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Decrease in Atlantic circulation?

Filed under: — group @ 30 November 2005 - (Français)

by Gavin Schmidt and Michael Mann

In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?

The first question that is asked is usually how these calculations are done. Due to the predominantly “geostrophic” nature of the ocean circulation (i.e. velocity is generally horizontally perpendicular to pressure gradients because of the Coriolis effect), you can calculate changes in North-South velocities by only considering the East-West changes in temperature and salinity. So given a section across the ocean (say 25°N), oceanographers can estimate the transport across that section. The error in these numbers is a little hard to know, but Bryden et al estimate around +/- 6 Sv (1 Sv is 106 m3/s, the Amazon output is around 0.1 Sv for perspective).

What did Bryden et al find? Their calculations indicate that the Gulf Stream itself has been remarkably stable over the almost 40 years, and this accords with other measurements of the Gulf Stream flow itself. Since what goes north must eventually go south (after taking into account the very small amounts of atmospheric transport and the amount of flow through the Bering Strait), all of the other changes will balance. They show that the amount of deep return flow seems to have gone down about 8 Sv (out of 25 Sv), and the amount of mid-ocean to surface transport has gone up by about the same amount. This corresponds to a roughly 30% apparent weakening in the so-called “Thermohaline Circulation” (see our previous discussion here). Since the surface flow is warmer than the deep flow, there is a consequent decrease in the northward heat flux of about 0.2 PW (or about 15%).

It should be stressed that should this be a sustained feature (and not affected by the +/- 6 Sv uncertainty estimated in the paper), this would be extremely significant. Modelling experiments suggest that this kind of decrease should be associated with a decrease in ocean temperatures in the North Atlantic of up to 2°C or so, and maybe 0.5° over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed. Alternatively, the models may not entirely be capturing the fairly complex oceanic processes involved. Continuous monitoring of this section has already been funded through the UK RAPID program and should provide much better data in the future, and a potential solution to this and other remaining puzzles.

It will take some time to integrate the findings of this study with other evidence of changes in North Atlantic ocean circulation, including the changes seen in salinity, changes in the so-called Atlantic Multidecadal Oscillation (AMO) (see e.g. Knight et al, 2005 and references therein) and other indicators of Atlantic climate change (e.g. Dickson et al, 2002). Right now, there isn’t an obvious synthesis of what these disparate studies are telling us.

While this is quite a serious issue, there are a few amusing points. Firstly, this study does present some awkward reading for some who hold that natural cyclical changes in the thermohaline circulation (rather than, say, anthropogenic influences), are responsible for the anomalous increase in Atlantic Hurricane activity in recent decades. Hurricane prognosticator William Gray (whose public statements we have commented on previously), has, in his recent senate testimony, confidently asserted that a putative increase in the intensity of the Atlantic Thermohaline circulation over recent decades was entirely responsible for this increase:

The Atlantic has large multi-decadal variations in major (category 3-4-5) hurricane activity. These variations are observed to result from multi-decadal variations in the North Atlantic Thermohaline Circulation (THC) – Fig. 4. When the THC is strong, it causes the North Atlantic to have warm or positive Sea Surface Temperature Anomalies (SSTA) and when the THC is weak, cold SSTAs prevail. Figure 5 shows these North Atlantic SSTAs over the last century with a projection for the next 15 years.

By Gray’s very clearly articulated reasoning, there should have been a downturn, not the observed upturn in major Atlantic hurricane activity over the past several decades (in the absence of other—including anthropogenic–influences on tropical Atlantic climate) if Bryden et al.’s results are correct. It will be interesting to see if Gray, and others, will change their line of argument in the face of this new study. Today, the last day of the official 2005 Atlantic Hurricane season, might be a fitting opportunity for them to do so.

Secondly, since the Gulf Stream itself is remarkably stable in these analyses, headline writers may have to adjust the standard ‘Gulf Stream may reverse’ titles that they normally come up with when dealing with this topic (though we note that The Independent succumbed anyway) . See here for a previous discussion.

And finally, for those of you who frequent some of the more contrarian websites, tried to pull a bit of a fast one in predicting ‘imminent’ headlines on this issue on Tuesday (Nov 29th). This was before the official release of the paper, but after the embargoed copies of this paper had been sent out, which as journalists they would have seen, but their readers had not. You didn’t need to be Nostradamus to predict the headlines here! They were trying to suggest that ‘alarmists’ will automatically blame the ocean circulation for the current wintry weather in Europe. Since the Bryden paper is talking about a multi-decadal trend, this week’s weather is obviously not relevant…

Harry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham, Slowing of the Atlantic meridional overturning circulation at 25° N, Nature, 438, 655-657. 2005
Dickson et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature, 416, 832-836. 2002
Knight, J.R., Allan, R.J., Folland, C.K., Vellinga, M., Mann, M.E., A signature of persistent natural thermohaline circulation cycles in observed climate, Geophysical Research Letters, 32, L20708, doi:10.1029/2005GL024233, 2005.

60 Responses to “Decrease in Atlantic circulation?”

  1. 1

    I’m confused by your analysis here with respect to hurricane intensity. It appears to me from the Bryden paper (and especially Figure 2b), that what they are basically saying is less warm water is going to Europe, and more is recirculating back down into the tropics. The circulation is not really getting stronger or weaker, it’s changing where it goes. This seems like it’s going to keep tropical waters hotter and thus promote more hurricanes (in agreement with what Gray says (if one interprets his statements as referring to the portion of the atlantic circulation – the subtropical gyre – that delivers more warm water to the tropics). The question of whether this change is due to GHG or some natural oscillation is somewhat orthogonal.

    [Response: The comment is premised upon some misconceptions. We must distinguish between the wind-driven gyre circulation and the thermohaline-driven meridional overturning circulation. To first order, these are typically considered to be independent. Whether this is entirely true is a matter that can be legitimately debated, but the Bryden et al findings actually appear to support this–they find significant changes in the meridional overturning circulation, but little if any change in the Gulf Stream (the western boundary current of the subtropical wind-driven gyre that must return any interior southward flow). So, firstly, their findings don’t support your speculation that changes in the wind-driven subtropical gyre might explain the observed tropical Atlantic SST warming. But this is beside the point. Gray doesn’t refer to the wind-driven gyre circulation in his comments, he specifically refers to the thermohaline circulation. The Bryden et al study specifically addressed this component of the circulation, and finds that it has likely decreased in recent decades, contradicting the public assertions by Gray. The other incorrect premise in your question is that the THC is a zero-sum game as far as tropical and extratropical North Atlantic upper ocean heat content (and SST) is concerned. Its not. The THC actually transports a positive net amount of heat northward across the equator from the South Atlantic to the North Atlantic. When the THC increases, the entire North Atlantic ocean can warm. High latitude North Atlantic warming is not coming at the expense of tropical North Atlantic cooling! For further details on this, see my comments below in reply to Isaac’s comment. -Mike]

  2. 2
    Isaac Held says:

    I am also confused by your reference to the hurricane issue in this context. My impression is that if you take North Atlantic temperatures, detrend them, and call the remainder the temperature signal of the AMO, then we are near a positive peak at present. Models suggest that the greenhouse signal should be pretty linear. (Admittedly, the response to changing aerosols confuses this issue.) It is also plausible that the temperatures lag the overturning by a significant fraction of a “period”, since it takes time to advect the warm water polewards, so one should expect weakening of the circulation at the time of peak temperatures. So part of the weakening could be AMO-related.

    [Response: Isaac, thanks for your comment. Let me do my best to address it. It is arguably impossible to accurately detangle a multidecadal oscillation from a long-term (probably not linear) forced trend in 100 years of data. The trend pattern confounds any attempts to cleanly isolate the oscillatory pattern in such a scenario, particularly when a positive peak in the oscillation is argued to occur late in the series. Given that, our best assessment of the relationship between the AMO surface pattern and the underlying THC variations is likely to come from analyses of long coupled model integrations with models that are known to produce multidecadal variability in the THC and an “AMO” of sorts. One such analysis was provided by Delworth and Mann (2000). Indeed, this paper (or Dick Kerr’s Science piece thereon) was the origin of the term AMO. A more recent analysis which is arguably more detailed with regard to the precise relationship between the meridional overturning streamfunction and the surface temperature changes associated with the AMO is provided by the recently published GRL article by Knight et al. Here, a multidecadal surface “AMO” signal tied to the Atlantic meridional overturning is shown to be robust in a 1400 year control simulation of the HadCM3 coupled model. More importantly in the context of your comment, the maximum positive meridional overturning streamfunction anomalies are found to be almost precisely in phase with the maximum SSTs over the entire North Atlantic basin, including the tropical Atlantic and Carribean (see Figure 3 in the paper; SSTs shown on the left side panel, associated meridional overturning streamfunction anomalies shown on the right side panel). There is no phase lag of the sort you allude to. Perhaps other future studies will come to different conclusions. However, based on this study, which is arguably the most definitive for the timebeing, positive tropical Atlantic SSTs associated with a THC-driven AMO should be contemporaneous with positive anomalies in the THC. In this context, Gray’s public assertions and Bryden et al’s quantitative findings cannot both be correct. -Mike]

  3. 3
    Ken says:

    I’ve noticed that climate scientists, as well as non-scientists, appear to be biased in how they interpret statistical or predictive uncertainty, e.g.: “It should be stressed that should this be a sustained feature (and not affected by the +/- 6 Sv uncertainty estimated in the paper), this would be extremely significant.” A +/-6 Sv range presumably implies that a flow rate decrease of 8+6 Sv is as likely as an 8-6 Sv decrease.

    [Response: It isn’t immediately clear (to me) whether the 6 Sv uncertainty is a one or two sigma, and whether it applies to the 8 Sv change directly. The fact that the 1998 results look very similar to the 2004 results is somewhat of a confirmation, but I’d need to talk to someone who is more familiar with these kinds of calculations before I could say more. -gavin]

  4. 4

    I have read this from the Guardian :

    “Alarm over dramatic weakening of Gulf Stream ”

    “Chris West, director of the UK climate impacts programme at Oxford University’s centre for the environment, said: “The only way computer models have managed to simulate an entire shutdown of the current is to magic into existence millions of tonnes of fresh water and dump it in the Atlantic. It’s not clear where that water could ever come from, even taking into account increased Greenland melting.””

    It is not well known that multi-year sea ice, as from the Arctic Ocean Ice pack, is nearly as pure as distilled water, free of salt, and makes a most delicious cup of tea. I think that many millions of tonnes already melted and much more left which may create Mr West’s “magic” .

  5. 5
    Gar Lipow says:

    Is decline in the Gulf Stream likely to equal freezing temperatures in Europe and the West Coast of North America etc. in this century? I had the impression that the “new ice age sceneario” had been pretty well refuted. Is it in fact likely? I’m a bit confused on that.

  6. 6
    Hank Roberts says:

    Someone has posted some of the pictures from the Nature article here:
    along with quotes from New Scientist that seem to be one of the publications confused about the Gulf Stream, and discussion I can’t make much sense of.

    Nice pictures though. Might be useful to refer to them in discussing this until and if Nature makes the article available.

  7. 7
    Arco Thompson says:

    I just read an article on an Alaskan News Site today referring to various animals ‘migrating’ (moving would be a better description) to Alaska
    []. They even found a snake, crushed in the road. Maybe
    the animals know something. (I’m a geologist, not a climatologist-but I
    know the Earth goes through cycles of heating and cooling).

    The article went on to describe the states plans to back
    exploration of a “Northwest Passage” across the Arctic, in cooperation
    with a Finnish company. Apparently other countries are also working on
    plans to exploit the route.

  8. 8
    Dylan Baumgartner says:

    I begin my comments/questions by stating that I am not a climate scientist. I have a B.S. in engineering – so I can be taught – but I won’t pretend to be an expert on climate theory.

    That being said, assume a 2-4 deg C increase in Global avg temp and a doubling of pre-industrial GHG by the year 2100. I can clearly understand that sea-level rise would result in a loss of real-estate (including many major cities); I can also understand that a faster than “normal” climate change might force a larger number of species into extinction. However, is it not possible that at least some good might come of it?

    I have read, extensively, the comments on this site about what climate change will do to the Earth. Again, I’m not an expert and I haven’t formed an opinion on what percentage of the warming is due to anthropogenic influence, but I have yet to see anyone suggest that anything good might result from warming.

    In fact, anyone who states that something good might result – increased growing season/increasing biomass for example – is promptly put down immediately. I have read in the comments section a variety of claims. I have questions about all of them. Here is a brief list:

    Increased global temp may result in:
    Increased frequency/violence of severe weather
    Drought/Desertification/loss of biomass
    Loss of fresh water
    Increased disease from vector-borne diseases
    Species extinction on a large scale.

    I find it curious that no one seems to believe that anything positive may occur. I also wonder about this: this site has been very clear (in dozens of articles and comments) that global climate is not chaotic. I’ll accept that argument as reasonable.

    However, weather is chaotic. I won’t say for certain, but I suspect that ecology (terrestrial and oceanic) are also chaotic systems – or at least extraordinarily complex and not necessarily well-modeled. Based on my limited knowledge of chaotic systems, I wish to ask the following: isn’t it possible (perhaps equally possible) that severe weather could become less-widespread in a warmer world? Isn’t it possible that biomass might increase as well?

    I would also like to argue against the idea that reducing anthropogenic GHG can be done at no cost or negative cost. If you live in other than a primitive society (and having a computer sort-of gives that away) then everything you see, do and touch is strongly related to petroleum-based products. Everything from fabric to plastic, lubricants, transportation, heating, electricity – almost all of these things are powered by petroleum. I am not arguing “do nothing” as an environmental policy. What I am arguing is that any actions taken need to be cost-effective.

    In order to be cost-effective, policy makers need quatifiable data. That means analyzing the negative (and positive) aspects of warming. That is not being done right now (to my knowledge.) If someone were to present a scenario where +2deg C in 2050 = $XXXB in losses – $YYYB in gain – and then proposed a solution costing less than the difference, we might see results. I realize that isn’t the job of climate scientists (entirely) but asserting “global disaster” as a reflex action will not help the cause.

  9. 9


    I think you missed the substance of my point. The survey line in the Bryden paper goes from Africa to the Bahamas. Ie it catches everything except the gulf current which goes west of the Bahamas. In figure 2b, it’s really clear that the amount of shallow (less than 1000m) southerly water transport across the survey line is increasing with each survey. That is pretty much true at every depth between 0 and 1000m. Brady et al summarize it as:

    …the mid-ocean southward transport above 1000m depth has increased from 13Sv in 1957 to nearly 23 Sv in 2004.

    My understanding is that’s the subtropical gyre right? It’s very clearly increasing from the data. And the water transport across the full survey line must roughly balance since very little goes through the Bering strait (0.8 Sv), the gulf current through the Florida Strait hasn’t changed much (33 Sv+-1.1), evaporation is negligible (0.1 Sv) and there’s nowhere else for the water to go. So the decrease in southerly flow of cold water below 1000m must be balanced by something – and the data show it’s increased southerly flow of shallow water. (I agree the heat flow need not be balanced across the survey line, but the water flow must be (modulo any small changes in flow in the Bering Straits, or between Florida and the Bahamas).

    It seems like that shallow south flowing water must be warm (it’s the portion of the Gulf stream that’s turned around and headed back south), and it’s headed right where hurricanes form.

    [Response: The classical (i.e. Stommel) theory of the subtropical gyre requires that the interior equatorward (Sverdrup) transport that takes place everywhere but a narrow strip along the western edge of the basin, precisely balance the poleward transport that takes place in a narrow boundary current along the western edge of the basin. Thus, the subtropical gyre circulation is a horizontal circulation with poleward mass transport along the western boundary, and equatorward transport everywhere east of that, and providing no net northward mass transport integrated across the basin (which is what Bryden et al have done). Bryden et al find that that western boundary current (what is properly called “the Gulf Stream”) has not changed in magnitude over several decades. If the classic Stommel model is correct, that means that the southward interior gyre flow (which is what you appear to be referring to) also must not be changing, since there would have to be an equal and opposite change measured in the Gulf Stream, and no change was found. What does appear to be changing, based on Bryden et al, is the meridional overturning component of the circulation, the thermohaline circulation, which describes a vertical overturning circulation with northward transport in the upper part of the ocean, and equatorward transport at depth. That’s about as clearly as I can explain this. -Mike]

  10. 10
    stephan harrison says:

    Aren’t the newspapers being a bit alarmist here? From a palaeo perspective the Atlantic circulation seems to be pretty robust to changes in meltwater production. Only the rapid drainage of ice-dammed lakes appeared to be sufficient to significantly affect circulation (eg drainage of Lake Ojibway and the 8200 year event). Even with the collapse of the Greenalnd ice sheet it’s not clear that this would provide the meltwater spike that lake drainage produces. Do the modellers have any perspective on this?

  11. 11
    mauri pelto says:

    Mike; As a glaciologist I focus primarily on higher latitude ocean circulation changes, not the tropics. One puzzle to me that you are driving at answering, or did answer and I am not seeing it, is:
    1. Since high hurricane activity is associated with periods of high sst in the tropics which we have now and did in the 1920′ and 1930’s
    2. Given that these two periods also were those of the most significant warming in our atmosphere.
    3. Why are many so sure that the AMO is a circulation cycle driving this, and that this is not merely a radiative forced warming, which in turn does influence the THC.

    [Response: You can find some answers to these and other related questions in our previous post here. -Mike]

  12. 12

    Re: Mike’s response to Comment 2.

    It seems quite unfair to attack Gray’s association between THC and Atlantic hurricanes by citing Knight et al. In fact, Knight et al. show a reconstructed THC with cycles that clearly match cycles of Atlantic hurricanes. Further, Knight et al. find that the THC has been strengthening since the 1970s, specifically: “Of particular note is the significant strengthening implied from the 1970s to the present day…”

    It certainly seems that Bryden et al.’s findings are not consistent with Knight et al. It hardly seems right for Mike to challenge Gray (and other hurricane natural cyclists) using the results of Bryden et al., when a paper that Mike is a co-author on (Knight et al.) serves to support Gray’s notion. So which is it? Are the past 30 years better described by Bryden et al. or Knight et al.?

    [Response: Mike’s point is that the AMO is not a direct measure of the THC and so the link that Gray assumes between the AMO and the THC is not justified. Given the widespread notion that the AMO and THC are however linked (though without direct measures of the circulation it was always difficult to be sure) there may be a contradiction between Knight et al and Bryden et al. Or, it might be telling us something very interesting about how the ocean actually works and that both are somehow consistent. – gavin]

    [Response: Chip’s comments are based on an incorrect understanding of the problem. I’ve already clearly explained above that one cannot diagnose the “AMO” signal directly from the observational record because any attempt to do from actual surface observations suffers from the trend contamination problem. This is why we turn to the model. The model defines the relationship between the surface AMO pattern and the THC in the absence of any such contamination, precisely because it is a control integration without any anthropogenic forcing. The model simulation quite clearly indicates that any natural “AMO” surface warming of the tropical Atlantic should be in phase with a strengthening, not a weakening, of the THC. Gray’s arguments are thus clearly inconsistent with our best understanding of the relationship between the AMO and THC in the absence of other (e.g. anthropogenic) impacts, if Bryden et al’s results are correct. End of story. – Mike]

  13. 13
    Hank Roberts says:

    By the way, the inimitable Benny Peiser (an anthropologist, whose CCNET emphasizes any argument against human responsibility for change in climate) has a page out on the recent thermohaline info.

    He seems to be a participant-observer from the climate-change-denial side, but unlike most he does give citations and extensive quotes, if you read past his often true-believer HEADLINE text.

  14. 14


    Thanks for your response. I understand the model that you describe of two circulations with no mass exchange, one wind driven at shallow depth, and one thermohaline at greater depth. However, the model that you describe is clearly inconsistent with Bryden’s data. If you look at their figure 2:

    you can clearly see the increase in the southward shallow depth transport (above 1000m) with each later survey. The paper itself says in the abstract:

    In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

    [Response: This is the real puzzle. I can think of no a priori reason why the compensation should be as they show – I would have expected the horizontal transports and vertical transports to be decoupled. It’s conceivable that something in their methodology is forcing this to be the case, but I can’t see it. -gavin]

  15. 15
    Coby says:

    Re Dylan’s comment #8:

    I think the essential thing to clarify when discussing cost/benefits of climate change is the timeframe. Long term effects of a warmer climate may well indeed be beneficial but I think that there is ample evidence as well as logical reasoning supporting the expectation of negative consequences overwhelming positive ones in the short term. What is short term? Well, for global climate I think short term is measured in one to a few centuries. For the biosphere, short term is millenia.

    Unfortunately, by most human measures, these are very long term timeframes. So while, tropical forest inside the arctic circle is not necessarily a negative on its own merits, the ecological turmoil that the planet will go through before we are nice and relatively settled again, at whatever climate, will prove very hard to endure. Cities built long ago will find that they have no viable water supply anymore. Agricultural industries refined and well established in a particular location will find that their fields can no longer grow crops. Populations living on the coastlines will have to move. These are the negatives associated with a changing climate if not necessarily with a new one.

    The other point I wanted to make with respect to a cost/benefit analysis of reduction and eventual near elimination of fossil fuel as an energy source is that this is already an unavoidable future. Fossil fuels are non-renewable, we will eventually have to find other sources of energy. So any cost that you wish to attribute to emissions reduction goals must be something that we will not incur in the future anyway. On top one that, I would insist on some kind of credit in the benefit column for the quantity of oil saved from burning and therefore available for the myriad other exceptional uses it has. Burning this resource when alternatives could be developed is a tragic waste already, aside from pollution concerns.

  16. 16

    small follow up on what I said in #4, BTW a lot of the Arctic Ocean ice is ejected exactly where the THC conveyor belt descends….

  17. 17
    Andrew says:

    In reply to Passage 8

    For a good, recent, book on the financial issues you asked about I would suggest;

    Mohan Munasinghe and Rob Swart’s “Primer on Climate Change and Sustainable Development” 2005 Cambridge University Press.

    I have no idea whether these two people are renowned in this field, however their analysis of the costing analysis and the best course for human beings and business leaders to go is in this book. It shows us what is Sustainable and what is unsustainable for the long and short term. And it bases all of this analysis on the latest IPCC report. As you can see it is a brand new book and so very upto date and has proved very useful while writing my dissertation this year.

  18. 18
    Dylan Baumgartner says:

    Thanks to Coby and Andrew for the remarks. I will check out the resources you suggested to help form my opinion. The near-term and long-term remarks were especially well thought out. Who is to say what technology will be present at the end of those time horizons? It was only 60 years ago that manned spaceflight was a fantasy, and 20 years ago Leukemia was a death sentence.

    Sorry to all for taking this slightly off-topic. I intend to suggest a forum on this issue (my questions, that is) just for more intelligent discussion/ Q&A on this topic. I hope to see all of you there.

    BTW – as predicted by some comments in this post: several outlets in the media are taking alarmist positions (not grounded in fact) on the change in Atlantic circulation.

  19. 19
    Tim F says:

    Modelling experiments suggest that this kind of decrease should be associated with a decrease in ocean temperatures in the North Atlantic of up to 2°C or so, and maybe 0.5° over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed.

    Is it possible that the models aren’t properly accounting for latitude-specific greenhouse inputs? If this new data is correct then it seems as though we have two processes working against one another.

  20. 20
    Almuth Ernsting says:

    Sorry for my ignorance on this, but I am really trying to understand in lay terms what is happening.

    What really puzzles me is that I was under the impression that the warm waters moving north near Scotland and Scandinavia have been exceptionally warm in the last few years. I remember reading that an Arctic expedition also in 2004 found SSTs off Spitzbergen several degrees warmer than expected (with a newspaper statement “at least the Gulf Stream must stil be running strong”). Here in Scotland we have had the three warmest years on record all in one row (presuming that 2005 will be one of them) – all of a sudden we stopped getting almost any normal or cool months. And then we had sudden almost total collapse in sand eels and thus seabird breeding failure off our west coast this year, something that, elsewhere, has been linked to high sea temperatures killing off cold water plankton. Warming in Iceland and northern Scandinavia sems to also have been exceptional – I read a meteriologist from Iceland say that the warming in the past three years has been close to what had been predicted there for the next 50-100 years.

    Does this mean that for the time being high SSTs in the northward drift of the THC are coexisting with a lessening of the circulation overall? Does it imply any time-lag (ie eventually there should be cooler SSTs in the northern North Atlantic, but only if the circulation remains weaker over some years)?

    Or do I just refer to the statement “right now there is no obvious synthesis of what these disparate stories are telling us”, and stop trying to figure it out for the tim being?

    Almuth Ernsting

  21. 21
    Chris Reed says:

    Sorry to refer to media articles (certainly not the best resource) but I only have the ‘net at work and haven’t been able track don any papers referring to this. So this is not a case of cherry picking, more one of using the only cherries I can find! But I do recognise that the end result can be the same even without malicious intent. ;)

    With regards the findings of Dr Peter Wadhams that were announced at a conference, but not subjected to peer reviewed publication in any journal (as far as I know). “,,2087-1602579,00.html” (can’t get link to work, sorry) Wadhamâ??s work relates to the ice-shelf off Greenland and an apparent reduction in subduction at this location. I know that the previous RC post was related to this, but having read it again I can’t find any actual refutation of Wadham’s findings and given his apparent expertise donâ??t feel that this can be overlooked.

    Then as I posted under that previous post, in the past Bogi Hansen of the Faroese Fisheries Laboratories has also published similar findings. These relate to the Faroe Deep Channel between the Faroe Islands. And a measured trend of flow reduction.

    Now Bryden’s work seems to fit into a pattern with these reports. My (probably flawed) understanding is that the Greenland subduction zones and the Faroe Deep channel flow both feed the south going deep flow that Bryden asserts has recently reduced by 30% (with an admittedly large error range).

    So we seem to have 3 independent sources that indicate linked changes. Given the 40 years of stability that Bryden’s calculations indicate, I find it hard not to see this as alarming. Not so much due to ‘Younger Dryas’ based scenarios, but because this does not seem to be in line with what I’d expect reading in IPCC TAR on this issue, here:

    Can someone tell me why I’m wrong?

  22. 22
    Timothy says:

    Re #21: I think one of the issues here is “with an admittedly large error range”. Unfortunately, at present our observations, of the ocean in particular, fall well short of what we would like them to be. So the [theoretical] errors in the measurements are of the same order of magnitude as the changes being reported [at least for ocean circulation]. The question does arise, when models and observations disagree, are the models wrong? Or is it that the observations are wrong/incomplete/inaccurate?

    I remember hearing one Oceanographer/Climate scientist declare that the Atlantic THC could have already ‘stopped’ and we wouldn’t know about it yet, such were the paucity of relevant observations.

    Now, in terms of the models we have an interesting situation where the models are being built in order to match with a present-day observed climate that is not stationary [but that is hoped to be stationary enough that it can be assumed to be stationary for model development]. Consequently, when a new model is developed it is often checked whether it can reproduce a ‘realistic’ Atlantic THC [and also other ocean circulations such as around the Antarctic].

    It is easy to imagine that if, in a ‘proto-model’, the Atlantic THC was found to be weak, that changes to the models science would be made to make the model Atlantic THC stronger and therefore a ‘better’ match with the limited observations available. It wouldn’t be surprising if models tended to have Atlantic THC that were too strong or stable if the current climate, that they are being tested and developed against, is less stable than had previously been thought.

    Unfortunately this isn’t an easy problem to solve since perfect observations simply don’t exist. [And given the funding problems…]

  23. 23

    #20, There are two things at play, atmosphere and ocean, one is related to the other but event locations may not be the same. The Arctic has had all time high’s in temperature recently, not surprising that Scotland gets a nip of this warming. The Ocean currents and atmospheric temperatures are related by long term heat exchanges and will respond to the warming atmosphere in its own way, not necessarily at the same locations as with the warmer air .

  24. 24


    Thanks for responding. If we take these axioms:

    a) The wind driven gyre fills the entire basin (between the rapid western boundary current and the slower southward returning currents in the rest of the Atlantic) , and extends vertically down to the top of the NADW.

    b) The global conveyor says there’s a net transfer of warm shallow water northwards.

    There is no way that these facts can be reconciled without requiring an imbalance between the mass transport in the two arms of the gyre. There’s nowhere else for northbound shallow water to sneak through. The more northerly transfer of shallow water one would like, the more imbalance one would expect to see. So if the conveyor is slowing down, one would expect that to show up as a decreasing imbalance between the western and eastern halves of the gyre.

    The question that remains is whether the required reduction in imbalance would be achieved by 1) a slowing of the Gulf stream, 2) a speeding up of the Eastern half of the gyre, or 3) some combination of the two. Bryden’s data suggest 2), but why should this be so?

    (For background, I have no training in oceanography, but do have a rusty PhD in Physics).

    It appears to me that the surface thermohaline currents in the conveyor must be mediated by sea-level inhomogeneities (long-lived, average ones). A given chunk of water moves because of the net forces on it, and so a net northward transfer of water not driven by friction must require that the average isobar surfaces slope downwards in the shallow ocean as you go north. In general, the regions of expanding warming upwelling water in the Indian Ocean, North Pacific, or wherever they are, must create slight bulges in the surface, and the regions of shrinking, cooling, sinking water in the Arctic must create slight depressions in the sea surface (again, I mean in a very low pass sense – obviously storms, tides, etc, create all kinds of short-terms signals obscuring this). I can’t find any confirmation of this in a quick search, but I don’t see how else it could be. (Surely someone must study sea level inhomogeneties and make maps of them? I can’t find it).

    Now in the Gulf stream, this isobar gradient would be expected to reinforce the frictional wind forces, while in the eastern atlantic, it would oppose the frictional wind forces (thus leading to the necessary imbalance between the arms of the gyre I noted above). However, the portion of the Atlantic that is the southern gyre is much much larger than the portion involved in the boundary current. We would expect isobar deviations to more or less radiate out symmetrically from areas of higher sea level. Thus this isobar gradient is able to have a far larger impact on the total transport through the broad eastern side of the gyre than it is on the very narrow western side of the gyre. Thus changes in the THC mainly show up as changes in the Eastern side of the gyre.

    I realize this is only a cartoon of the complex physics that must go on. However, if basically correct it would explain what Bryden et al have observed.


    [Response: Martin Visbeck is an expert in this particular area of the science. I’d simply refer you to his comments below. – Mike]

  25. 25

    Dear Gavin and Mike
    I am a rare visitor of your site, but like the forum that allows interested folks to ask questions. The paper by Bryden reports on a particular calculation done to infer the volume transport of the Atlantic Ocean along 25N at five instances in time (now granted this is expensive so we cannot get to do many of these types of transects). The method he applied is only briefly described, but seems to be using a fixed zero reference level. This way of getting at transports was done in the pre 1990s area of oceanography and has been shown by many subsequent researchers to be not optimal. More modern approaches use inverse methods to calculate the reference velocity for each station pair under a set of physical contraints (Such as the Ganachaud and Wunsch reference). This sounds a little complicated for the non experts, but needs to be done since no DIRECT measurements of flow were taken.

    An even more up to date way to get transports in the ocean is to use ocean models constraint by observations (so called data assimilation or ocean state estimation in several forms). Both of these methods have demonstrated to be more accurate and can yield significantly different results for two reasons:

    1) The assumed level of zero motion turend out to be not optimal or non existant.

    2) The particular week, when the measurements were taken had a transport that was higher or lower than the say annual average. (aliasing)

    Bryden reports an error bar of 5Sv which is based on an invese model result, and could be even higher for his method. His method does not allow to calculate errors easily. So I would not be to confident in that number for his method.

    The sampling error can be large as results from realistic ocean model consistently show. To alleviate this problem the same group has one of the largest arrays of moorings in the water to get daily measurements of this transport (RAPID/MOCA array funded jointly by the UK and USA). They have not analyzed the first year of data yet, but in my lab we have looked at results from a similar set of moorings at 15N (Uwe Send’s work) and find rather significant variability on weekly to monthly time scales (but no trend over the 4 years of data). This togther with the model findings should raise some flags (as also Quadfasel did) that the results might not stand the test of time.

    Secondly, someone else in the comments and Quadfasel in the N&V piece mentioned the direct flow measurments in the Faroe – Shettland channel, one of the two main pathways for dense water out of the Nordic Seas, as supporting evidence. That record showed a declining transport by up to 15% until 1999, when the Science paper was written. Bogi Hansen has recently shown in several seminars that the updated time series until 2003 sends a different message: The flow is back up to the 1995 level. i.e. no change over the last decade… (unfortunately these updates usually does not make a Science or Nature paper in particular if the bottom line of the argument has shifted…)

    Similar signals of interannual variability without a discernable trend come from direct flow measurements in the western boundary current at the exit of the Labrador Sea (Fritz Schott and colaborators) (north of Brydan’s section) and Uwe Send’s record at 15N to the south. In my assessment that is an issue that needs to be looked into in a larger synthesis paper, and not just a single point analysis.

    None of the data assimilation models show a strongly declining trend in the mass flux at 25N in fact many of them show a weak increase overall…

    And Gavin you are right … the way the calculation is done mass needs to balance (since flow is only inferred via thermal wind and a reference level…) So the compensation is part of the method.

    My personal take is that this remains an open issue in oceanography and climate research. We simply have not all the tools and observing systems in place to give these numbers with confidence. One thing is clear, though, that only the combined model-data synthesis will give us robust and belivable answers. This paper unfortunatly does not attempt to do that.

    [Response: Martin, Much appreciated, thanks – gavin]

  26. 26
    Stephen Berg says:

    I am thinking that the slowdown in the Atlantic circulation is a contributing factor to the record hurricane season this year, due to the stockpiling of warmer water in the subtropical zone.

    Drs. Mann, Schmidt, or other RC mods, is this connection justified?

    [Response: See replies to comments #1 and #2. This is actually the opposite of the conventional wisdom, and the opposite of what proponents of a THC/Hurricane connection propose. As noted in our piece, Gray and others advocating such a connection, argue that a recent increase in the intensity of the THC is responsible for the upturn in Hurricane activity. This argument at least has some plausibility to it; model simulations of multidecadal “AMO”-type variability show that the entire North Atlantic basin (including the tropics) are warmest when the THC is at its strongest. So clearly, the argument that the thermohaline circulation is driving the observed increase in Hurricane activity in recent decades has to go out the window if Bryden’s claim that the THC has actually slowed over this time frame, is correct. On the other hand, there are a number of inconsistencies in various indicators that give us some pause in drawing definitive conclusions as to what exactly the THC is doing. See Martin Visbeck’s sober analysis above. -Mike]

  27. 27

    Given that the editors have allowed through the well-posed if off-topic comment #8, let me take the chance to add a few related ideas.

    Climate change is normal, and within some range there probably is no best climate. However, there is a best rate of climate change; arguably the closer to zero it is the better. With modest climate change, appropriate infrastructure can be built, appropriate crops can be planted, and population densities, human and otherwise, can adjust themselves gradually to available resources. It’s worth pointing out that human civilization emerged in the period of unusual climate stability that prevailed over the last 6000 years.

    Also, regarding the economic consequences of climate change, I think there is a fundamental flaw that is pervasive in thinking about the impacts of climate change.

    The climate community is asked to deliver a formal consensus document about the state of knowledge on climate change every five years or so. (This is an extraordinary process. I don’t know of any other scientific discipline that undertakes such a thing. Most people close to the field think it is a remarkable and mostly successful process.) However, that consensus process has two aspects that make the picture look more sanguine than it is.

    First, the projections almost invariably end abruptly in the year 2100. Hopefully, the world itself will not end abruptly in that year. Admittedly, beyond a century our projections of what will happen to the perturbed climate system become very hazy, but this is because after that date the consequences may become so large that conventional wisdom starts to fail, not because anthropogenic climate change will end on that date. David Archer sometimes asks the question as to whether there is some reason that our obligation to future generations should be seen to terminate abruptly in 2100, and points out that there is no consensus in this regard, e.g., with respect to nuclear wastes. Why should fossil fuels not be held to as high a standard?

    Second, the projections most attended to are the ones that in the view of the community, the most likely outcome. The costs associated with climate change are thus estimated as the costs of the most likely change.

    A moment’s thought by anyone familiar with statistical reasoning will show that the expected cost of the outcome is not the same as the cost of the expected outcome, since cost increases nonlinearly with the extent of the disturbance. (Consider that if temperature changes by 100C, the cost is more than 100 times the cost of the temperature changing by one degree, to take an extreme example.) Thus, for economic policy purposes we ought to take a cost-weighted mean of the plausible scenarios, rather than the cost of the plausibility-weighted scenarios. That is to say that the large risk of changes that are much bigger than currently estimated is rationally more important than the small risk or benefit if the changes are smaller.

    This is not how matters appear in public discussions. Those who like to postulate that nothing serious is happening are often posed against the middle of the scientific consensus as if they were two poles of a debate, where in fact we are at best hearing from the moderate middle and the more Polyannish tail of informed opinion. Where is the other tail? The possibilities for disaster far greater than the expected result deserve at least as much attention in policy formation as the possibilities for damage much smaller than expected, but they never seem to get that attention. Cost/probability weighting of risks and benefits is well-understood and is often raised in issues where economic and environmental interests are at contrary purposes, but for some odd reason it gets little attention in climate change.

  28. 28
    Ken Robinson says:

    In reading the original post, it would be easy to draw the conclusion that William Gray is a contrarian whose views on climate and climate change should be lumped in with Michael Crichton’s (and here I make no assertions regarding the veracity of Crichton’s statements). The post also describes the recent increase in North Atlantic hurricanes as “anomalous”.

    In researching the topic a bit more deeply, I came across the following article on the National Oceanic and Atmospheric Administration (NOAA) website, published on November 29, 2005.

    It seems that there is consensus among NOAA researchers that the recent upswing in hurricanes is entirely natural. I’m not a climatologist, simply an interested layman. However, I would assume that given the organizations for whom these people work, they qualify as hurricane experts. As such, I tend to give their opinions significant weight.

    A few excerpts from the article include the following:

    “As a result, the North Atlantic experiences alternating decades long (20 to 30 year periods or even longer) of above normal or below normal hurricane seasons. NOAA research shows that the tropical multi-decadal signal is causing the increased Atlantic hurricane activity since 1995, and is not related to greenhouse warming.”

    “There is consensus among NOAA hurricane researchers and forecasters that recent increases in hurricane activity are primarily the result of natural fluctuations in the tropical climate system known as the tropical multi-decadal signal.”

    “NOAA’s Seasonal Hurricane Outlooks are Based on Two Tropical Climate Factors
    NOAA began issuing seasonal Atlantic hurricane outlooks in 1998. These outlooks are a collaborative effort from scientists at the NOAA Climate Prediction Center, NOAA Hurricane Research Division and NOAA National Hurricane Center. NOAA research shows that two prominent climate factors strongly control the key inter-related set of conditions that determine if tropical storms will form in the main development region during August through October. These climate factors are the tropical multi-decadal signal and the El Nino/Southern Oscillation (the El Nino/La Nina cycle).”

    “NOAA scientists now understand, monitor and predict these climate factors and their combined affects in a way that was not possible a decade ago. As a result, NOAA can often confidently predict how conditions will develop across the tropical Atlantic as the season progresses,” said Gerry Bell, NOAA’s lead seasonal hurricane forecaster at the Climate Prediction Center in Camp Springs, Md. As a result, these outlooks help the nation better prepare for hurricanes.

    “Max Mayfield, director of the Tropical Prediction Center at the National Hurricane Center in Miami, Fla., also heightened awareness of the tropical multi-decadal signal when testifying at a congressional hearing earlier this year (Sept. 20, 2005). He stated that hurricane activity in the Atlantic ebbs and surges in cycles, each of which lasts several decades.”

    “Research by NOAA scientists Gerry Bell and Muthuvel Chelliah, currently in press with the Journal of Climate, describes the tropical multi-decadal signal and shows that it accounts for the entire inter-related set of conditions that controls hurricane activity for decades at a time. Their study also shows that the tropical multi-decadal signal is causing the observed multi-decadal fluctuations in Atlantic hurricane activity since 1950.”

    “These results expanded upon a 2001 study in Science by hurricane meteorologist Stanley Goldenberg at the NOAA Atlantic Oceanographic and Meteorological Laboratory’s HRD in Miami, Fla.; Chris Landsea, the NOAA Science and Operations Officer at the NOAA National Hurricane Center in Miami, Fla.; Alberto M. Mestas-Nunez of the University of Miami and William M. Gray of Colorado State University, which suggested that “decades-long cycles in sea-surface temperatures and wind shear in the tropical Atlantic closely matched the cycles of major hurricane formation in that region.” That study also showed the recent increase in hurricane activity is nothing new. In fact, “Atlantic Ocean temperature data shows that this is just the latest manifestation of a long-running hurricane cycle that dates back to at least 1870,” said Landsea.”

    “There is consensus among NOAA hurricane researchers and forecasters that recent increases in hurricane activity are primarily the result of natural fluctuations in the tropical climate system known as the tropical multi-decadal signal. The tropical climate patterns now favoring very active hurricane seasons are similar to those seen in the late 1920s to the late 1960s. The current active hurricane era began in 1995, meaning the nation is now 11 years into an active era that could easily last several decades (20-30 years or even longer). We can expect ongoing high levels of hurricane activity – and very importantly high levels of hurricane landfalls – as long as the active era continues.”

    By the way, the Chris Landsea mentioned in the article is the same Chris Landsea (great name for a hurricane researcher, don’t you think?) who resigned earlier this year from participation in the IPCC’s Fourth Assessment Report (he had been asked to do the writeup on Atlantic hurricanes). His open letter to the community can be found here:

    In his letter, Landsea refers to the large body of evidence (ie more than just one study) supporting the consensus among hurricane researchers that is there is no detectable human signal in the hurricane record.

    In the original post, Gavin and Michael state that “It will be interesting to see if Gray, and others, will change their line of argument in the face of this new study.”

    Here’s my opinion, for what it’s worth. The “others” mentioned in “Gray, and others” are in fact highly qualified hurricane experts with substantial expertise and a significant history of publication in respected journals. For that matter, Gray himself is a bona fide expert with a long history of published research. This fact should, I believe, be pointed out.

    Second, Bryden’s study may be interesting, and may merit further investigation. However, it seems to me that a single study, as yet unreplicated and with very unclear implications, is by itself insufficient to overturn the current viewpoint held by experts in the field. “Changing their line of argument in the face of this new study” would seem highly premature at best.

    So, is global warming causing more or bigger hurricanes? Until I hear otherwise from the hurricane science community, I would say no.

    [Response: We discussed this previously, and will return to the point once the season (which one could argue has been extended by the rather anomalous Hurricane Epsilon) has finished. – gavin]

  29. 29
    James Annan says:

    While I don’t disagree with the general point in Michael Tobis’s post (that we should try to account for uncertainty which could matter due to the nonlinearity of costs as a function of change), I find it hard to swallow his claim that “we are at best hearing from the moderate middle and the more Polyannish tail of informed opinion”. It seems quite clear to me that there is a significant element of alarmism in many of the predictions and speculation that is presented within the mainstream literature. One could start with the widespread use of the A2 scenario (which “just happens” to be the most extreme of the main markers), or the 1% per annum tests that are still regularly presented with dates as if they refer to a realistic forecast. It seems equally arguable that there is a bias towards presenting something close to the worst posssible case, and then using this to argue that “something must be done” without attempting to account for how unlikely this worst case is. Of course this approach is a nonsense far beyond what Michael Tobis is complaining about, since there is no hard upper bound to the uncertainties.

  30. 30

    Comparing two measurements with a possible uncertainty of +/- 6Sv each should result in a total uncertainty of +/-12Sv (ossibly more considering less sophisticated equipment in 1957). Given a total error bar of 24Sv, measured differences of 12Sv could well be explained by measurement uncertainty alone IMO.

  31. 31
    Timothy says:

    Re: #29. I thought that the use of the high CO2 scenarios and the 1% tests [or even 2-4%!] was because of the fact that with climate ‘prediction’ we are still dealing with signal to noise problems, etc. I feel that the only thing we can conclude with confidence [from the models] is that the climate will warm [more than it already has done] if we don’t take action to reduce emissions.

    Personally I don’t want to find out what the consequences may or may not be [disapearance of Tibetan glaciers leading to starvation of billions, etc], but I think that we are a long way from deriving confident predictions of impact changes due to global warming from GCMs. They simply aren’t good enough [and won’t be for at least the next two decades given likely computer power].

    What we can see from stabilisation scenarios and the possibilities of positive feedbacks in the carbon cycle is that we don’t have two decades to wait to get detailed information to enable us to make a near-perfect cost-benefit analysis.

    Instead we have a situation of risk management. There is a non-negligible risk that we could disrupt the world’s climate sufficiently that the world’s economy and society would be disrupted extremely seriously. It’s a sensible insurance investment to reduce our perturbation to the world’s climate.

    The point is that no-one knows what will happen. The contrarians can’t be certain that it will be easier/cheaper to adapt to changes because we aren’t capable of making that sort of prediction

  32. 32
    Janne Sinkkonen says:

    Re: #30. Just a minor statistical note without knowing how these numbers relate to reality: If you compare two measurements with standard deviation of 6, the standard deviation of the difference is not 12 but sqrt(2)*6 = 8.5. (Assuming independent noise, etc., of course.)

  33. 33
    Dano says:

    James [#29]:

    In my little town, I often end up in the paper for some reason or another. Usually the coverage is fine, but last week I was surprised at the headline; plus, the content highlighted a sidebar rather than the main issue. I wrote it off to looking for circulation increases, as the headline was above the fold and thus would appear in the newpaper dispenser’s window. In this context, the implication for highlighting a sidebar rather than the main is not so important as the circulation is a few tens of thousands – plus, the politics behind the headline isn’t really going to affect the outcome one way or the other, as the outcome was determined before the headline appeared.

    On a larger scale, however, this sort of focused headline reality of what James was addressing in #29 has larger effects. It is these effects that Michael was talking about above, and referred to in the ‘650K years’ thread. James’ point about alarmism, IMHO, is a result of headline-seeking, for whatever reason – ego, politics, circulation increase, etc. Michael’s point is that the headlines are purposely manipulated for mendacity or obfuscation, with facts or frames being manipulated. In my case, the manipulation had no bearing on the outcome; in Michael’s case, the manipulation may well have bearing on outcomes.

    Both James and Michael are insisting upon more rational presentation of issues and facts, but (especially today in our information-saturated world), appeals to values and emotion grab attention and impel action far more effectively than rational, Platonic discussion. The mendacicization that angers Michael begins with appeals to values that are portrayed as being under attack. Whether they are acutally under attack or not is not the point, as action is impelled by this portrayal. You two are are not quite the same page, but the pages face each other, anyways.

    What Gavin and Michael imply in the beginning of this post is that the results are preliminary, and subsequent comment in #25 reinforces this preliminariness. It is clear that it is too early to tell what is going on out there [still another instance where more data are needed]. But this doesn’t sell papers.

    What we can say now is that there are a lot of dogs barking, but whether there is something in the tree remains to be seen. It is natural to have one dog start to bark if they hear another barking. What matters is what you tell everyone you saw when you went out to check: “they weren’t barking at anything”, or “I couldn’t tell what they were barking at” (provided you end your sentences with preps.), or “another critter in the tree. We’re gonna have to do something.” Whether anyone listens to you when you come back to tell your spouse why the dogs were barking depends upon your credibility and whether everyone knows that those dang dogs bark even when a leaf drops out of the tree.



  34. 34

    Let me concede James Annan’s point that sometimes the press gravitates to the scary scenarios. In fact that will keep us on-topic, as the current article will surely be blown out of proportion into a suggestion of glaciers returning to London.

    On the whole, though, the he-said she-said of the media tends to put equal weight on the comments of a committed contrarian and a mainstream scientist. This is clever framing of the debate by the contrarians, leaving the public with the sense of two equally valid positions. Reasonable people often shrug and conclude that “the truth lies somewhere in between” the center and the don’t-worry fringe. This amounts to a success of the economic interests behind the contrarian position, as the public is left with a far more equivocal view of the situation than the science actually indicates.

    I would prefer that the press, in their pursuit of balance, interview three people, one at the mainstream, one alarmist and one denialist. A genuinely symmetric report would present the scientific mainstream in the middle, and the political fringes as fringes.

    A genuinely rational analysis would go beyond symmetry and would risk-weight these positions. There is a possibility that we have grossly overestimated the net future impacts due to anthropogenic perturbation of the climate system, and will therefore erroneously indulge in the sort of policy that denialists invariably call “draconian”. There is also a possibility that we have grossly underestimated those impacts, and will therefore undercorrect our behavior and suffer cataclysmic consequences. The latter risk is more important. So given equal scientific weight to their arguments, the argument of the alarmist is rationally of greater policy consequence and should be given more attention, without it being cast as part of the mainstream.

    I would have to say that, in North America at least, that is nothing like what emerges from the press.

    To take an example I find of particular interest, the IPCC carbon emissions scenarios account for economic activity. As I understand matters, they don’t account for large releases of methane from tundral clathrates. The criticisms of the scenario set usually seen suggest that the curves are too steep, but the clathrate feedback may leave them too shallow. There is no carbon cycle in the GCMs, so it is essentially left out of the predictions.

    I believe that that tundral clathrate releases have played a substantial role in the rapid deglaciations on the 100KA cycle. While this is controversial, in any case there must be some sort of feedback from temperature to atmospheric carbon to account for certain aspects of the glacial record. If that feedback is operational today, human emissions may kick off natural emissions, and make matters much worse than they would appear from the IPCC scenarios. Yet the debate as seen by the policy sector appears as “are the scenarios realistic, or are they overstated?” What if they are understated? Where is the discussion of that possibility? It comes up in discussions at geophysics faculties, but it somehow doesn’t reach the public discourse.

  35. 35

    As written above:

    “a decrease in ocean temperatures in the North Atlantic of up to 2°C or so, and maybe 0.5° over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed.”

    May be circulation has indeed changed and that World wide warming is masking the cooling signal as expected by models. It is a similar argument to the ongoing Hurricane debate which I am looking forward to take on again whenever RC starts another session . Hurricane contrarians not so clever ploy is to say that the world’s warming climate has nothing to do with hurricanes. It goes likewise with western Europe, warming up with the rest of the world, and so the Gulf stream heat supply may be replaced by tthe lack of cooling from the Arctic and Siberia, it would be a different climate, but no Icy Canadian Blizzards for East Anglia, unless the Arctic is normal. The models with Gulf stream failure must have picked up this lack of cooling from the North?

    November Resolute Bay Nunavut Canada mean temperature anomaly was around +4.5 C. No Gulf stream here, just incredible warming. And the same it seems all over the polar region, must wait for official November data, but if the Arctic becomes permanently warmer this suggests that a gulf stream failure may be catastrophic not as much for Europe but much worser for the South Atlantic.

  36. 36

    Not much sense in applying (unknown) statistics and distributions with respect to a single result IMO. Shouldn’t this be handled rather straightforward instead?

  37. 37
    Janne Sinkkonen says:

    Re: 36: You already referred to error bars, which do not exist without underlying distributions. Multiplying the correct number by 1.4 seems to me cumbersome and misleading rather than straightforward.

  38. 38

    Re 34 Michael Tobis’ phrase “scary stories” of course comes from the Stephen Schneider dilemma – whether to report what can be proved and be ignored or to report one’s fears in order to obtain the public’s attention. However, with a peer review system, it is not possible to take the second option and as a result, the dangers of anthropogenic global warming have gone by default.

    Michael is also right to point out that the fossil fuel funded sceptics have cleverly kept the argument on whether global warming is happening and on its cause, thus diverting attention away from the dangers. But apart from Wally Broecker, no one is really mentioning them.

    For instance, we now know that the Greenland ice sheet is melting, and that as it melts the height of the surface of the top of the ice sheet will reduce increasing the temperature there. This means that if the climatic conditions remain unaltered, then the melt of the Greenland ice will accelerate and sea level will rise by 5 metres. See This means that not only will most of Florida disappear beneath the waves, so will much of southern England including London. Since the melting is being caused by carbon dioxide, the only way to stop it would be to reduce carbon dioxide levels below those of today. Is that politically or physically possible? Letâ??s face it; it is already time to say “Bye-bye London!”

    How have we got into this state? Some blame the press and their â??False Objectivity of Balance.â?? I blame the scientists with their â??False Balance of Objectivityâ?? :-(

  39. 39
    Timothy says:

    Oh Dear. The Guardian, which is normally better than most when it comes to science topics, has produced a lamentably bad click-though ‘interactive’ guide to the slow down of the “Gulf Stream”. Apparently…

    “As the water is warmed at the tropics, it rises up and heads north giving off heat as it goes. By the time it arrives of the coasts of Greenland and Scandinavia it is cold, relatively heavy and so sinks back down. This cold water is then drawn south to replace the water rising at the equator.”

    They don’t say what thermal source warms the water at the sea bed causing it to rise at the equator… However they do get the supposed mechanism for weakening of the THC due to global warming right. [Any info on whether attempts at water budgets from observations have been made to calculate what the freshwater anomaly into the Arctic/North Atlanitc has been over the last decade or so? I know that Ocean modellers generally have little confidence in climatologies of Ocean precipitation so I’m guessing that the uncertainties on the budget as a whole are going to be pretty large – they’ll also be uncertainties due to how much Arctic sea-ice mass has been lost].

    They also completely miss out any of the interesting uncertainty discussed here about the measurements and multi-decadal oscillations in the Ocean.

  40. 40
    smyyga says:

    Last week I saw a presentation about still unpublished (under peer-review) latest results about the mass-balance of the Greenland ice-cap done using spaceborne synthetic aperture radar (SAR). The loss-of-mass resulting from ice-dynamics alone has increased three-fold during the last 10 years! The mass-balance of the whole greenland ice-sheet was estimated to be the following:

    Year Mass balance (km^3/year)

    1996 -93
    2000 -119
    2005 -215

    Most of the increase is on the eastern-side of Greenland, i.e. potentially hazardous for the Gulf-stream. I think this is scary, I have more details in case you’re interested.

  41. 41
  42. 42
    smyyga says:

    The numbers I quoted are from measurements (i.e. not model-based estimates) presented last week in this presentation:

    Most of the mass-loss is apparently from ice-dynamics, which was apparently not considered at least in the first study you linked to. Even if increased precipitation would thicken the ice-sheet at its middle, the speeding up of the output glaciers would dump more freshwater into the North Atlantic. Largest mass-change is observed in Southeast Greenland (64M km^2) which has accelerated from -30+-5 km^3 in 1996 to -70+-5 km^3 in 2005 (I think these numbers ignore increased melting altogether).

  43. 43
    Steve Bloom says:

    Re #42 (Smygga): The second study analyzed direct measurements of the surface (which account for snow accumulation and *minor* loss factors such as surface melt and blowing snow), but used a 1999 study to estimate the major loss factors of iceberg discharge and basal melting. I didn’t bother looking this latter study up, but it’s early enough to have missed the recent big acceleration in the glaciers.

  44. 44

    Re #43 (Steve Bloom) The Greenland ice is now starting to melt and if CO2 levels continue to rise it will melt even faster. You, and even serious/senior scientists like Berger (who says it will take levels of 700 ppm for the Greenland ice to melt) may argue that more research is needed, but it is obvious to any right thinking citizen that the game is up!

  45. 45

    How much ice is on Greenland total?

  46. 46
    Chris Reed says:

    Re #22, Thanks for the reply Timothy.

    I accept what you say, but I’d feel happier if there were not such an apparent pattern in the North Atlantic region. I’d like a raft of papers that show ‘nothing odd, no problem here’. The lack of observed temperature impact in the N. Atlantic Basin is reassuring; perhaps it’s closer to the 8% than 30% or 56% side of the range in Bryden’s work. But I find myself in the odd position of being an ex-sceptic who’s now reverting back to form ‘it’s probably just natural variability’. ;)

    BTW With regards funding, I came across this last week, . So yes the ball has started rolling – although the results will probably take some time. Personally I’ll be lobbying for more support for such programmes.

  47. 47
    Chris Reed says:

    re #25, Thanks Martin, for pointing out that the article I’ve ref’d to in my post 21 (Re Bogi Hansen of Faroese Fisheries Lab) is out of date.

  48. 48
    Mauri Pelto says:

    The Greenland Ice Sheet has an area of 1,736,095 km2 according to the USGS, the density of an ice sheet is about 900 kg/cubic meter. The mean thickncess reported varies from 1500 to 1700 m thick. this yields a volume of 2.5 million cubic kilometers. It must be pointed out that the ice has been thinning more appreciatively in west Greenland of late and that ice sheet melting can only contribute a moderate amount of freshwater volume each year. Freshwater spikes requires extenisve sustained calving which the ice sheet is not capable of. A loss of ice from Greenland of 372(±37) km3 yr-1 in 1998-2003 was reported by:

    Hanna, E., Huybrechts, P., Janssens, I., Cappelen, J., Steffen, K., Stephens, A. [2005]

    Runoff and mass balance of the Greenland Ice Sheet: 1958-2003
    Journal of Geophysical Research, 110, D13108.

  49. 49
    smyyga says:

    So a 3-fold increase in output glacier velocity does not count as “extensive sustained calving”? The Kangerdlussuad Gletscher shows also a 10km ice-front retreat since 2000 and the Helheim Gletscher shows a 5km retreat within the same period.

    The paper you quote ( ) estimates the overall mass-balance of Greenland being -36(+-59)km^3/year between 1998-2003, which is significantly less than the -215 km^3/year estimate from 2005. Hopefully 2005 was an exceptional year, luckily with Radarsat and Envisat we can now monitor the whole Greenland several times a year. Interesting times :-/

  50. 50
    Mauri Pelto says:

    #49 The contribution of these large glaciers seems just that in terms of ice and water volume, but put in the context of sverdrups, is another question. I would leave to an oceanographer to say whether it would be insignificant overall. Helheim Glacier is five kilometers wide and at the calving front less than 1 km thick, from 2001-2005 it retreated 8 km, you are looking at less than a contribution of 40 km3 for a five kilometer retreat over a span of four years. 10 km3 per year is equal to 0.3 sverdrups of output. In a paper I published in 1990 we established the calving flux of the Jakoshavns Isbrae, the most productive glacier in Greenland at 40 cubic kilometers per year,it is retreating after 40 years of balance. That was the name of the paper we published “Equilbrium state of the Jakobshavns Isbae, Annals of Glaciology (17). Of course we are concerned only with acceleration, but an acceleration of this narrow front of 30% which has occured at least over a short period in recent years is an added contribution of 12 km3. The mean contribution of the glacier has ranged from 1.0 to 1.5 sverdrups from 1950-2003, with an increase of 0.3 sv from 2001-2003.

    [Response: Are you sure about your conversion factor? 1 Sv = 106 m3/s = 10-3 km3/s = 365*24*3600*10-3 km3/year = 3.1*104 km3/year, so 10 km3/year = 3.2*10-4 Sv, a factor of 1000 out. – gavin]