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Gulf Stream slowdown?

Filed under: — gavin @ 26 May 2005 - (Français)

There has been an overwhelming popular demand for us to weigh in on recent reports in the Times Britain faces big chill as ocean current slows and CNN Changes in Gulf Stream could chill Europe (note the interesting shift in geographical perspective!).

At the heart of the story was a statement at the recent EGU meeting by Peter Wadhams from Cambridge University, that convection in a normally active area of the Greenland Sea was much reduced last winter. Specifically, in an area where a dozen or so convective ‘chimneys’ form, only two small chimneys were seen. (Unfortunately, I can’t seem to be able to find a relevant abstract of Dr. Wadhams talk, and so I have to rely on the Times’ news reports for the specifics).


Convective chimneys in the seas bounded by Greenland, Iceland and Norway occur when intense cooling of the ocean, usually associated with a low-pressure system passing through, breaks down the normally stable ocean layers and causes the now colder, denser water to convect and mix down to a relatively deep layer. This area of the world is one of only a few places where the underlying ocean column is marginally stable enough that this process can occur in the open ocean and lead to convective chimneys going down 2000 to 3000 meters. The deep water masses formed in this way are then exported out of the area in deep currents that eventually make up “North Atlantic Deep Water” (which also contains contributions from the Labrador Sea and entrainment of other water masses). This process is part of what is called the ‘thermohaline’ or ‘overturning’ circulation and is associated with a significant amount of heat transport into the North Atlantic, which indeed keeps Britain and the rest of the North Atlantic region 3 to 6 degrees C warmer than they otherwise would be. The figure gives two model estimates for the impact of this circulation (Stocker, 2002).

This heat transport is often associated with the Gulf Stream in the media and among the public. However, my pedantic side obliges me to point out that the Gulf Stream is a predominantly wind-driven western boundary current that moves up from the Gulf of Mexico along the US coast to Cape Hatteras, at which point it heads off into the central Atlantic (see also this letter by Carl Wunsch). It then turns into the North Atlantic Drift which is really the flow of water responsible for the anomalous northward heat transport in the Atlantic. 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. Indeed climate models generally (though not exclusively) forecast a slowdown in this circulation by 2100. This occurs mainly as a function of increased rainfall in the region which strengthens the ocean layering and reduces the amount of convection in the region. It is probably futile to insist on it at this point, but a collapse of the overturning circulation is not the same as a collapse or reversal of the Gulf Stream (which as I mentioned above is predominantly wind-driven).

Getting back to the statement by Peter Wadhams though, how does this relatively small-scale observation get translated into headlines forecasting changes in the Gulf Stream and chilly times ahead for Europe? The major problem is that the background story and the climate model results are now very well known, and any scientific result that appears to project onto this storyline therefore gets a lot of attention.

However, it is a long way from the Greenland Sea to the Gulf Stream and some important points did not get a mention in the news stories. Firstly, we know that there is a great deal of decadal variability in how much and where deep convection takes place. Indeed, it was reported by Schlosser et al (1991), that based on CFC measurements, very little convection had occured in the Greenland Sea over the previous 7 years. Subsequently, convection was renewed. Similarly, convection in the Labrador Sea (the other main component) has also oscillated, possibly out of phase with the convection in Greenland. Studies by Dickson et al (1999, 2002) showed that properties of the deep water overflowing the Denmark Strait (between the Greenland Sea and rest of the Atlantic) appear to be related to patterns of variability like the North Atlantic Osillation, and this may help explain some of the variabilty.

To be sure, there are some long term trends that are becoming discernable. There is a freshening of the North Atlantic visible since the 1950s. Long continuous records of temperature and salinity at Ocean Weather Station M in the Norwegian Sea indicate that the deep water has also warmed noticeably. However, monitoring networks are now starting to be put in place (Osterhus et al, 2005) and better integrated data will be available in the future. It is important to bear in mind that while the changes being seen are indeed significant given the accuracy of modern oceanography, the magnitude of the changes (a few hundredths of a salinity unit) are very much smaller (maybe two orders of magnitude) than the kinds of changes inferred from the paleo data or seen in climate models. Thus while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of the Europe is a ways off yet.


84 Responses to “Gulf Stream slowdown?”

  1. 51
    grundt says:

    Who knows..
    We can have some control over a lab experiment, but we are utterly ignorant about thousands or millions of parameters affecting system Earth. That´s why I respect so much Climate Scientists. They don´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.

    http://somalinet.com/news/world/Somalia/506

    Is this evidence of GW?
    ” Somalis think this unusual weather and last nightâ??s previously unheard of snowfall are part of the global warming phenomena. “

  2. 52
    Timothy says:

    #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.

  3. 53
    Peter Hearnden says:

    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.

  4. 54

    Stefan,

    Thanks for your response to #43.

    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.

  5. 55

    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…

  6. 56
    Michael Jankowski says:

    Re#44,

    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).

  7. 57
    David Wojick says:

    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]

  8. 58
    David Wojick says:

    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

  9. 59
    Katie Mcgarity says:

    why does the gulf stream have an effect on europe’s and the United States weather

  10. 60
    dave says:

    Re #58:

    Rahmstorf and Zickfeld (2005) states

    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.

  11. 61
    eric says:

    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.

  12. 62
    Gil says:

    Re# 56

    Hi Michael,

    Thanks for your reply and your thoughts. Here are 2 permafrost related links for you that point out observed phenomena.

    Alaska scientist warns of impact of permafrost thaw
    http://news.yahoo.com/news?tmpl=story&u=/nm/20050526/sc_nm/environment_alaska_dc_1

    Permafrost may be shrinking Arctic lakes
    http://abcnews.go.com/Technology/wireStory?id=819056&CMP=OTC-RSSFeeds0312

    Re: 46

    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.

  13. 63
    Gil says:

    re: 46

    Hi Stefan,

    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.

    Thanks for pointing it out to me.

    Cheers

    Gil

  14. 64
    John Finn says:

    Re: #62 – Permafrost Thaw

    The linked article states

    Over the past 30 years, soil temperatures have risen 1 degree to 3 degrees Celsius ….

    And sure enough when you check the temperature record at Fairbanks (see link below), it definitely seems warmer than it was 30 years ago

    http://data.giss.nasa.gov/cgi-bin/gistemp/gistemp_station.py?id=425702610000&data_set=1&num_neighbors=1

    But no warmer than it was in the 1940s. The following link is the Anchorage temperature record which is the next nearest station I could find with an up-to-date record.

    http://data.giss.nasa.gov/cgi-bin/gistemp/gistemp_station.py?id=425702730000&data_set=1&num_neighbors=1

    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….

  15. 65
    Michael Jankowski says:

    RE#62,

    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.

  16. 66
    Michael Jankowski says:

    PS: Re#62,
    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.

  17. 67
    David Wojick says:

    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.

  18. 68
    Steve Latham says:

    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.”

    http://www.enn.com/today.html?id=7920

  19. 69

    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]

  20. 70
    Steve Bloom says:

    Re #65:
    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.

  21. 71
    Timothy says:

    #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].

  22. 72
    Michael Jankowski says:

    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.

  23. 73
    Michael Jankowski says:

    RE#70,

    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?”

  24. 74
    Michael Jankowski says:

    Re#68/72PS:

    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).

  25. 75

    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]

  26. 76
    John Finn says:

    RE: #68, #74

    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.

    None of this has anything to do with AGW.

  27. 77
    Nigel Williams says:

    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.

    Thanks

    Nigel Williams

  28. 78
    Nigel Williams says:

    ..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.

    Nigel Williams

  29. 79
    Joseph O'Sullivan says:

    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:

    http://www.whoi.edu/mr/pr.do?id=5098

    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.”

    The science article: Curry and Mauritzen, Dilution of the Northern North Atlantic Ocean in Recent Decades, Science 2005 308: 1772-1774, its at:
    http://www.sciencemag.org/cgi/content/abstract/308/5729/1772

    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.

  30. 80
    Steve Bloom says:

    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.

  31. 81
    Michael Jankowski says:

    Re#65 and #80 Steve,

    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.”

    It may surprise you, but it wouldn’t surprise these researchers http://www.agu.org/pubs/crossref/2002/2002GL015191.shtml

    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.

  32. 82
    Nigel Williams says:

    Jet-stream dynamics.

    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?

  33. 83
    Nigel Williams says:

    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.

    Does such a coupling exist?

  34. 84
    Stephen Berg says:

    “How Much Excess Fresh Water Was Added To The North Atlantic In Recent Decades?”

    http://www.sciencedaily.com/releases/2005/06/050619121942.htm


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