there was a book!)
Comment by Michael Jankowski — 27 May 2005 @ 12:28 PM
It’s important to know the heat budget of each latitudes band.
For example without heat meridian transfer(atmospheric and oceanic) the annual mean temperature of the 50-60N band should be -14°C.
Fortunately this transfer exists, but a reduction of it leads to a very logic decreasing of the temperatures.
The equilibrium budget latitude is about 42N (with my own calculations)
But the real question of this topic is : “Is there ,as Wadhams says it, a decreasing of the GS and/or the NAD and how can we realize it?”
I’m sure Art Bell will be back on the radio suggesting we’ll all be eaten by chupacabra before the THC shuts down anyway.
Comment by Benjamin Harrison — 27 May 2005 @ 2:44 PM
I would have thought that any interruption to this circulation would be some cause for alarm. As I understand it some of the flow is salinity driven and was interrupted in the recent past by a huge influx of fresh water from an ice dam breaking.
Surely the melting Arctic ice cap is changing the salinity of the northern Atlantic and could affect this circulation as well.
One wonders how competent science writers could confuse THC with the gulf stream? It would be interesting to track this entrenched confusion back to its origins. Also, how much of the climate change hysteria in Europe and especially the UK is due to this fallacy? It may help to explain the difference in European and American atitudes.
There is no confusion between GS and THC.
And even in this case is it important?
I don’t think so.
It’s important from another way to know that the evaluation of deep water in Arctic zone is about 20 to 30 millons m3/s.
It’s the most important source of deep water in the world,and it generates the most of the conveyor belt.
It’s also important for the oceanic and atmospheric CO2 balance.
The great flow of Arctic deep water comes mainly from THC and is fed with NAD.It prooves the great sinking of water in this zone and the great oceanic heat transfer.
So I repeat that a slow-down of THC leads to a slow-down of the GS and to temperature decreasing in western Europe.
It’s not too difficult to understand and to admit for american and european people.
Re #22, no one seems to agree with you Pascal. The consensus is that European warmness is due to a combination of atmospheric circulation and a wind driven GS, neither of which is driven by the THC. I would add that the GS has a cycle time of (I think) around 35 years while the THC takes around 1000 years to cycle so it is hard to see how the very slow latter could drive the fast former. But then it seems like everything in climate science is contentious.
[Response: Well, I mainly do. It is not true that the THC has no impact on Eurpoean temperatures (see my earlier responses and those from Stefan, and the above figure). This doesn't imply that the THC 'drives' the GS, only that what we see as the GS is a combination of the two factors. Thus response times in the North Atlantic have many time scales that interconnect the wind-driven and themohaline circulations ranging from decades to potentially centuries. -gavin]
The timescale is evidently uninteresting to policy makers and short term climate modelers — but isn’t an anthropogenic change in thermohaline circulation something to worry about (for example, the post by D. Archer a while back).
It will take centuries or millenia for the ocean to approach a new equilibrium and a thermal anomaly to penetrate marine sediments, but the resulting effects upon the marine carbon cycle could be catastrophic. At some point we will be resigned to simply watching the dominoes fall. Right now we have an opportunity to stop pushing them over.
I think that atmospheric circulation is also linked to the GS.
This current brings 1.5PW (10^15W) of heat through Atlantic ocean.
this represents around 50 % of the total transferred heat between Equator and North Pole.
This heat leads to great quantities of water vapour introduced in the atmospheric circulation.
So the GS has a significant influence on the atmosphere and its circulation.(It’s not the only one!!)
It’s clear that THC does’nt drive the entire GS (I never said it) , there are ,as Gavin said it ,loops and gyres which have a great participation in the heat transfer.
But,maybe someone is agree with me to say that about 20 millions de m3/s are sinking in Arctic zone ,or it’s also false?
If it is true ,from where does the water come from?
The rain ?
I don’t think this is the case.
So a reduction of THC leads to a slowdown of GS.It is not a question of timescales it is a question of addition and substraction of flows.
Example (the numbers are not true) near Florida the GS flow is 40 millions m3/s.At 35-40N there is a loop branch of about 20 millions de m3/s (that returns in tropical GS) and the DNA branch of 20 millions de m3/s.
If the DNA branch (more THC-driven than wind-driven) is slowing down to reach 10 millions m3/s ,what is ,for you, David,the flow of GS near Florida?(in the absence of any feed-back)
For the original subject of this topic “Gulf Stream slow-down” it comes from an article from Peter Wadhams in Sunday Times.
I’m not sure that Peter Wadhams is right particularly for this story of “huge chimneys”.
I believed that the sinking of dense water was more diffuse than in chimneys.
Has someone some ideas about it?
There is not very much chance that the THC will shut down / slow down in the foreseeable future. Indeed it happened a few times in the past, e.g. at the 8.2 ky event, when an enormous flow of sweet water from lake Agassiz entered the North Atlantic, due to the collapse of an ice barrier.
But the extra quantities of fresh water involved today and in the far future are of another magnitude. If we take the largest decline (1,500 km3/year) in Arctic ice cover of the last decades together with the increase in precipitation over the Arctic (500 km3/year), then this is completely dwarfed by the 115,000-230,000 km3 in one year which is supposed to have slowed down (not even stopped) the THC. The quantity involved was larger than the amount of water that is refreezing every year in the Arctic, thus probably preventing (to a certain extent) the formation of cold saline water.
Further, according to the Colorado State University forecast for the coming hurricane season (point 8 ), the number of hurricanes in the Atlantic is directly related to the strenght of the THC (but I suppose they point to the Gulf Stream part in this case), according to them:
“Since 1995, the THC has been flowing more strongly, and there has been a concomitant increase in Atlantic major hurricanes in the tropical Atlantic.”
[Response: The definition of 'THC' in the CSU forecast is their own particular diagnostic which may not actually have very much to do with the THC as we are discussing here. It is simply the SST off of Europe, which although it may be affected by the THC, does not define the THC. There are situations, for instance of general global warming where this temperature could increase, and yet not imply any change in the THC. This kind of imprecision in terminology (from CSU) is unfortunate and does not add to the clarity of the discussion. The truth is that we are very poorly served by the current observational network, and cannot give precise estimates of what the THC 'is', particularly not on annual time scales.- gavin]
I recall reading something in the past few years about some scientists whose models implied the presence of the Rocky Mountains had more of an impact on European warmth than any ocean currents. Is anyone familiar with this?
[Response:Seager et al, as referenced above and as discussed in Rhines and Hakkinen. With respect to a previous point, that since the Rockies aren't going anywhere, they are unlikely to be responsible for any variations in European warmth. - gavin]
Comment by Michael Jankowski — 28 May 2005 @ 5:22 PM
I am not a scientist, but have a great fascination with what is really happening in terms of climate change and have followed the discussions on this site and a number of others with interest. What is notable is that over the last decade, the majority of scientists appear to be continually renewing their predictions concerning the impact of global warming, and each time this is done, the speed of climatic change is anticipated as being quicker than previously expected. It is therefore not unreasonable to assume that the level of climate change currently occurring is still being underpredicted. It is also my understanding that the oceanic and atmospheric circulations are intrinsically linked, so that changes in one are likely to lead to changes in another. In other words, a slowing of the North Atlantic Drift, though not eliminating the influence of the Jet Stream, may well change the course in which the Jet Stream flows.
Comment by Stephen Prosser — 29 May 2005 @ 3:48 AM
ref 28: horray for common sense and people like Steven Prosser! Scientists like to think digitally, right or wrongs. The rest of the human race use what nature provided – the best analogue technology that money can buy, that means imprecision and assumption leading to scenario building and making of judgements. In contrast the purity and precision often offered by empirical researchers more often than not leads to confusion in the big picture and attention to detail that camoflages what is really important. I too am not a scientist, but everybit as concerned about what’s going on as any scientist. It’s my planet too!
Science should be aware of and always remember the ‘Titantic Principle’: “Evidence alone is never enough. Evidence when its most needed always arrives… simply too late.”
I am not alone in regarding much of scientific debate as little more than mere discussion regarding the relative movement of chairs on the sloping deck of the titanic, whilst the main event goes on elsewhere… unbeknown to those on top! Don’t take this personally, but we can always learn from those we yet don’t fully understand.
The planet is working hard to attempt to return equilibrium to its heat budget. [Why it is out of balance is merely academic and at this stage irrelevant.] For this balancing act [work] to take place at all, energy flows must have… a sink and… a source. The oldest sink on this planet was decommisioned by natural forces some 10,000 years ago. Once stirred into motion, the coldest sink current in the world, dormant for 10,000 years, will transform the map of oceanic and atmospheric current flows, thereby making the necessary rebalances by whatever [unknown] means necessary to maintain the second law of thermodynamics by absolute right! It will also make a nonsense of current flows of wisdom based on purely on early 21st century logic [a carryover from an inappropriate 20th century]. An imbedded population of 1.0 billion people in Europe have a right to expect that science provides timely answers and solutions not bicker and debate about movement of arbitary deck chairs!
Please widen your debates and keep your eye firmly on the big picture! Good luck.
Comment by Trevor McMinnis — 29 May 2005 @ 8:53 AM
Re the Response to my #23. My mistake (nobody said this was easy to understand). So European warmth vis a vis Labrador has three major components (any others?). 1. Direct atmospheric circulation heat transport. 2. The wind driven component of the GS. 3. The THC heat transport which may be part of the GS. Any idea what the relative proportions are?
Regarding 3, I am still unclear about the hydraulics. Does the THC flow via a separate stream, apart from the GS, or does it contribute its heat to the GS, such that the GS would be cooler if the THC were absent? I ask because the velocity of the THC is so much less than the GS (100 times less?) that I cannot see how it can be part of the GS on a flow basis.
[Response: Velocity isn't the correct thing to compare because of the way these things are averaged. Total mass flux is better. The THC is about 20 Sv (1 Sv = 106 m3/s), and the amount of flux in the GS (in the upper part of the ocean) is around 40 Sv, so the THC contibuted about half of what we see in the GS. (Both these numbers are approximate, but are qualitatively ok). -gavin]
excuse me David but I don’t understand your argument.
Why do you speak about THC velocity?
Is it the velocity of the dense water sinking?
If it is the case I’m agree, this velocity is clearly very much smaller than the DNA velocity.
But it is important to account the flow ,which is the area * velocity.
For example ,if the area of sinking is 20000 km2 and the velocity is only 1 mm/s (0.04 inch/s) the flow of THC is 20 millions m3/s which is the NAD flow.
To get this sinking velocity the water must be a very little more dense than deeper water.
This little more dense water is obtained by the salinity and supercooling.
If there is an unusual mixing with fresh water and/or warming of SST in Arctic (actually observed) the sinking may stop.
I don’t know the density difference which is the driving force of THC but I think it is very weak and the amount of necessary fresh water or rewarming is, maybe, not so great that it is said in#26.
Thanks Gavin, I get the point (in your response to my comment #14) that your intention here is to discuss changes in the ocean/atmosphere system that could cause a cooling of European climate, and that both observational and model evidence point to a weakening of THC as the most likely candidate. Actually, as an ocean/climate interactions student, I am happy to say that I basically agree with you on this. My point, which I hope you would likewise concede, is that even with a weakened or shut-off THC, western Europe would still remain warm relative to other land masses at the same latitudes (possibly even warmer than British Columbia, as it is now), based primarily on atmospheric circulation patterns. I am afraid that these news broadcasts, movies, etc. may give the public the impression that if the Gulf Stream weakens, western Europe will equal Siberia (climatologically speaking), and that is simply not true. After all, as you say, the Rockies aren’t going anywhere anytime soon, so regardless of what happens to THC, the persistent crest in the Jet Stream over the eastern North Atlantic and western Europe should continue to bring warm (though perhaps not as warm) maritime air to the northwestern part of the continent.
[Response: I am happy to concede that. In fact these two media stories were the first I have ever seen in which Siberia is quoted as potential outcome. That is not on the cards. -gavin]
The Rockies aren’t going anywhere soon, but remember that the waves that the Rockies drive are not just passively emitted by the Rockies! They depend on the winds blowing over the Rockies, and the propagation characteristics of the atmosphere downstream. The propagation characteristics, and the way they affect the temperature pattern of the waves, is dependent on the pole to equator temperature gradient, among other things — which gets lower, in a global warming situation. This will all affect the European climate change pattern.
On the matter of whether THC shutdown could wipe out European agriculture, it is true that there is no support in current GCM’s for that happening. THC shutdown in a global warming scenario offsets some of the global mean-warming, which might look like a good thing. The real thing to fear in an altered THC scenario is an increase in climate variability — a decade of drought, a century of heat waves, twenty years of cold winters, forty years of flood. It’s that, rather than the “ice age” scenario, that is the thing to be concerned about. That’s the message we tried to convey in the NAS book.
All that notwithstanding, we do have the uncomfortable fact that,at the Younger Dryas, SOMETHING (partly involved with THC shutdown) caused a sudden drop in temperatures that, if it happened today, would have very dramatic repercussions. One has to be careful to distinguish the extreme drop in Greenland with the more moderated drop over Europe, but still, it is far from clear at present that any real GCM, with the ocean-atmosphere dynamics properly represented, yields a temperature change of comparable magnitude to the YD. There’s a case to be made that current GCM’s are still missing something, and are insufficiently sensitive to climate forcings. It’s not by any means an ironclad case, but it’s more than a loose end.
[Response: I agree with Ray - regional cooling is only the most well-known result of a THC shutdown, not neccessarily the most serious one, especially if it happens only after a lot of global warming has occurred which offsets it. Other likely consequences (e.g. rapid dynamical sea level changes, shift in the inter-tropical convergence zone and hence tropical precipitation patterns) are discussed in the Rahmstorf and Zickfeld editorial essay mentioned above, and the references therein. - Stefan]
Trevor has asked one valid question, “What would the map of oceanic currents have been during the last iceage?” This question presumes that the inputs for such a reconstruction are obtainable; it also presumes that particular aspects of the previous ice age(s) are relevant to global warming scenarios. I’m a fish geneticist so I won’t bother commenting on ‘paleo-ocean current-ology’, but it seems to me that glaciation would result in a reduction of fresh water inputs to the North Atlantic (during the ice age) and would therefore be quite different from the mechanism in question (which is related to early phases of global warming). Perhaps someone in the know could present a reference or two for Trevor and I so that we can understand something about oceanographic reconstructions and their potential relevance.
Trevor: hang in there dude, and have some faith that scientists are generally keeping open minds. After all, finding something that nobody else has found would be very rewarding (wrt publication and otherwise). We’re doing what we’re trained to do and what we do best. When there are enough data suggesting that our working hypotheses are incorrect, a new paradigm will emerge that is hopefully closer to The Truth. But without serendipity, the only way these data emerge are through the normal work of scientists. You and other members of the public may not have much confidence in our work (and the oil industry and their hired guns play on that), but I think you’d have to admit that we’ve done quite well in the past. Scientists are making good progress doing science, maybe better progress than others are making in other disciplines (e.g., policy), and almost certainly more progress than if we tried to do that other stuff.
I think I read somewhere that if the THC slows or closes down, then the ocean’s ability to take up atmospheric CO2 will diminished — I think because less nutrients are churned up, causing phytoplankton to diminish. I’m not sure about this, but if it’s true then it would be another positive feedback loop in the GW process. I don’t suppose that’s been added to the models.
Comment by Lynn Vincentnathan — 30 May 2005 @ 3:34 AM
You guys should do a piece on the conveyor and Heinrich events. Those of us not among the climatocogniscienti need some grounding.
Re the Response to #30. Flow is important for at least two reasons. First, if some of the northbound return flow of the THC is actually carried by the wind driven GS then that fraction will not stop if the THC shuts down. The much greater wind driven GS velocity means that the THC pressure gradient is not driving the embedded THC flow at this point. From your numbers this could include all of the THC flow. Second, if there is northbound THC flow that is not part of the GS it probably has to be below it, otherwise the GS would cut it off. At that depth there is very little heat transport. The uphot of all this is that most of the ocean heat transport that warms Europe is probably due to the wind driven GS and would not stop if the THC shut down.
[Response: Let me try again. Assuming we can think of these things as quasi-indpendent, you can consider the THC to be the flow pattern you get by integrating the flow across the basin, and the wind-driven part as what you get by integrating through depth. These are independent of each other, yet if I look at the upper-level Gulf Stream transport it will contains a bit of both - some of the mass flux is balanced by the deep western return flow, and some through the shallow eastern return flow. Each part contributes to the heat transport. If the deep return flow slows, then the mass transported by the Gulf Stream decreases, and the associated heat transport decreases. That this does effect European temperatures is seen in the model simulations above, as well as from evidence in the paleo-record. Does this imply that heat transports go to zero? No, because the wind driven part is still there (as is the part related to planetary waves), but it does mean that they may be noticeably less. - gavin]
Re#27, Thanks Gavin (I’ll smack myself on the forehead and give myself a nice big “duh!”)
Re#28, My perception is actually the opposite – each new iteration of modeling and analysis seems to result in a downward projection of both the magnitude and pace of global warming.
Comment by Michael Jankowski — 30 May 2005 @ 1:20 PM
Re #28 Stephen Prosser. Assuming all other things are equal then I would expect that a shutdown in the THC would lead to more heat transport by the atmospheric circulation as the temperature gradient would be greater. Consequently I would expect a more active storm track and more intense cyclones. Therefore the equilibrium temperature change would be somewhat less than that approximated by simply removing the heat trasported by the THC. Also, climate models generally show much greater warming at higher latitudes [due to the ice-albedo feedback I guess] so with a THC shutdown in a global warming scenario the impact would again be lessened. [The greater high latitude warming produces a weakening of the storm track in somee models global warming experiments].
What I’m more interested in is: Where does all this extra heat go that is no longer being brought to the North Atlantic? If you look at the figs at the top you can see some red in the SH. Due to the THC the Atlantic has a net SH -> NH heat transport so a collapse of the THC would see more heat left in the South Atlantic. My main worry would be what impact this would have on:
*1 – Antarctic ice melt. [Presumably increased -> ocean sea level rise]
*2 – Tropical convection patterns. [Tropical convection being very sensitive to SST]
Basically having London become more like Vancouver is not a major concern. It’ll cost in terms of having to make the infrastructure more resilient to colder winters, but it’s not what I would call ‘dangerous’ climate change in terms of what the UNFCCC commits us to avoid. Sea level rise that overwhelms the Thames Barrier and floods London, or changes to tropical convection which affect agriculture for millions of African subsitence farmers, however I would consider dangerous.
Clearly these types of questions are second-order questions compared to the first-order question of whether or not the THC will collapse/weaken appreciably, and therefore harder to address, but they are more important from an impacts point of view.
This has been a fascinating discussion. However, I haven’t yet seen anything on the contribution made to changes in salinity of the North Atlantic by the melting of ice-rich permafrost. About 50% of Canada and Russia are underlain by permafrost and this is, in places, more than 1km deep. Not only is melting likely to release significant quantities of GHG, there will also be an increase in the discharge of Arctic rivers. Has this been modelled?
Comment by stephan harrison — 31 May 2005 @ 6:52 AM
#40 – GCMs certainly deal with rivers, although how they deal with catchment basins vary.
When I’ve heard people talking about increased river flow in Arcitc rivers it’s normally been on the basis of greater precipitation rather than due to permafrost melt. I’m not sure whether this is because it’s thought that the increase due to greater precipitation is greater than the increase due to melt or whether because there is more confidence in an increase in precipitation [and its magnitude] compared to that of melting permafrost.
Re #39: “Basically having London become more like Vancouver is not a major concern. It’ll cost in terms of having to make the infrastructure more resilient to colder winters…”
How many times do Canadians have to state we don’t live in igloos! Vancouver’s summer and winter temperates are very similar to London’s, to within a degree or two. The main difference is we have sunnier dryer summers, and almost twice as much rain in the winter. Snow is rare.
Comment by Bruce the Canuck — 31 May 2005 @ 4:52 PM
The increase of Arctic river discharge was 7% in the period 1936-1999, or 128 km3 on a total of app. 1800 km3/yr. But total precipitation (including rain/snow on the Arctic ocean) is estimated to have increased with 500 km3/yr.
One should compare that with the amounts of water which melt and refreeze in the Arctic. In the last decades, the maximum summer melt is around 21,000 km3, but practically the same anount refreezes in winter (the Arctic winter ice volume has a smaller decline than summer ice). Thus even if the THC should shut down or reduce in summer due to an increased melt (of which I have not found any evidence), it should restart in winter, when ice forms and salt concentrates in the cold remaining water, giving it a higher density.
Thus (the increase in) precipitation over the last decades is far lower than what melts and refreezes as Arctic floating ice. The amounts which slowed down the THC at the 8.2 kyr event (115,000-230,000 km3) to the contrary where higher than what could be refreezed in winter, thus preventing/reducing the densification of the polar waters.
A rough calculation says that one need a 300-fold increase in precipitation in the Arctic to have a similar problem as with the 8.2 kyr event.
[Response: There's two errors in this reasoning.
(1) You compare a seasonal cycle (which over the year integrates out to zero) with a mean flux. To drive a mean deep water formation, you need a mean surface flux. The annual cycle of melting and refreezing does nothing, but a net sea ice export does (as it continually exports freshwater and thus provides a mean salinity enhancement). Of course convection occurs only during a few days in the year when the highest surface water densities can be reached, but the THC operates on longer time scales, it responds to the mean fluxes sustained over a number of years.
(2) You compare a volume of freshwater in km3 (the catastrophic drainage of Lake Agassiz which triggered the 8k event) with a freshwater flux in km3/year (enhanced precipitation, river and meltwater runoff). - stefan]
Gosh .. DaVinci must be rolling over in his grave He lived at a time when the greatest minds were engaged in argument over how many angels could fit on the head of a needle. Leonardo never stopped observing .. and always asked why. Why as important to him as how. Though he may wonder if anything has actually changed, for the good of us all, I wish he were here now. ‘Some’ of what he would observe would include the following:
Up to 80% of the krill are gone in southern oceans.
Southern ocean temperatures are down.
Northern ocean temperatures are up.
Alaska (and much of northern Canada) has increased in temperature to the point that it is greatly affecting the permafrost.
Some plants are blooming way too early, some way too late.
Birds are disappearing in huge numbers in England .. and all over the world.
There are droughts all over the planet.
Seems that the West Antarctic Ice Sheet isn’t as stable as we thought just 3 years ago.
Warm-water phytoplankton has moved north many hundreds of miles.
Rice and grain harvests are down, planet wide.
The Gulf Stream is slowing.
By some estimates, up to 40% of the artic ice is gone.
Ocean dead zones are happening earlier and are getting bigger .. much bigger.
There are more polar stratospheric clouds observed now than ever before.
Storms are forming earlier, are greater in number and greater in strength.
It’s cold where it should be warm, and warm where it should be cold.
8 of the last 10 years have been the warmest on record since we started keeping records!
This is only a smattering of what he would see today.
Surely you all know that one has a cascading effect on the other and that the whole is MUCH greater then the sum of its parts.
I don’t want to sound alarmist. That is not my aim as I know some of you will immediately conclude. That being said, I would much rather be a bit freaked out and proven wrong then to walk blindly off a cliff to my doom. If ANYONE of you had a lump on your arm, you would go see your doctor right away. You would not wait till you had 20 lumps before you went to see him. The world .. our world .. over which we are stewards HAS MANY LUMPS!!! Stop arguing about who’s theory is right or wrong, about the need for more data and better forecasting models. The planet is telling you something .. and its not whispering. It’s yelling it out. Its time for all of us to stop thinking in absolutes and start ACTING on what we are witnessing!
[Response: Dear Gil, as scientists we are the doctors who need to make the diagnosis - thus we have to discuss all those "lumps" in every detail. Like, is the Gulf Steam actually slowing? I'm not sure it is.
Is what we already know enough to act and start reducing our emissions? There I agree with you - in my judgement it is. But that involves a value judgement. Our role as scientists is not to make this judgement for you; it is to investigate the facts and lay out what we know and what we don't know as clearly as we can, so that everyone can make their own judgement. And that's why we have to discuss all those details... - stefan]
#42 – I’ve had a look around the place [Met Office website and Canadian Weather service website] and, although I’ve not been able to find anything that shows things very clearly, it looks as though winter mean and minimum temperatures are about 5 degrees lower in Vancouver than in London. That’s about the same difference as between London and the Scottish Highlands. As I said, I don’t think it is a major concern, but… it would have implications for transport infrastructure [for example], which grinds to halt in the South East pretty much whenever there is any snowfall. [Because at the moment snowfall is very rare so it isn't worth spending the money in the equipment to be able to deal with it...].
I did find one page though that said that the climate in Vancouver was a good reason to move there, so I guess it can’t be that cold…;-).
I do understand this. Please know that I have the highest degree of respect for scientists as a whole and to the commitment, dedication and effort it takes to become one. Many of my close friends are scientists; some of them work for me. I get into this with them as well.
My concern is that there is too much analysis. Analysis leads to paralysis. While I understand that you need to discuss the ‘lumps’ in details, I offer to you for your consideration that my doctor, while he would indeed seek out additional information, would nonetheless based on the observed phenomena (the lumps) attempt some remedial action. If he found that course of treatment to be ineffective, he would make the required changes. Stated otherwise, you can’t steer a parked car!
I will grant that no one knows for sure if the Gulf Stream is slowing, but, it does appear to be. I know that it is impossible to predict exactly what the weather will do on a global level due to the enormous complexity of the equation and the huge numbers of variables. It is however possible to make some guesses. (I know this is hard to do in academia as one can be ostracized for this type of thinking. Carl Sagan did not receive much support form the scientific community at large when he suggested there must be extra solar planets. Turns out they are the rule, not the exception.) That being said, we can learn from what we see. Just before writing this, I note that it has snowed in Somalia for perhaps the first time in recorded history. I see that the droughts in Australia are getting worse as the rains have failed and that they are experiencing some of the highest temperatures on record. I live in Victoria BC and the temperature here hit 30 Celsius last Friday (way above normal) and was close to 40 inland.
I appreciate that it is not the scientists that will make the changes we need. That falls into the hands of our governments (God help us) and indeed, into our own hands as individuals. I am VERY grateful that you do what you do Stefan. I just encourage you all to be a bit less concerned with absolute data and more concerned with the obvious changes that are occurring across this planet. I agree with Trevor when he says: it is better to be roughly right than precisely wrong! We need loud voices now. The time for sitting on the fence has past. Speak and the world WILL listen.
[Response: Gil, I think we agree on one point: by debating every symptom and every piece of evidence in great detail (important as this is to science), we risk losing sight of the "big picture". We don't know for sure what the Gulf Stream is doing - but there are some basic facts about global warming that science is sure about. Scientists like to discuss the new and not yet universally agreed results, and the media like to focus on the controversies - repeating what we have known for over a decade is simply not "news". So perhaps it is important for us scientists to remind people every now and then of the basic uncontroversial facts, which are not disputed in the scientific community any more (only by a few industry lobbyists). (I've tried that e.g. in this article.) -stefan]
#45, Regarding temperature, it could seem there is not much to worry, if things to be taken in account are only the ones you point.
But regarding ecology, crops, all the issues which imply adaptation, migration, etc., it could be (and is already being) disastrous.
It takes a long time to adapt to all the changes.
#44 Gil, all you include in your list is, as you say, only a fragment.
All this reminds us we are no more than one more animal species on Earth.
I know in medical statistics, if you have a whole lot of weak evidence, from a lot of different measures or theories, all suggesting there’s something going on not yet well understood, it’s more convincing that if you have only one piece of weak evidence that’s easy to dismiss as being that one-in-twenty accidental blip.
Re#48, as stated in the article, they’ve been anticipating the possibility of severe and early flooding for some time as the result of a recurrence of El Nino. They’ve been on flood alert since the beginning of the year. One could certainly argue that it’s worse (or that El Nino is made more frequent) due to GW.
It didn’t seem to get a lot of press, IMHO. However, if the opposite were true – if we in 2005 were experiencing a much higher than normal level of tornadoes – do you think you and/or a lot of other people would also consider this a sign of the negative impacts of GW? I’ve already seen a number of articles where people were worried that 2005 would have wacky summer weather and either a lot of hurricanes or very strong hurricanes (or both) due to GW. I can only imagine where the finger-pointing would be if tornadoes were well above average so far in 2005. But since they’re well-below average? Nothing but silence.
If the Yangtze floods arrived a month late, would that also potentially be evidence of GW?
For a slightly difference perspective, here’s an article about the flooding back in 1998 edition.cnn.com/WORLD/asiapcf/9808/25/china.floods.01/
with a lot of the blame being put on land-use changes. The same goes for this one in 1999, which provides some more specific details about some of the changes in the 50′s and 60′s http://www.wsws.org/articles/1999/sep1999/chin-s09.shtml . This report on 1998 flooding does include the GW argument but mainly discusses land-use changes http://www.usembassy-china.org.cn/sandt/fldrpt.htm . Even solar variability is mentioned along with other “freak” weather events (some of which may or may not be linked to GW).
Comment by Michael Jankowski — 3 Jun 2005 @ 10:00 AM
We can have some control over a lab experiment, but we are utterly ignorant about thousands or millions of parameters affecting system Earth. ThatÂ´s why I respect so much Climate Scientists. They donÂ´t have all the tools needed. It is not possible! Many people expects from them what is impossible.
Snowfall in Somalia reported
Wed. June 01, 2005 10:36 am.
#50 – Very good points. When you have extreme events it is important to consider natural variability and to work hard to avoid the propensity of humans to see patterns even where there aren’t any.
[As an aside it reminds me of the story about Israeli fighter pilots having more girls as children. This isn't neccessarily significant because it doesn't take into account all the groups of people - e.g. British bus drivers - who don't father/mother unusual proportions of girls/boys. If you subdivide the population into enough different groups your bound to find one group that exhibits "extreme" behaviour in one regard i.e. in having an unbalanced ratio of girls:boys babies]
I think, then, that it was the intention of this site to provide the sober scientific analysis of the facts behind the hype of the press stories, such that people wouldn’t over-react to the latest natural disaster and then over-compensate when we had a year with below-normal hurricanes/tornadoes and assume that the scientist’s had shouted “Wolf!”
Having said that…I remember hearing that analysis of two recent extreme events that affected the UK, [September 2000 floods and 2004 heatwave] shows that these were extreme events an increased risk of which would be consistent with global warming.
Also, because, by their very nature, extreme events are rare, it would not be possible to have a meaningful statistical trend until a large amount of global warming had already happened. Given that we have already observed [comparitively small] changes in the mean parameters I don’t think it is necessary to wait to observe changes in the extreme statistics before we take action to limit global warming.
I think that there is an issue with the way that the science is communicated in the media. I think it has left a lot of people confused at the moment.
Re #50, indeed fair points – though if you follow weather as I do you’ll know few ‘chases’ seems to be newsworthy this year. But, and I think you’re doing this, equally there is no need to try find a way of showing every weather event doesn’t have any element of anthropogenic effect within. Clearly if we accept that we’re adding ghg’s to the atmosphere, then there is an anthropogenic effect on the weather and the climate. As ever its not the effect that’s really in question, but how much and how fast it’s magnitude will increase by.
About item (1), the amounts which melt and refreeze in the Arctic give an impression of the relative amounts involved, where an increase of precipitation is marginal, compared to the the masses involved.
But you are right, the total change of in/outflow over the years is more important.
For the inflow, the Gulf stream heat content seems to be related to the NAO, which was strongly positive in the last decades, until recently. The same can be said about the inflow of warm saline waters of the NADC (north atlantic drift current, which is the extension of the Gulf Stream via the north atlantic current) into Nordic waters. And at last, the Norwegian Current, the Spitsbergen Current and the East Iceland Current carry warm, relative saline waters into the Arctic seas. The Norwegian Current is mainly responsable for the loss of Arctic ice in the Barentz Sea.
That for the relative increase of warmer, saline currents toward the Arctic. The opposite East Greenland Current brings cold, less salty water and lots of ice from the Arctic back into the Atlantic Ocean. Ice volume is estimated to be between 1,000 and 4,000 km3/yr and shows a large seasonal variation. The net trend in the past decennia is a cooling of the (deeper) Atlantic around South Greenland, which points to more cooler water/ice export from the Arctic.
Thus there is evidence that the flow of heat/water in and cold/water out the Arctic has increased, but there is not much evidence that this weakened the THC. Remains to be seen what the (decreasing) NAO will bring in the near future.
The link between the NAO and the Gulf Stream seems to be confirmed, as there is a cooling trend visible of the sea surface temperatures in the stream over the past years. See the temperature charts in the UKweatherworld discussion (mid page)
About item (2), the Lake Agassiz outflow is supposed to have lasted near one year, thus the one-time outflow was a near yearly outflow. But as the amounts were so huge, the sweet water may have influenced the top layer of the Arctic Ocean for many decades, if not centuries.
A related article in Nature may be of interest. The researchers tried to simulate what happened during the earlier meltwater pulse 1a and during the Younger Dryas. The meltwater pulse 1a didn’t stop the THC, but during the Younger Dryas, most of the meltwater/iceberg discharge was in the Arctic Ocean, where the huge freshwater spill into the Greenland-Iceland-Norwegian seas could reduce/stop the deep water formation.
In my previous reaction for item (2), I made a mistake, the 8.2 kyr event meltwater discharge was in the Atlantic Ocean (via the Labrador Sea), not in the Arctic. The influence of the freshwater inflow seems not only a matter of quantity, but also where it happens…
All of those items you listed may or may not be related to global warming, which may or may not be significantly enhanced due to anthropogenic GHG emissions. They may also be due to other human-related activity (such as the disappearance of birds). This is the one that stands out to me: “Alaska (and much of northern Canada) has increased in temperature to the point that it is greatly affecting the permafrost.”
“Increased in temperature” since when? I hope you’re not getting your information from the NY Times blitzkrieg of a few years ago, which not only used an incorrect temperature conversion between C and F but which used cherry-picked bookends.
I’ve seen other headlines about permafrost concerns, etc, but my cursory reads often show it as a “concern due to global warming” and not something necessarily being observed. And in some articles where I have read that it is being observed, often historical data shows those cities and/or regions to have been warmer in the past century, which would seem to indicate (1) the permafrost issue isn’t new or necessarily unnatural and/or (2) there is a substantial lag between permafrost melting and rising temps (ie, the current permafrost melt is mostly or all natural).
Comment by Michael Jankowski — 5 Jun 2005 @ 4:24 PM
Re the reply to my #37, let me try again. Unfortunately I do not understand this sentence — “you can consider the THC to be the flow pattern you get by integrating the flow across the basin, and the wind-driven part as what you get by integrating through depth.”
The THC and GS are not two “patterns” they are two mechanisms, driven by different forces. I don’t know what function you are integrating, nor why. I am looking at the fluid mechanics.
The THC is driven by sinking water. This water flows southward at great depth. The return flow is probably diffused through the entire ocean cross section, so its velocity is extremely low. (I doubt the actual molecules ever return.) Moreover, most of this flow probably occurs at some depth, where the temperature is close to zero, so its heat transport is also very low.
The GS is a wind driven gyre of relatively high velocity. It is shallow so transports a lot of heat northward. It may well help to replenish the THC but that mass flow is not driven by the THC pressure gradient so would not stop moving if the THC stopped. That part of the THC is swept along by the GS, a free ride if you like.
Thus I think your statement that “if the deep return flow slows, then the mass transported by the Gulf Stream decreases, and the associated heat transport decreases” is simply false. This simple first order model suggests that the THC mechanism delivers very little heat to Europe and none via the Gulf Stream. Stopping the THC should therefore have little effect, unless one goes on to look at feedbacks, etc., but we need to agree on the basic model first.
In fact I get the impression that the simple fluid mechanical model I have outlined is not included in the climate models. That would be interesting indeed. But if my model is wrong please say how.
[Response: You're not exactly wrong, but you are not getting the big picture either. I presume you agree that mass is conserved? In this situation, we can consider the flow in the Atlantic to be a closed system (i.e. losses/gains from rain, evaporation, export through the Bering Strait etc are all small compared to the mass transports in the GS and return flows). Therefore, at any particular latitude (say 24 N) the total amount of northward mass flow is pretty much equal to the amount of southward mass flow (otherwise mass would be accumulating somewhere else). Almost all of the northward flow is associated with the relatively shallow GS on the western side of basin. The southward flow is partly associated with a deep western boundary current (below the GS) and a more diffuse but larger surface flow on the eastern boundary. Velocities are larger in the western boundary currents purely because of the curvature of the Earth (the beta effect). If for some reason the deep western current slowed, mass balance implies that either the GS must also slow, or the eastern (southward flowing) current must increase to make up the slack. What would happen is that the GS would slow.
Note also that it is the NET heat transport that is important - i.e. the heat associated with the mass going north minus the heat associated with the mass going south. Given the big differences in temperature between the upper branch and the deep western boundary current, that is a large number. However, the temperature contrast with the eastern return flow is much less, so the heat transport associated with that is not as efficient. -gavin]
Re the Response to #6 on risk assessment, I have read the Rahmstorf and Zickfeld (2005) and have two problems with it.
First, I don’t see how you can get objective probabilities for future events out of a climate model. Probabilities are fundamentally statistical entities and there are no statistics here. Unless you are doing it with ensembles which I don’t think works. Can you refer me to the mathematical justification for inferring these probabilities?
Second, even if you could somehow derive these probabilities, to get a risk assessment you would also need the probability of the underlying model hypotheses being correct. I know of no way to derive the probability that a scientific hypothesis is correct. In fact this is the general problem with environmental risk assessment. Even where we do have underlying statistics, as with exposure to toxins, the dose-response uncertainty is not quantifiable. Most uncertainty is not quantifiable. The mathematical concept of probability and the ordinary language term “probability” are only metaphorically related.
[Response: You are using a frequentist concept of probability, we used a Baysian or subjective concept of probability. The latter is the appropriate one for the type of uncertainty we are dealing with, namely epistemic uncertainty (which is uncertainty arising from lack of knowledge about a process, rather than some kind of random variability which could be measured by statistics). Example: you are in the car with a friend in an unfamiliar part of town looking for an address. Your friend says: "I think there's a 50% chance that it's the next street coming up". Her "50%" are an expression of how unsure she is - after looking at the map again, she might be 90% sure. They have nothing to do with statistics or frequency - it does not mean that if you drive past there 100 times, it will be the right street 50 times... Nevertheless this is a workable concept of probability, which satisfies certain axioms which allow for some calculations, Bayesian updating, etc. There is a host of literature on the different concepts of probability and uncertainty - google "epistemic uncertainty" and you'll find lots. We made it clear in our article that we were talking about subjective probabilities. - Stefan]
However, this is an important topic and I pass on the following: Call for Presentations — The U.S. Climate Change Science Program (CCSP) will hold a workshop on Climate Science in Support of Decisionmaking on November 14-16, 2005, at the Crystal Gateway Marriott Hotel in Arlington, Virginia. The workshop will explore uses of observations, modeling, studies of climate and related environmental processes, and derived tools to inform decisionmaking. Current information about the workshop is available at http://www.climatescience.gov/workshop2005/default.htm
The report also disappoints in a more fundamental way: it fails to understand the issue of future ocean circulation changes as an issue of risk assessment, rather than one of climate prediction. In climate prediction, the question is “What is most likely to happen?” In a risk assessment, the question is “What could go wrong, and what would the consequences be?”
A major ocean circulation change can be viewed as a possible “accident” of climate change – comparable, say, to a nuclear power accident. Climatologists have long discussed it as an event that is unlikely but could have serious consequences – a so-called low probability/high impact risk (e.g., IPCC, 2001).
Here it’s stated as a “what if” kind of assessment – there is no attempt to get “objective probabilities for future events out of a climate model” as it is stated above. Also, there is agreement that there doesn’t seem to be a good way to assess a specific risk:
The top panel shows the probability of a major change of the North Atlantic Current, given a specified global temperature rise until 2100. Like the risk of an accident in a particular new class of power station, such a probability for a single future event cannot be calculated objectively; it is an expression of a subjective assessment of the likelihood based on limited available knowledge.
Nonetheless, based on “the modeling framework of Zickfeld and Bruckner (2003)”, Stefan and Kirsten Zickfeld go ahead and start talking about specific probabilities based on the assumptions shown in Figure 1 of the paper. These numbers are stated to be “illustrative only, but not unreasonable”. I’m having trouble with the reasonable-ness of these numbers, especially ocean change probability as a function of degrees warming, which is a “subjective assessment” as stated. Given this post and all the comments, I’d say that the knowledge available for calculating a THC shutdown is definitely limited.
That’s not to say we shouldn’t worry about the risk but assigning a probability to this event (at this point in time) seems to be very speculative. My subjective view is that so many disasters will happen with the climate before there is a significant threat of a THC shutdown that such an event, should it occur, would only be icing on the cake. Also, I doubt that what constitutes a dangerous level of warming requiring action can be defined in the near future by assessing the risk of a THC shutdown given some level of GHGs in the atmosphere. See Dangerous Climate Impacts and the Kyoto Protocol by Oppenheimer and O’Neill (14 JUNE 2002 VOL 296 Science) for some discussion of this.
I feel obliged to add my 2 cents from the paleoclimate data perspective. I think that the following (from this post) overstates things a bit: “There is good evidence from past climates, theoretical studies and climate models that large changes, a slowing down or even a complete collapse, in the North Atlantic Drift and the thermohaline circulation can happen”. In particular, how to interpret the paleoclimate data is not straightforward. What is true is that there is very very strong evidence from paleoclimate data (deep sea sediment cores) for changes in the distribution of chemical tracers that must reflect changes in the deep circulation in the Atlantic. While these are consistent with theoretical ideas and climate models that suggest slow downs, or even collapses, in the thermohaline circulation, they by no means demonstrate that either of these has happened. The tracer changes provide even less constraint on how much the heat transport (and therefore climate) may have changed.
Stefan, thank you also for your notes. I have not yet read your article as I have had a lot on my plate but will read it tonight.
I would like to revisit something you said though.
(Our role as scientists is not to make this judgement for you; it is to investigate the facts and lay out what we know and what we don’t know as clearly as we can, so that everyone can make their own judgement.)
I agree with you to a point. It is your job to investigate and present the data. But, who better to make a judgement than those who know? Seems to me that if we (joe average or joe media) make judgements, those judgements are called into question by the scientific community. As well, it is my experience through observation that when scientists do make a bold statement, they are more often than not taken to task for it by the scientific community. I suspect that they also have a lot of support within that community but those supporters are pretty quiet. The worst of it is to leave it to politicians to make the judgements. I know there MUST be some good ones out there with a lot of integrity but thats just a theory ;-)
Thing is, the dinosaurs lived here for many millions of years. Compared to them, we are rookies. The big difference between us and them (other than size LOL) is that we have the ability to look forward and plan. So, we know what killed them. Something slammed into the earth. We also know it will happen again. Why then do we have such a small number of scopes looking for NEOs ? If I was king, I would sacrifice the purchase of a few F18s to fund such efforts. If we become extinct over from an NEO, then we will have proven ourselves of no greater intellect than the dinosaurs or even worse, we will have shown ourselves unworthy of that great gift. My point is that one can use the same thinking as it relates to the things we are now observing on our planet. And we dont need a zillion scopes to do it. All we need to do is pay attention to the big picture and make whatever predictions and preparations that are necessary to ensure our survival. Like I said before, we need loud and credible voices. No one is going to listen to me.
By the way (to all who are reading this), I dont subscribe to the notion that we (mankind) and our industrial activities are the primary reason for the changes we are observing. Clearly, we have an impact. But all of our bad habits of polluting our atmosphere dont even hold a candle to a few major volcanic events. We are, in the big picture, bit players. And NO this does not mean I think we have licence to keep doing the bad things we do. We need to be more responsible and accountable. However, if we are at the beginning of a natural cycle, we are not going to change it. All we can do is adapt so that we can survive.
The link you pointed me to (The Climate Sceptics) is a great piece. Very informative and instructive. I suppose I am kind of in the attribution sceptic group in that I believe that the changes we are witnessing are not entirely man made. However, your article has given me new things to consider, and consider them I will.
Anchorage seems to be quite similar to Fairbanks, ie. cool between 1950-1970 and warmer in the 1940s and now.
I wonder why the scientist chose a 30 year period to illustrate the warming? It can’t be because he was using “selected data” because it’s only climate sceptics who do that sort of thing. It’s a bit of a mystery, though….
I think #64 touches upon your first link well, but I would add this from the Alaska Climate Research Center climate.gi.alaska.edu/ClimTrends/Change/4903Change.html .
Note especially Figure 2 and the related statement, “In 1976, a stepwise shift appears in the temperature data, which corresponds to a phase shift of the Pacific Decadal Oscillation from a negative phase to a positive phase.” Basically, this (natural) shift seems to account for the increase in temperature in Alaska over the past 30 years.
When this was published (data thru 2003), the 30-yr change in Fairbanks was actually a drop of 0.2 degrees F – and this includes the PDO phase shift climate.gi.alaska.edu/ClimTrends/Change/7403Change.html
Since the phase shift, Fairbanks has actually dropped 0.5 deg F climate.gi.alaska.edu/ClimTrends/Change/7703Change.html
So it’s tough the blame the last 30 yrs for permafrost issues in/around Fairbanks, or imply that AGW is responsible.
I am not sure of the location of the trans-Alaska pipeline or, more specifically, where the soil temperature measurements were made. But as the above links show, the air temperature data trend seems to conflict with the permafrost temperature trend for much of Alaska.
Comment by Michael Jankowski — 8 Jun 2005 @ 11:11 AM
Tough to comment on the Siberia article because no specific locations are nailed-down in the link. But towards the end of the article, a series of “tundra ponds” in Alaska near Council is mentioned. Council is 15 miles east of Nome. According to the links I posted in #64, Nome warmed 2.9 deg F from 1949-2003. For what it’s worth, most of the stations have warmed more than Nome since 1949, and Nome has cooled since the PDO shift. So these tundra ponds should/may be recovering somewhat. How much blame can be attributed to natural variation since 1949 an/or the PDO shift, and how much blame (if any) can be attributed to AGW since 1999 is next to impossible.
I’m not sure why they were even mentioned the tundra ponds because the article then points-out that “tundra lakes” are tied to weather and not climate.
Comment by Michael Jankowski — 8 Jun 2005 @ 11:34 AM
Re the reply to my #57, I think we are making progress. (Aside: my field is the logic of complex issues, so I am used to this kind of confusion. In fact I have a diagnostic system of 126 kinds of confusion. It takes time and work to get clear about one’s differences, something all too lacking in the climate change debate.)
I am starting with not one, but two closed systems. Then we can ask how they may be joined. Mass and energy are both conserved. The first system I have called the Gulf Stream or GS, but that may have confused some. I mean the entire gyre, not merely the GS as named, so let’s call it the GSC or Gulf Stream Circulation. This is a wind driven system, predominantly shallow. The second system is the THC, which is driven by sinking deep water, so is mostly deep.
Each system can be thought of as a set of closed streamlines, mass driven by energy. The energy systems for these two sets are independent so if one, the THC, stopped the other would not be affected. At least not as far as this analysis goes. Indirect effects due to climate change are not considered. Like the wind changing, etc.
The exception to this independence rule would be where the GSC and THC shared some streamlines. As I said, I think there is not much of this since one system is essentially shallow and the other essentially deep. Moreover, given the speed differences most of the energy in a shared streamline would probably come from the GSC, not the THC. I even think maybe the energy lost by a flow from the GSC to the THC is regained on the return flow, which must occur, so if the THC stopped the GSC would not notice. The GSC streamline would just go elsewhere, probably staying shallow.
Finally, to return to the original issue, I think that most of the water borne fraction of the heat transport that makes the UK and Europe anomalously warm is in the GSC. My argument therefore is that slowing or stopping the THC should have very little effect on that warmth, so far as this analysis goes. Perhaps one of my assumptions is incorrect. Has anyone modeled this mechanism? It’s pretty simple.
This link is for Mike Jankowski. This isn’t a very scientific posting but it seems we’re pretty far off the topic of the gulf stream anyway. (I don’t know how you could scientifically use data on village movements without comprehensive spatial and temporal data on other sites to determine a general pattern for permafrost wrt global warming, but I thought it might interest you.) Here’s a quotation from the story:
“In all, 184 Alaska villages are in serious danger of erosion or flooding, according to a government report issued last year.”
In addition to #67, see the work of Moerner (page 9-10) about the historical changes in Gulf Stream position/strength and European climate and the link with solar changes like the Maunder Minimum.
(note that the comments of figures 3.9 and 3.10 in Moerner’s work were reversed)
[Response Ferdinand, for someone who likes to think he's sceptical, you sometimes appear to endorse the least supported argument. The example you give here is wishful thinking at it's best, and at it's worst, complete rubbish. While LOD (length of day) is indeed a diagnostic of ocean and atmospheric circlulation, it is an extremely complicated diagnostic, and to think it simply reflects Gulf Stream position is naive in the extreme. Where is there any model or theory to back this claim up (other than Moerner's own musings)? An analysis of a coupled model's mass field variations would be a good start... - gavin]
That Alaska climate site is suspect in the extreme. For starters, they draw conclusions about the state-wide climate based on a mean of temperatures primarily from southern Alaska. It’s hard to describe this as anything but bad science. Note that the one station they do identify as “Arctic” shows a 2.7 degree increase since 1977. Regarding the permafrost issue, again the temperature data aren’t very meaningful as so many of the stations are in southern areas where there is little or no permafrost. This seems to be the key page describing the Alaska permafrost research: http://www.gi.alaska.edu/snowice/Permafrost-lab/proj_influ/pr_influ.html. The research has definitely focused in the northern Arctic zone of Alaska, so there indeed appears to ne little meaningful overlap with the Alaska climate site data. Perhaps the lesson in all of this is that Alaska is so large that it’s unreasonable to refer to the whole of it as a region for climate purposes.
While we’re on the subject, a quick googling of PDO with global warming turned up a bunch of interesting links indicating that the situation with the PDO is way more complex than our Alaskan climate friends indicate. See, for instance, this review of the current state of thinking about the PDO at http://scholar.google.com/scholar?hl=en&lr=&q=cache:h8EsBo-rkw8J:www.cgd.ucar.edu/~cdeser/Docs/jclim_minobe-pdv.pdf+link:wnDOgs-PrIgJ:scholar.google.com/. Apparently the current belief (discussed starting on page 18, and like most such statements accompanied by a caveat that much more research is needed) is that the PDO itself is closely linked to global warming, which is to say we can expect it to spend a lot more time in the positive phase as global warming progresses. That said, it would be a mistake to conclude that any specific additional warming in southern Alaska can be expected in the near term due to the persistence of the PDO. What would be a surprise would be a flip back to the cool phase.
So, the Alaska climate site statement referring to the 1977 PDO shift as “natural” is misleading in the extreme in that the effect of global warming on the PDO warm phase would be with regard to its persistence and possibly its timing. (By the way, I had a look at the CV of the Alaskan site’s staff climatologist: A 2002 climatology PhD from UMinnesota, she has yet to publish a peer-reviewed study of any kind, her dissertation was on the subject of snow fences, and her BA and MS were in meteorology. Her boss is a meteorologist. Draw your own conclusions.)
Given all of this, I suspect that the current permafrost melting in Fairbanks despite the local temperature having been pretty flat since 1977 is simply a result of the PDO-related warming having persisted long enough to do the job. Probably they can expect to see a lot more of the same.
#67 – “…The exception to this independence rule would be where the GSC and THC shared some streamlines. As I said, I think there is not much of this since one system is essentially shallow and the other essentially deep….Perhaps one of my assumptions is incorrect.”
Your assumption that the THC is “essentially deep” is incorrect. The THC in the Atlantic is sometimes called the Meridional Overturning Circulation [MOC] since all non-wind driven ocean circulations are thermohaline circulations [THC] and so some people feel there is a need to choose a more ‘unique’ name for the Atlantic circulation.
The “Overturning” part of the name is the important part. The flow from north to south of the MOC is “essentially deep”, but the flow from south to north is not. [As gavin said in his answer to #57 above "Almost all of the northward flow is associated with the relatively shallow GS on the western side of basin."] Therefore the GS and the MOC do coincide in the western surface boundary current [from the Caribbean to the North Atlantic].
The deep flow from north to south is slower than the shallow flow because it occurs in a greater cross-sectional area of the basin. This is university undergraduate degree level fluid dynamics [I came across it in my second year - it's an application of Bernoulli's equation: analogous to the way that river flow speeds up when it goes over rocks on the river floor that effectively narrow the river channel].
Re#68 – The village of Shishmaref is 100 miles north of Nome and 126 miles southwest of Kotzebue. These areas have cooled since 1977 according to the links in my other posts, which seems somewhat contradictory to what they’re seeing if the changes were due to temperature alone (unless there’s a long lag time).
As far as the link goes, as you said, it wasn’t very scientific. An anomalous erosion period of 2001-2003 is hard to blame solely on AGW, IMHO. It seems like way too short of a period to make such an assertion. And as the article says, they’ve had less snowfall and a greater number of powerful storms (not specific details), which could also explain the increased rates of erosion. Those could also potentially be blamed on AGW, or they could just be short-term variability.
Comment by Michael Jankowski — 13 Jun 2005 @ 10:58 AM
It isn’t just some random climate site – it’s a state-established and state-funded climate research center based at the University of Alaska Fairbanks.
I imagine their goal is not publish peer-reviewed journal articles. Based on their mission statement, they are more of a data collection and reporting agency.
Yes, some of their conclusions are a bit crude (such as taking an average of the statewide weather stations). But if you feel they are that suspect (“to the extreme”), you should probably take that up with the State of Alaska.
I am only particularly concerned with data at specific stations (such as Fairbanks itself). Is this data also “suspect in the extreme?”
Comment by Michael Jankowski — 13 Jun 2005 @ 11:16 AM
Temperature for Kotzebue climate.gi.alaska.edu/Climate/Location/TimeSeries/Kotzebue.html , snowfall climate.gi.alaska.edu/Climate/Location/TimeSeries/Data/otzSn , snow depth snow depth climate.gi.alaska.edu/Climate/Location/TimeSeries/Data/otzSd
Temperature for Nome climate.gi.alaska.edu/Climate/Location/TimeSeries/Nome.html , snowfall climate.gi.alaska.edu/Climate/Location/TimeSeries/Data/omeSn , climate.gi.alaska.edu/Climate/Location/TimeSeries/Data/omeSd
Granted, Shishmaref could see extremely different conditions (100+ miles from each station, 5 miles from the mainland, etc). But IF it’s reasonable to assume that those stations are indicative of the trends for Shishmaref, then taking into account local temperatures, snowfall, and snow depth (1) I don’t see a justification for the claims of decreased snowfall and (2) it’s very difficult to say the anamalous erosion of 2001-2003 is directly to recent temperatures (more likely a lag, or due to jump after the PDO shift). So it seems tough to link Shishmaref’s woes to AGW. The permafrost issue seems to be due to the PDO, and I’d take a look at the storms for the erosion issue (storms which some may try to attribute to AGW).
Comment by Michael Jankowski — 13 Jun 2005 @ 11:30 AM
Re the comment on #69:
Gavin, I have no opinion about the LOD as source/result of changes in flow/ocean levels, which appears to me as mainly of academic interest. The reason I have pointed to Moerners work is that the graphs give a good idea of what happened in the past with the Gulf Stream. These graphs are based on observations of sea levels and climate in Europe in the past centuries.
The good correlation with solar activity may be caused by the interaction between solar wind intensity and the earth’s magnetic field, but that is the opinion of Moerner. Or it may be caused by any other mechanism (like the influence of solar changes on the jet stream position) which enhance the simple direct insolation change which is incorporated in several current climate models…
[Response: The only graph I see claiming to be a history of GS variations is labelled 'LOD' - thus I assume that Moerner is equating GS variation with LOD changes (possibly I'm wrong, but you would need to find the original publications to check). This is tenuous at best, and so the likelihood of this being a correct history of the GS is small. To be sure, this is a difficult history to construct, but I am unaware of any recognised method that can do this at annual, decadal or even centennial timescales. -gavin]
Aren’t the problems at Shishmaref simply a case of coatal erosion due to ‘longshore drift’. Much of the East coast of England has suffered in the same way. Anyone who saw the David Dimbleby programme on BBC1 on sunday night will have seen him discussing the vulnerability of various ‘at risk’ communities. Some years ago a whole town (can’t remember it’s name) was swallowed up by the sea.
It is interesting to note the now-persistent failure of Wadhams’ Chimneys in the Greenland Sea. At the other end of the ThC by Antarctica the deep current (having surrendered its heat south of the Falklands) appears to swing northwards from the vicinity of the Ross Sea ice shelf to run up past the east of the New Zealand continental shelf. The ocean in that area appears to shelve conveniently from the Antarctic coast out to 5000 to 6000 metres depth – which would provide a useful ramp for chimneys from the Ross Ice Shelf to slide down – providing the southern impetus to the ThC circulation.
My query is .. Have ‘Wadhams Chimneys’ ever been observed in the Ross Sea? My cursory reviews of some of the available imagery finds that the intervals between images are such that I cannot discern the sort of circulation in the forming autumn ice pack off the shelf that would be consistent with 10km diameter chimney-pots. I note that the iceberg B-15a seemed to stall in the area for quite a while – suggesting perhaps that there is not much current driving force near the ice shelf. Is there any other evidence of marked changes in this southern leg of the ThC?
I also note that some commentators have suggested that the slow-down of the ThC (esp the ‘Gulf Stream’) will take quite a long time – many years – perhaps decades. But surely, each element of the ThC is driven by a real head difference. This head difference may be caused by density, temperature, wind and/or coriollis effects, but never the less every element is moved by the relative position of the immediately adjacent elements in the stream. If there is a large head difference between neighbouring elements, then the flow is fast. If there is a small or zero difference, then the flow is slow or nill.
So I presume that the ‘slow-down’ of the ThC will be induced by the lack of driving forces such as the Greenland Sea and Ross Sea chimneys. But may I respectfully suggest that – importantly – the knowledge that each element in the ThC stream has of the condition of its up-stream and down-stream neighbours will proliferate through the ThC stream at the speed any wave travels through the open ocean; viz the speed a tsunami proliferates.
Thus each element of the ThC will have become ‘aware’ of the failure of the Chimneys WITHIN ABOUT 24 HOURS of their failure. The ‘shock wave’ will be subtle – hardly as marked as a valve slamming shut on a closed pipeline – but never the less it will have sent its message, and the message will have been received loud and clear by every element of the ThC stream. And every element in the ThC will have responded immediately, in terms of the adjusted driving head at its position in the stream.
One could imagine the entire ThC letting out a gentle sigh, (Perhaps:- ‘So long, and thanks for all the fish!’) and drifting gracefully to a halt – pending the announcement of a new set of operating parameters.
..and of course the Larsen Ice Shelf and its neighbours are earlier contributors to the impetus for the southern leg of the ThC running eastwards towards the Ross Sea. The break-up of Larsen B and the acceleration of the previously supported glaciers there suggests that (like in the Greenland Sea) a change of some significance is afoot there too.
This was another informative post. RealClimate is great for addressing some of the issues that are brought up in the media and for being very current. I like to think that RealClimate is keeping me well informed. To return the favor here is a paper published in Science today by researchers at the Woods Hole Oceanograhic Institution. For the non-scientist, Woods Hole is a big deal in oceanography. They summarize the findings on their own website:
Here’s a quote:
“Given the projected 21st century rise in greenhouse gas concentrations and increased fresh water input to the high latitude ocean, we cannot rule out a significant slowing of the Atlantic conveyor in the next 100 years. I emphasize that we are talking about century timescales to witness measurable changes in the ocean transports of mass and heat across the Greenland-Scotland Ridge-we are not suggesting that the Gulf Stream will shut down.”
I also want to thank Gavin and Raymond Pierrehumbert for their helpful responses to my attempts to apply what I have learned about climate science in the “Tropical Glacier Retreat” post.
Comment by Joseph O'Sullivan — 17 Jun 2005 @ 8:20 AM
Re #73: Remember that this whole discussion started with your argument in #s 56 and 65 that “this (natural) shift seems to account for the increase in temperature in Alaska over the past 30 years” so that “it’s tough the blame the last 30 yrs for permafrost issues in/around Fairbanks, or imply that AGW is responsible.” It now seems clear enough that the only explanation for the unusual permafrost melting at Fairbanks is the duration of the current positve phase of the PDO over the last 30 years, which in turn can very much be linked (albeit not with absolute certainty) to anthropogenic global warming. It’s significant that the current permafrost melting involves areas that were not affected substantially by the prior positive PDO phase earlier in the century (and presumably ones before that, although probably there would begin to be problems with direct observations of those).
Going back and carefully reading through the climate part of the Alaska site, I see that I unfairly blamed them for providing the basis for your statements. Their language is actually carefully neutral, which would make sense given that their staff appears to lack tenure protection and that they have to function in the contentious atmosphere of Alaskan global warming politics. I do think they shouldn’t have provided that statewide average without explaining its lack of utility, but on the other hand it was on the same page with a map showing that the station locations are clustered in the south. Of course I have no issue with any of their temperature records, which are more extensive that those of any similar site I’ve seen.
Speaking for myself, I participate in this site because it provides me an opportunity to learn about global warming and climate in general. I clearly have a bias (in that I believe based on the evidence I’ve seen that AGW is both real and dangerous enough to warrant strong action), but when I see a reference to a climate mechanism I know nothing about (the PDO in this instance), I tend to want to look at the available literature before leaping to conclusions based on my bias. In this case, I spent several hours reading about the PDO, not enough to understand all the details but enough to satisfy myself that there is a reasonable basis for linking PDO duration and AGW, and that the statements of the Russian researcher in turn linking unusual permafrost melting to AGW also have a reasonable basis. In future, I would appreciate if you would try to adopt something of the same approach.
Your single PDO-related link in #65 provides a single reference that apparently supports the idea that most CGCMs show global warming inducing an anomaly pattern similar to that of the warm phase of ENSO. The authors then say that (my caps), “The PDO MAY have positive trend in the future.” Yet you proclaimed that “the current belief is that the PDO itself is closely linked to global warming.” I don’t see how you lept from what the authors of the article suggested “may” be true to something that is the “current belief.” I also don’t see how “may” becomes “closely linked.” Furthermore, the authors used the specific wording “in the future,” and we’re talking about current/past events in Alaska, aren’t we? Surely in your “several hours of reading about the PDO,” you found something better than a few paragraphs from an apparently unpublished article tenuously stating a possible future link between GW and the PDO to come to your conclusions that the “the current belief is that the PDO itself is closely linked to global warming.” Surely you have more to present than that to support your claim, “…the duration of the current positve phase of the PDO over the last 30 years…can very much be linked (albeit not with absolute certainty) to anthropogenic global warming.” FYI, a duration of 30 yrs isn’t unreasonably large. According to this research http://www.agu.org/pubs/crossref/2005/2005GL022478.shtml, typical recent periods of oscillation have been 50-70 yrs. A 30-yr phase within a 50-70yr period is certainly not unusual.
Check here, if you’d like – tao.atmos.washington.edu/pdo/ – it’s a site RealClimate linked to several months back, so you don’t have to worry about any bias from the skeptical crowd . My interpretation of what the site says with regard to CGCMs and PDO seems vastly different from the authors in the link you provided implied.
“What would be a surprise would be a flip back to the cool phase.”
We dipped in the cool portion of the PDO Index for much of the years 1998-2002. It wasn’t prolonged enough for anyone to conclusively say the phase had shifted (and the PDO Index has since returned to positive), but it shouldn’t/wouldn’t be a surprise to have a phase shift in the next few-to-several years.
FWIW, your final sentence has no place in a forum such as this. There are many disagreements here, but I think there would be a nearly unanimous agreement on that. In the future, I think posters would appreciate if you would refrain from such an approach.
Comment by Michael Jankowski — 17 Jun 2005 @ 5:20 PM
Can someone please outline the influence of ocean temperature on jet-stream function? In particular if the loop of the ThC in the north Pacific changes character (energy transport), will that impact on the standing wave over the Rockies that moves the surface currents north-east over the Atlantic?
OK.. the reason I am asking, is because I would imagine that if there is a coupling between North Pacific water energy and the energy of the jet stream (and I would think the moisture levels in the fohn wind over the rockies would be important) then that would be reflected in the surface flow rate of the ‘gulf stream’, which would then reduce water levels in the Denmark Sea, which would then reduce the driving force in the Greenland Sea and over the Greenland-Scotland Ridge, which would reduce the impetus of the ThC.. etc etc.. in a downward spiral.
The travel time for a wave to do the full circuit of the ThC (a distance of about 65000km and assuming 4000m average depth) is about 90 hours. (A better calc than my earlier 24hrs, above). The section from Greenland to the North Pacific takes about 48 hours, and the jet stream will run the 5000km from the Pacific to the gulf stream in about 24 hours, so the total travel time in the coupled circuit is only about 3 days.