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Est-ce que le Gulf Stream ralentit ?

Classé dans: — gavin @ 26 mai 2005 - (English)

par Gavin Schmidt (traduit par Pierre Allemand)

Nous avons été submergés de demandes d’évaluation des rapports récents du Times
Britain faces big chill as ocean current slows” (la Grande Bretagne soumise à un refroidissement important en raison du ralentissement de courants océaniques) et de CNN “Changes in Gulf Stream could chill EuropeLes changement du Gulf Stream pourrait refroidir l’Europe), (notez l’intéressant glissement de perspective géographique !).

(suite…)

L’origine de l’histoire, est une affirmation, au cours d’une réunion de l’EGU de Peter Wadhams de l’Université de Cambridge, selon laquelle la convection, dans une zone normalement active de la Mer du Groenland avait été très réduite pendant l’hiver dernier. Plus précisément, dans une des zones où une douzaine de ‘cheminées’ convectives se forment, on avait observé seulement deux petites cheminées. (Malheureusement, je n’ai pas été capable de trouver un résumé des propos du Docteur Wadhams digne de confiance, et j’ai dû m’en remettre à l’article du Times pour les détails).


Les cheminées convectives des mers bordées par le Groenland, l’Islande et la Norvège se mettent en route lorsque le refroidissement intense de l’océan, normalement associé à un passage de basses pressions, détruit la stabilité des couches normalement stables de l’océan, et provoque la convection et le mélange de l’eau refroidie et rendue plus dense pour former une couche relativement plus profonde. Cette région du monde est un des seuls rares endroits où la colonne inférieure des couches océaniques est assez stable pour que ce processus puisse se produire au large, et conduise à des cheminées convectives descendant jusqu’à 2 à 3000 mètres.
Les masses d’eau profonde issues de ce phénomène sont ensuite évacuées de cette région, pour former éventuellement les courants profonds des “North Atlantic Deep Water” (eaux profondes nord atlantique) (auxquels participent aussi des eaux provenant de la Mer du Labrador, et d’autres masses d’eaux). Ce processus constitue une partie de ce qu’on appelle la ‘circulation thermohaline’. Il est associé aux quantités de chaleur relativement importantes transportées dans l’Atlantique nord, qui maintiennent la Grande Bretagne et le reste de l’Atlantique nord 3 à 6 °C plus chauds que ce qu’ils devraient normalement être. La figure illustre les chiffres estimés à partir de deux modèles pour l’impact de cette circulation. (Stocker, 2002).

Ce transport de chaleur est souvent associé au Gulf Stream dans les médias et le grand public. Cependant, mon coté pédant m’oblige à préciser que le Gulf Stream est essentiellement un courant périphérique provoqué par les vents d’ouest qui remontent depuis le Golfe du Mexique le long des côtes américaines vers le Cap Hatteras d’où il bifurque vers l’Atlantique central (voir aussi cette lettre de Carl Wunsch). Il devient ensuite le Courant Nord Atlantique qui est, de fait le courant responsable du déplacement de chaleur anormal vers le nord de l’Atlantique. Il y a de fortes chances, en se fondant sur des études théoriques et des modèles climatiques des climats passés, pour que d’importants changements, un ralentissement ou même un arrêt complet puissent se produire dans le Courant Nord Atlantique et la circulation thermosaline. Les modèles climatiques prévoient en général indéniablement (mais pas exclusivement) un ralentissement de cette circulation vers 2100. Ce phénomène est dû à l’accroissement des précipitations dans la zone, ce qui renforce la formation de couches océaniques et réduit le mouvement de convection. Il est probablement futile d’insister sur ce point, mais un arrêt de la circulation du grand vecteur océanique n’est pas la même chose qu’un arrêt ou un renversement du sens de circulation du Gulf Stream (qui, comme je l’ai mentionné plus haut, est essentiellement gouverné par les vents).

Pour en revenir aux affirmations de Peter Wadhams, comment ces observations à petite échelle peuvent être traduites en titres de presse concernant des prévisions de changement du Gulf Stream et un rafraîchissement à venir pour l’Europe ? Le problème majeur est que le sujet et les résultats du modèle climatique sont maintenant bien connus, si bien que n’importe quel résultat scientifique concernant ces prévisions retient énormément d’attention.

Néanmoins, il y a beaucoup de chemin entre la Mer du Groenland et le Gulf Stream, et un certain nombre de points importants n’apparaissent pas dans ces nouveaux scénarios. Tout d’abord, nous savons que la variabilité décennale concernant l’amplitude, la profondeur de la convection et sa profondeur est importante. En fait, il a été rapporté par Schlosser et al. (1991) que des mesures concernant les CFC avaient montré une convection très faible dans la Mer du Groenland au cours des 7 années écoulées. Par la suite, la convection a repris. De façon similaire, la convection dans la Mer du Labrador (l’autre élément majeur) a aussi oscillé, peut-être sans relation de phase avec la convection du Groenland. Les études de Dickson et al. (1999, 2002) ont montré que les propriétés des eaux profondes circulant dans le Détroit du Danemark (entre la Mer du Groenland et le reste de l’Atlantique) étaient reliées à des ensembles de variabilité comme l’Oscillation Nord Atlantique et que cela pouvait expliquer une partie de la variabilité.

Il est indéniable que des tendances à long terme deviennent discernables. Un refroidissement de l’Atlantique nord est visible depuis les années 1950. De longues mesures continues de la température et de la salinité à ‘Ocean Weather Station M’ (NdT : Station météo océanique M) en Mer de Norvège indiquent que les eaux profondes se sont notablement réchauffées. De plus, des réseaux de mesures commencent à se mettre en place (Osterhus et al, 2005) et des mesures mieux intégrées entre elles seront disponibles dans le futur. Il est important de garder à l’esprit que si les changements observés sont, évidemment, significatifs compte tenu de la précision de l’océanographie moderne, l’amplitude des changements (quelques centièmes d’unité de salinité) est beaucoup plus petite (peut-être deux ordres de grandeur) que celle rapportée par les données paléocéanographiques ou observée dans les modèles climatiques. Ainsi, bien que la surveillance continue de cette zone climatique soit clairement justifiée, l’imminence d’un refroidissement en Europe est encore bien lointaine.


84 Responses to “Est-ce que le Gulf Stream ralentit ?”

  1. 1
    Andre Bijkerk says:

    On the other hand we have:

    Boccaletti, et al (2005) The vertical structure of ocean heat transport. Geophys. Res. Lett., 32, L10603, doi:10.1029/2005GL022474, May 17

    who claim:

    “One of the most important contributions the ocean makes to Earth’s climate is through its poleward heat transport: about 1.5 PW or more than 30% of that accomplished by the ocean-atmosphere system (Trenberth and Caron, 2001). Recently, concern has arisen over whether global warming could affect this heat transport (Watson et al., 2001), for example, reducing high latitude convection and triggering a collapse of the deep overturning circulation (Rahmstorf, 1995). While the consequences of abrupt changes in oceanic circulation should be of concern, we argue that the attention devoted to deep circulations is disproportionate to their role in heat transport. For this purpose, we introduce a heat function which identifies the contribution to the heat transport by different components of the oceanic circulation. A new view of the ocean emerges in which a shallow surface intensified circulation dominates the poleward heat transport…..”

    [Response: Your point being what exactly? This paper is an assessment of the current climate poleward heat transport. The post is discussing the possibilty of one aspect of that transport being disrupted. As stated above, the wind-driven component is unlikely to change much, and so while the shallow, wind-driven circulations may actually transport more heat (and of course the atmosphere transfers even more), the variability in the heat transport can still be dominated by the variability in the overturning. -gavin]

  2. 2
    Pascal says:

    hello

    I’m agree when you speak about the wind-driven (firstly) and thermohaline (secondarly) forces for the GS.
    But if I “understand” the thermohaline circulation , the wind-driven force is more difficult to unbderstand.
    We have a cycle, by definition , with the thermohaline circulation ,but how is realized the return with the wind-driven force ?
    I think that ,to plunge down the water of the GS (or North Atlantic current) must be ,it’s an evidence , more dense then the present water.
    To be more dense ,this water must be more saline ,because its temperature cannot be colder than the present water.
    In all the cases it’s a thermohaline circulation not entirely induced by the thermohaline force.
    If this return does’nt exist we can get an accumulation of warm water ,then a decrease of the stream owing by hydrostatic force.

    [Response:The wind-driven circulation is harder to understand, yes, but is nonetheless well understood. You just need to do some dynamics, whereas the THC bit seems more obvious. But if you want a very rough idea... (others will no doubt correct me) then the wind-driven circ ends up being a large gyre, which concentrates into a narrow current along the western boundary, with a much broader return in the rest of the basin - William]

  3. 3
    Steve Gaurin says:

    This is a good overview of the Gulf Stream – thermohaline circulation (THC) story, but in addressing the question in this post I fear you have left out a discussion of the major factor that keeps northern Europe warm compared to similar latitudes in eastern North America. Atmospheric circulation patterns, specifically the predominant shape of the Jet Stream, controlled largely by orographic forcing of the Rocky Mountains, have more to do with keeping northern Europe warm than does the Gulf Stream or THC. The shape and location of the Rockies help to cause a persistent wave in the Jet Stream, with a trough in over eastern North America which brings cold continental air, via northwesterly winds, to northeastern North America, and a crest over the eastern north Atlantic and western Europe which brings warm maritime air, via southwesterly winds, to northern Europe. The ocean plays a role in that the seasonal storage and release of heat by the Atlantic Ocean mixed layer, coupled with wind advection, lend northern Europe a maritime climate with mild winters, but this is true whether the Gulf Stream operates or not.

    This is argued very clearly by one of your colleagues from across the Hudson River at LDEO, Richard Seager.
    See Seager et al., 2002. Is the Gulf Stream responsible for Europe’s mild winters?, Quarterly Journal of the Royal Meteorlogical Society, v. 128, 2563-2586.
    Also see this news release from Columbia U: http://www.columbia.edu/cu/news/03/02/richardSeager_research.html

    I understand that this post is meant to address variation in one side of the story (the Gulf Stream), but if we are to educate the public away from the Gulf Stream = warm Europe idea, we must give atmospheric circulation its due as the dominant forcing. I agree that THC largely determines the amount of ocean heat transport to the North Atlantic, but this affects both sides of the Atlantic, not just Europe. According to Seager et al. (2002) THC appears to account for the North America vs. Europe winter temperature difference only in the highest latitudes, north of about 60 degrees N, due to the fact that the heat transport limits sea ice cover there.

    [Response:You are partly right. Two issues are sometimes confused:
    (1) What keeps the eastern side of the Atlantic (say, Britain) warmer than the western side (say, Labrador)? That is largely atmospheric circulation and the general presence of an ocean upstream, i.e. maritime vs. continental air masses, as was affirmed in the paper by Seager et al. (2002). (But note they have a mistake in calculating heat storage vs. heat transport contributions, underestimating the latter, as explained by Rhines and Häkkinen (2003))
    (2) What keeps the northern Atlantic and surroundings warmer than the northern Pacific and surroundings? That is indeed largely the ocean heat transport associated with the THC, in lay-persons terminology often referred to as the Gulf Stream.
    Hence, traditionally the effect of the ocean heat transport is illustrated by comparing temperatures on the eastern side of both oceans: e.g. the classic review paper by Weaver and Hughes (1992) compares Bodö (Norway, 67°N) with average January temperature of -2°C to Nome (Alaska, 65°N) with average January temperature of -15°C. Another way to look at data is to plot the deviation of temperatures from the zonal average, as in Fig. 3 here.
    A good way to estimate the effect of the thermohaline part of the heat transport is to shut it down by dumping a lot of freshwater into the north Atlantic in a climate model, which stops deep water formation there. Two examples are shown in the figures in Gavin's post; it leads to several degrees cooling over Europe but also on the western side of the Atlantic. Hence, the Gulf Stream does contribute a lot to Europe's warmth, and I see no reason why anybody would want to "educate the public away" from this idea. - Stefan]

  4. 4
    Thor Olson says:

    It’s news like this that really catches my attention.

    (While most of Europe has shivered through an unusually cold March, a snow festival in Arctic Greenland has been postponed indefinitely because of a “heat wave.” The 11th annual international Snow Sculpture Festival in Nuuk was scheduled from March 18 to 21, when the average temperature in Greenland’s capital would usually be well below freezing.

    “The snow has been melting because of the mild weather and last week we had several days of rain,” Nuuk Tourism manager Flemming Nicolaisen said.)

    In your opinion, how stable are the estimates of fresh water comming from Greenland and mixing with the North Atlantic Current?

    [Response: How stable are the estimates, or how stable are the fluxes? The estimates are quite variable because of the difficulty in measuring these things in a difficult part of the world and the complexity of the processes (ice berg calving; under ice shelve melting, snow blowing, under glacier melt etc.). Thus assessing how variable these fluxes are is doubly difficult. The integrated measures from large-scale ocean salinity measurements are quite good, but it is difficult to work out exactly where that freshwater has come from. With respect to your first point, remember that weather is quite variable - especially around the North Atlantic, and I doubt that rain in March is unprecedented. - gavin]

    [Response:Here's a simple back-of-envelope consideration for the future: if the Greenland ice sheet melts completely over the next ~1,000 years (Jim Hansen argues in the current Climatic Change that the time scale could be centuries), this would contribute an average flux of ~0.1 Sv of freshwater to the surrounding ocean. Modelling uncertainty currently is such that in some climate models, this amount of freshwater (without any other forcing) would shut down deep water formation, in some it wouldn't. The reasons for those model differences are not yet understood (we're working on it but it turns out to be a hard problem).
    For global warming scenarios, additional forcing comes into play: surface warming and enhanced high-latitude precipitation, which will also reduce density of northern surface waters (an effect which alone has shut down deep water formation in some model experiments, e.g. Manabe and Stouffer 1993, 1994).
    In most future global warming simulations with climate models no meltwater from Greenland is included so far. -Stefan]

  5. 5
    Mike Doran says:

    The following matches some of what I have been talking about on the electrical aspect of cloud behaviors, minus the CO2 impact from gas exchange on surface lows and the bio modulations:

    Advances in Space Research. Article in Press
    http://tinyurl.com/b6w52

    The physical mechanism of the solar variability influence on electrical and climatic characteristics of the troposphere

    G.A. Zherebtsov, V.A. Kovalenko and S.I. Molodykh,

    Abstract
    Possible mechanisms of solar-climatic connections, which may be of importance over short and long time intervals, are discussed. The variations of energetic balance of Earth’s climatic system for the last 50 years are estimated. It is ascertained that the imbalance between the flux of solar energy that comes to the Earth and radiates to space is of 0.1% for the last ten years. The significance is analyzed for the possible influence of variations of solar constant upon the energetic balance of the atmosphere. The physical mechanism of the influence of solar activity on climatic characteristics and the atmospheric circulation is suggested and theoretically substantiated. The mechanism is based on the redistribution in lower-troposphere of condensation nuclei by the vertical electric field. This electric field is determined by the ionosphere-Earth electric potential, which in the Polar Regions is controlled not only by tropical thunderstorms and by the galactic cosmic-ray intensity but also by solar cosmic-ray fluxes. The height redistribution in the atmosphere of condensation nuclei with a change of the electric field of the atmosphere is accompanied by a change in total latent heat (phase transition of water vapor), by changes in radiation balance, and by subsequent changes of the thermobaric field of troposphere. The results of analysis of thermobaric field variations for the periods of invasion of abnormally powerful solar cosmic ray fluxes and magnetic storms confirm the reality of manifestation of heliogeophysical disturbances.

    ….

    [Response:Edited for length... -gavin]

  6. 6
    dave says:

    Re: due to overwhelming popular demand

    The THC is probably not shutting down anytime soon, as the post tries to point out. Somebody needs to say here that hysteria about abrupt shutdown of the “gulf stream” (sigh) is counterproductive with respect to critical thinking about the real ongoing effects of climate change like Tropical Glacier Retreat which few people seem interested in. This nonsense from CNN:

    Without the influence of the Gulf Stream and its two northern branches, the North Atlantic Drift and the Canary Current, the weather in Britain could be more like that of Siberia, which shares the same latitude.

    Oh, No! it’s the “The Day After Tomorrow”! Smallpox, dirty bombs, THC shutdown!

    Abrupt changes happened in the late Quaternary and there’s a scary lesson in this, but no modeling studies have predicted, to my knowledge, any shutdown of the THC in this century. Here’s a good page Questions and Answers about Abrupt Climate Change from Lamont-Doherty.

    [Response:I fully agree with your statement: "The THC is probably not shutting down anytime soon." And anybody will agree that hysteria is counter-productive. Nevertheless, the risk of triggering ocean circulation changes as a result of global warming cannot be ruled out at present (which is why you use the word "probably"), and it needs to be studied and discussed. This is an issue of risk assessment, as we discuss in detail in Rahmstorf and Zickfeld (2005). The issue is: what is the risk, and what risk are we willing to accept? Would you accept a 1% risk of shutting down the THC? Or a 5% risk? - Stefan]

  7. 7

    I absolutely agree that the press is mistaken in conflating the Gulf Stream with the thermohaline circulation. On the other hand, it’s not quite right (Carl Wunsch notwithstanding) to say that the Gulf Stream is an entirely separate thing. The fact is that most of the mass flux (both North and South) in the thermohaline circulation takes place in boundary currents, and not in nice broad zonally uniform sheets like you might think of by looking at the streamfunctions. You’d get something like a Gulf Stream even if you just had buoyancy forcing, though it would be a good bit weaker than the present Gulf Stream. Rui Xin Huang has written about this. What Carl means, no doubt, is that you’d still have a Gulf Stream even without THC, since you’d still have the wind driven part.

    Now, since the problem is nonlinear, the heat flux in the THC part of the “Gulf Stream” and the wind driven part don’t just superpose, so it’s not a priori clear to what extent one can think of them as separate entities. I’ve never really dug into this, but I’m not aware of the interaction issue having been treated.

    [Response: Of course you are correct, one would see differences in the Gulf Stream transports in the case of a THC change (though no reversal). It should be easy enough to look at some of these model runs and see how much the wind-driven part changes though.... Maybe I'll look into it at the weekend! - gavin]

  8. 8
    Pascal says:

    I’m apologized to insist ,but I repeat my question about the becoming of the wind-driven part of the oceanic currents towards the north-east Atlantic ocean.
    Is there a plunge-down favoured by THC or another thing?
    From another part I’m very surprised to see the comments and your introduction (gavin) in this topic .
    Clearly there is a (voluntary?) minoration of the THC and of the role of the GS itself.
    I don’t understand why.
    I think that the heat transport towards the North Pole by the GS (generic term for atlantic currents towards the North) is a significant part of the total heat transport.
    This calculation can be made in considering the flow and the delta temperatures between mid-latitudes and high-latitudes of the NAD and by comparing this delta to the theorical necessary energy to obtain the high-latitude recorded temperatures.
    It’s not a very precise calculation but it allows, I think, an approach of the scales of sizes.

  9. 9
    Chris Reed says:

    Many thanks for posting this, I was one of those “clamouring” for more info. ;)

    You haven’t made any reference to Dr Bogi Hansen (et al) of the Faroese Fisheries Laboratory. I’Ve had difficulty tracking down references to any more recent work. But as an explanation of what they’re up to: “ICES 1999 Annual Science Conference CM 1999/L:19 Nordic Seas Exchanges The Deep Overflow through the Faroe Bank Channel” In this paper he doesn’t make any firm commitment to trends, but notes “a slight indication of a decreasing trend in ISOW transport” ; “as yet the ADCP measurements in the Faroe Bank Channel are of too short duration to allow any conclusions on this important question.”

    I can’t find the source used to support this later story but on 21 June, 2001 Alex Kirby of BBC news reported “They (Hansen et al) estimated that the flow had fallen by 20% in the last half century, with most of the decrease in the last 30 years. And they say the rate of decline has accelerated within the last five years.” The same finding was also reported by Caty Weaver of Voice of America on 25 July 2001.

    Due to intensive reading since January I’ve gone from sceptic, to being staggered at the pace of change. One thing I’ve noticed is that estimates “seem” to continually roll forward the issues raised by projection scenarios as our scientific understanding improves (this is just an impression from my reading I’ve yet to do a timeline to fully satisfy myself on this “apparent” trend). Whilst deferring to your expertise: I find myself in the curious position of having gone from an uninformed sceptic of Climate Change, to wondering if this is another change whose pace the experts are underestimating.

  10. 10
    Trevor McMinnis says:

    “All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.”
    - Arthur Schopenhauer (1788-1860)

    It’s a pity science just couldn’t jump to stage three… without the tiresome delays that individual scientists invent to promote their own predominant [and usually wrong] assumptions. In the nineties, scientists were predicting triple crop rotations per year as a result of enhanced weather from global warming. Then the warning was turbulence! Now the warning is fear of cold. Admit your intellectual fraility – virtually all judgements are transient and nearly all eminent scientists are eventually proven to be way off the mark. I for one, and probably much of public opinion, believe it is better to be roughly right than precisiely wrong!

    In the margins between one era and another, uncertainty is king! There are no experts, “the best of the last era, becoming worst of the next!So especially true of science and scientists!! It is amazing to hear so many scientists definitely state exactitudes based on next to no absolute understanding of what is truly going on! This is what gives science a bad name.

    When I was at school in the fifties, the oceanic warm and cold currents were clearly marked and learnt as fact. They are still there, the same today – just! Scientists – ask this of yourselves – precisely what would the map of oceanic currents have been during the last iceage? The differences might just explain why ‘The Times’ could be more right than you think! If you don’t know the differences… you have no right to pass judgements on so-called non-scientific thinking. If we are about to change eras, conventional wisdom will be the last to recognise it!

    In defence of the ‘Times’ any publication aiming to tap into growing public interest must be good – as climate change accelerates up a public’s cause for concern ‘Richter scale’.

    Judgements based solely on precision of the day generate the ‘ass’ fronting all assumption! Much public concern with science is based upon its arrogant intolerance for new insight, total reliance upon evidence and incestuous drive for self-publication. Open minds always beat closed ones!

  11. 11
    Pascal says:

    thank’s for your response William (#2)

    The entire circulation is not, undoubtedly, only salinity-driven.
    I’m agree that there are wind-driven gyres and loops in the central Atlantic even in the North.
    To illustrate what I said ,concerning the relations between the two circulation forces ,please look at this link :
    http://www.cosis.net/abstracts/EAE03/07512/EAE03-J-07512.pdf

  12. 12
    dave says:

    Re #6 Stefan’s comment

    I agree completely.

    No amount of risk of a major change in the conveyor is acceptable but it is too late for that. If the press would frame the issue as a risk analysis, as you do, then there would be no issue. I did not mean to imply (and did not, I think) that the issue should not be studied and assessed.

  13. 13
    Eli Rabett says:

    WRT #9 – Part of the effect of things being predicted by theory and then found is that after a theory predicts something, people go looking for it. The converse is also true, after something is found, people go looking for a model. A good (bad) example is the Antarctic spring ozone hole (cold fusion).

  14. 14
    Steve Gaurin says:

    Thanks for the reply, Stefan (comment #3). I have downloaded and skimmed the Rhines and Hakkinen (2003) response to Seager et al. (2002), but it will take a more carfeul reading for me to digest its contents. On first glance, it does seem to present meaningful challenges to certain areas of Seager et al.’s approach. I’d be interested to hear if Seager has responded in kind – anybody know?

    Anyway, to address a couple points in your response to my previous comment …

    1) The data you cite from Weaver and Hughes (1992) merely supports my last statement (paraphrased from Seager et al., 2002), that THC (or more specifically, the North Atlantic Drift) keeps European latitudes north of 60 deg N warmer than other land areas at those same latitudes. So, while it is instructive to say that the North Atlantic Drift, which is indeed a remote branch of the Gulf Stream, has a dominant influence on keeping Scandinavia warm, I’m not convinced that the same can be said for it in terms of keeping Britain (between about 50-60 deg N) warm, as Gavin implies in the original post. Comparing temperatures in Britain to temperatures in British Columbia, Canada would be more meaningful.

    [Response: That would be more meaningful, and it does give the same qualitative answer. Britain is around 5 degrees warmer than the mean for it's latitude, BC is not. Look at Stefan's excellent fact sheet, and as he points out above figure 3 in particular. -gavin]

    [Further Response: It is true that the effect is strongest in high latitudes. Have a look at Fig. 10 of Seager et al. 2002, which shows the January cooling in their model after switching off ocean heat transport. This cooling exceeds 20 ºC in northern Norway, and it is "only" 3-6 ºC in Britain, Germany and more generally in central and southern Europe. Clearly much less than in the far north, but clearly not peanuts. Other models get similar results. So I think one should say: the Atlantic ocean currents keep the whole of Europe warmer than it would otherwise be by several degrees, with the effect getting stronger the further north you go. - Stefan]

    2) In terms of “educating the public away” from misleading ideas, well, I think that is part of our job, of pursuing scientific truth and honestly reporting our findings about the natural environment. That said, however, I could have chosen my words more carefully in my previous comment. In more precise terms, I don’t think it is necessary to completely disavow the public of the notion that the Gulf Stream contributes to western Europe’s mild climate; rather I would argue that we need to emphasize “contributes” over “determines” and include the atmospheric side of the story, which is at least as important as the oceanic side.

    [Response: Part of the education has to be differentiating the factors that contribute to current climate conditions (i.e. the 33 C greenhouse effect, the atmospheric poleward heat transport), and the things that are most likely to contribute to variations. For instance, there is no evidence that, with the current configuration, atmospheric heat transports have vastly different modes of behaviour - and so they are unlikely to suddenly flip into a new state. However, such changes have been modelled (and observed in the climate record) for the ocean transports. - gavin]

  15. 15
    Michael Jankowski says:

    ***Oh, No! it’s the “The Day After Tomorrow”!***

    I actually caught the authors of the book on the radio the other day, discussing this very same topic/these very same articles. They thought it was worse than they ever could imagine, something they expected to take centuries but was taking place in just a period of several years. One of them discussed reading one the articles and then saying, “It hit me right between the eyes. Between…the…eyes!”

    The last thing I heard was one of them basically implying that after the THC shuts-down, the world will have co-existing glaciers and desert with basically nothing in-between and no agriculture. The way they talked, I envisioned each one of them doubled-over in pain. I think one of them said it was the worst news he’d ever seen before.

  16. 16
    Trevor McMinnis says:

    Any takers for the map of oceanic currents during the last ice age?

  17. 17
    Michael Jankowski says:

    (Re#13…maybe it was the screenplay writers, not book authors…not sure there was a book!)

  18. 18
    Pascal says:

    (ref#14)
    It’s important to know the heat budget of each latitudes band.
    For example without heat meridian transfer(atmospheric and oceanic) the annual mean temperature of the 50-60N band should be -14°C.
    Fortunately this transfer exists, but a reduction of it leads to a very logic decreasing of the temperatures.
    The equilibrium budget latitude is about 42N (with my own calculations)
    But the real question of this topic is : “Is there ,as Wadhams says it, a decreasing of the GS and/or the NAD and how can we realize it?”

  19. 19
    Benjamin Harrison says:

    re: 17

    There is a specific book that followed the movie. There is also a book that inspired the movie in the first place, this monstrousity here:

    http://www.amazon.com/exec/obidos/tg/detail/-/0743470656/qid=1117219090/sr=8-1/ref=pd_csp_1/104-5441331-8592764?v=glance&s=books&n=507846

    …entitled “The Coming Global Superstorm.”

    I’m sure Art Bell will be back on the radio suggesting we’ll all be eaten by chupacabra before the THC shuts down anyway.

  20. 20
    Ender says:

    I would have thought that any interruption to this circulation would be some cause for alarm. As I understand it some of the flow is salinity driven and was interrupted in the recent past by a huge influx of fresh water from an ice dam breaking.

    Surely the melting Arctic ice cap is changing the salinity of the northern Atlantic and could affect this circulation as well.

  21. 21
    David Wojick says:

    One wonders how competent science writers could confuse THC with the gulf stream? It would be interesting to track this entrenched confusion back to its origins. Also, how much of the climate change hysteria in Europe and especially the UK is due to this fallacy? It may help to explain the difference in European and American atitudes.

  22. 22
    Pascal says:

    There is no confusion between GS and THC.
    And even in this case is it important?
    I don’t think so.
    It’s important from another way to know that the evaluation of deep water in Arctic zone is about 20 to 30 millons m3/s.
    It’s the most important source of deep water in the world,and it generates the most of the conveyor belt.
    It’s also important for the oceanic and atmospheric CO2 balance.
    The great flow of Arctic deep water comes mainly from THC and is fed with NAD.It prooves the great sinking of water in this zone and the great oceanic heat transfer.
    So I repeat that a slow-down of THC leads to a slow-down of the GS and to temperature decreasing in western Europe.
    It’s not too difficult to understand and to admit for american and european people.

  23. 23
    David Wojick says:

    Re #22, no one seems to agree with you Pascal. The consensus is that European warmness is due to a combination of atmospheric circulation and a wind driven GS, neither of which is driven by the THC. I would add that the GS has a cycle time of (I think) around 35 years while the THC takes around 1000 years to cycle so it is hard to see how the very slow latter could drive the fast former. But then it seems like everything in climate science is contentious.

    [Response: Well, I mainly do. It is not true that the THC has no impact on Eurpoean temperatures (see my earlier responses and those from Stefan, and the above figure). This doesn't imply that the THC 'drives' the GS, only that what we see as the GS is a combination of the two factors. Thus response times in the North Atlantic have many time scales that interconnect the wind-driven and themohaline circulations ranging from decades to potentially centuries. -gavin]

  24. 24
    g says:

    The timescale is evidently uninteresting to policy makers and short term climate modelers — but isn’t an anthropogenic change in thermohaline circulation something to worry about (for example, the post by D. Archer a while back).

    It will take centuries or millenia for the ocean to approach a new equilibrium and a thermal anomaly to penetrate marine sediments, but the resulting effects upon the marine carbon cycle could be catastrophic. At some point we will be resigned to simply watching the dominoes fall. Right now we have an opportunity to stop pushing them over.

  25. 25
    Pascal says:

    I think that atmospheric circulation is also linked to the GS.
    This current brings 1.5PW (10^15W) of heat through Atlantic ocean.
    this represents around 50 % of the total transferred heat between Equator and North Pole.
    This heat leads to great quantities of water vapour introduced in the atmospheric circulation.
    So the GS has a significant influence on the atmosphere and its circulation.(It’s not the only one!!)
    It’s clear that THC does’nt drive the entire GS (I never said it) , there are ,as Gavin said it ,loops and gyres which have a great participation in the heat transfer.
    But,maybe someone is agree with me to say that about 20 millions de m3/s are sinking in Arctic zone ,or it’s also false?
    If it is true ,from where does the water come from?
    The rain ?
    I don’t think this is the case.
    So a reduction of THC leads to a slowdown of GS.It is not a question of timescales it is a question of addition and substraction of flows.
    Example (the numbers are not true) near Florida the GS flow is 40 millions m3/s.At 35-40N there is a loop branch of about 20 millions de m3/s (that returns in tropical GS) and the DNA branch of 20 millions de m3/s.
    If the DNA branch (more THC-driven than wind-driven) is slowing down to reach 10 millions m3/s ,what is ,for you, David,the flow of GS near Florida?(in the absence of any feed-back)
    For the original subject of this topic “Gulf Stream slow-down” it comes from an article from Peter Wadhams in Sunday Times.
    I’m not sure that Peter Wadhams is right particularly for this story of “huge chimneys”.
    I believed that the sinking of dense water was more diffuse than in chimneys.
    Has someone some ideas about it?

  26. 26

    There is not very much chance that the THC will shut down / slow down in the foreseeable future. Indeed it happened a few times in the past, e.g. at the 8.2 ky event, when an enormous flow of sweet water from lake Agassiz entered the North Atlantic, due to the collapse of an ice barrier.

    But the extra quantities of fresh water involved today and in the far future are of another magnitude. If we take the largest decline (1,500 km3/year) in Arctic ice cover of the last decades together with the increase in precipitation over the Arctic (500 km3/year), then this is completely dwarfed by the 115,000-230,000 km3 in one year which is supposed to have slowed down (not even stopped) the THC. The quantity involved was larger than the amount of water that is refreezing every year in the Arctic, thus probably preventing (to a certain extent) the formation of cold saline water.

    Further, according to the Colorado State University forecast for the coming hurricane season (point 8 ), the number of hurricanes in the Atlantic is directly related to the strenght of the THC (but I suppose they point to the Gulf Stream part in this case), according to them:
    “Since 1995, the THC has been flowing more strongly, and there has been a concomitant increase in Atlantic major hurricanes in the tropical Atlantic.”

    [Response: The definition of 'THC' in the CSU forecast is their own particular diagnostic which may not actually have very much to do with the THC as we are discussing here. It is simply the SST off of Europe, which although it may be affected by the THC, does not define the THC. There are situations, for instance of general global warming where this temperature could increase, and yet not imply any change in the THC. This kind of imprecision in terminology (from CSU) is unfortunate and does not add to the clarity of the discussion. The truth is that we are very poorly served by the current observational network, and cannot give precise estimates of what the THC 'is', particularly not on annual time scales.- gavin]

  27. 27
    Michael Jankowski says:

    I recall reading something in the past few years about some scientists whose models implied the presence of the Rocky Mountains had more of an impact on European warmth than any ocean currents. Is anyone familiar with this?

    [Response: Seager et al, as referenced above and as discussed in Rhines and Hakkinen. With respect to a previous point, that since the Rockies aren't going anywhere, they are unlikely to be responsible for any variations in European warmth. - gavin]

  28. 28
    Stephen Prosser says:

    I am not a scientist, but have a great fascination with what is really happening in terms of climate change and have followed the discussions on this site and a number of others with interest. What is notable is that over the last decade, the majority of scientists appear to be continually renewing their predictions concerning the impact of global warming, and each time this is done, the speed of climatic change is anticipated as being quicker than previously expected. It is therefore not unreasonable to assume that the level of climate change currently occurring is still being underpredicted. It is also my understanding that the oceanic and atmospheric circulations are intrinsically linked, so that changes in one are likely to lead to changes in another. In other words, a slowing of the North Atlantic Drift, though not eliminating the influence of the Jet Stream, may well change the course in which the Jet Stream flows.

  29. 29
    Trevor McMinnis says:

    ref 28: horray for common sense and people like Steven Prosser! Scientists like to think digitally, right or wrongs. The rest of the human race use what nature provided – the best analogue technology that money can buy, that means imprecision and assumption leading to scenario building and making of judgements. In contrast the purity and precision often offered by empirical researchers more often than not leads to confusion in the big picture and attention to detail that camoflages what is really important. I too am not a scientist, but everybit as concerned about what’s going on as any scientist. It’s my planet too!

    Science should be aware of and always remember the ‘Titantic Principle’: “Evidence alone is never enough. Evidence when its most needed always arrives… simply too late.”

    I am not alone in regarding much of scientific debate as little more than mere discussion regarding the relative movement of chairs on the sloping deck of the titanic, whilst the main event goes on elsewhere… unbeknown to those on top! Don’t take this personally, but we can always learn from those we yet don’t fully understand.

    The planet is working hard to attempt to return equilibrium to its heat budget. [Why it is out of balance is merely academic and at this stage irrelevant.] For this balancing act [work] to take place at all, energy flows must have… a sink and… a source. The oldest sink on this planet was decommisioned by natural forces some 10,000 years ago. Once stirred into motion, the coldest sink current in the world, dormant for 10,000 years, will transform the map of oceanic and atmospheric current flows, thereby making the necessary rebalances by whatever [unknown] means necessary to maintain the second law of thermodynamics by absolute right! It will also make a nonsense of current flows of wisdom based on purely on early 21st century logic [a carryover from an inappropriate 20th century]. An imbedded population of 1.0 billion people in Europe have a right to expect that science provides timely answers and solutions not bicker and debate about movement of arbitary deck chairs!

    Please widen your debates and keep your eye firmly on the big picture! Good luck.

  30. 30
    David Wojick says:

    Re the Response to my #23. My mistake (nobody said this was easy to understand). So European warmth vis a vis Labrador has three major components (any others?). 1. Direct atmospheric circulation heat transport. 2. The wind driven component of the GS. 3. The THC heat transport which may be part of the GS. Any idea what the relative proportions are?

    Regarding 3, I am still unclear about the hydraulics. Does the THC flow via a separate stream, apart from the GS, or does it contribute its heat to the GS, such that the GS would be cooler if the THC were absent? I ask because the velocity of the THC is so much less than the GS (100 times less?) that I cannot see how it can be part of the GS on a flow basis.

    [Response: Velocity isn't the correct thing to compare because of the way these things are averaged. Total mass flux is better. The THC is about 20 Sv (1 Sv = 106 m3/s), and the amount of flux in the GS (in the upper part of the ocean) is around 40 Sv, so the THC contibuted about half of what we see in the GS. (Both these numbers are approximate, but are qualitatively ok). -gavin]

  31. 31
    Pascal says:

    ref#29
    excuse me David but I don’t understand your argument.
    Why do you speak about THC velocity?
    Is it the velocity of the dense water sinking?
    If it is the case I’m agree, this velocity is clearly very much smaller than the DNA velocity.
    But it is important to account the flow ,which is the area * velocity.
    For example ,if the area of sinking is 20000 km2 and the velocity is only 1 mm/s (0.04 inch/s) the flow of THC is 20 millions m3/s which is the NAD flow.
    To get this sinking velocity the water must be a very little more dense than deeper water.
    This little more dense water is obtained by the salinity and supercooling.
    If there is an unusual mixing with fresh water and/or warming of SST in Arctic (actually observed) the sinking may stop.
    I don’t know the density difference which is the driving force of THC but I think it is very weak and the amount of necessary fresh water or rewarming is, maybe, not so great that it is said in#26.

  32. 32
    Steve Gaurin says:

    Thanks Gavin, I get the point (in your response to my comment #14) that your intention here is to discuss changes in the ocean/atmosphere system that could cause a cooling of European climate, and that both observational and model evidence point to a weakening of THC as the most likely candidate. Actually, as an ocean/climate interactions student, I am happy to say that I basically agree with you on this. My point, which I hope you would likewise concede, is that even with a weakened or shut-off THC, western Europe would still remain warm relative to other land masses at the same latitudes (possibly even warmer than British Columbia, as it is now), based primarily on atmospheric circulation patterns. I am afraid that these news broadcasts, movies, etc. may give the public the impression that if the Gulf Stream weakens, western Europe will equal Siberia (climatologically speaking), and that is simply not true. After all, as you say, the Rockies aren’t going anywhere anytime soon, so regardless of what happens to THC, the persistent crest in the Jet Stream over the eastern North Atlantic and western Europe should continue to bring warm (though perhaps not as warm) maritime air to the northwestern part of the continent.

    [Response: I am happy to concede that. In fact these two media stories were the first I have ever seen in which Siberia is quoted as potential outcome. That is not on the cards. -gavin]

  33. 33

    The Rockies aren’t going anywhere soon, but remember that the waves that the Rockies drive are not just passively emitted by the Rockies! They depend on the winds blowing over the Rockies, and the propagation characteristics of the atmosphere downstream. The propagation characteristics, and the way they affect the temperature pattern of the waves, is dependent on the pole to equator temperature gradient, among other things — which gets lower, in a global warming situation. This will all affect the European climate change pattern.

    On the matter of whether THC shutdown could wipe out European agriculture, it is true that there is no support in current GCM’s for that happening. THC shutdown in a global warming scenario offsets some of the global mean-warming, which might look like a good thing. The real thing to fear in an altered THC scenario is an increase in climate variability — a decade of drought, a century of heat waves, twenty years of cold winters, forty years of flood. It’s that, rather than the “ice age” scenario, that is the thing to be concerned about. That’s the message we tried to convey in the NAS book.

    All that notwithstanding, we do have the uncomfortable fact that,at the Younger Dryas, SOMETHING (partly involved with THC shutdown) caused a sudden drop in temperatures that, if it happened today, would have very dramatic repercussions. One has to be careful to distinguish the extreme drop in Greenland with the more moderated drop over Europe, but still, it is far from clear at present that any real GCM, with the ocean-atmosphere dynamics properly represented, yields a temperature change of comparable magnitude to the YD. There’s a case to be made that current GCM’s are still missing something, and are insufficiently sensitive to climate forcings. It’s not by any means an ironclad case, but it’s more than a loose end.

    [Response: I agree with Ray - regional cooling is only the most well-known result of a THC shutdown, not neccessarily the most serious one, especially if it happens only after a lot of global warming has occurred which offsets it. Other likely consequences (e.g. rapid dynamical sea level changes, shift in the inter-tropical convergence zone and hence tropical precipitation patterns) are discussed in the Rahmstorf and Zickfeld editorial essay mentioned above, and the references therein. - Stefan]

  34. 34
    Steve Latham says:

    Re Trevor McMinnis (#s 10, 16, 29):

    Trevor has asked one valid question, “What would the map of oceanic currents have been during the last iceage?” This question presumes that the inputs for such a reconstruction are obtainable; it also presumes that particular aspects of the previous ice age(s) are relevant to global warming scenarios. I’m a fish geneticist so I won’t bother commenting on ‘paleo-ocean current-ology’, but it seems to me that glaciation would result in a reduction of fresh water inputs to the North Atlantic (during the ice age) and would therefore be quite different from the mechanism in question (which is related to early phases of global warming). Perhaps someone in the know could present a reference or two for Trevor and I so that we can understand something about oceanographic reconstructions and their potential relevance.

    Trevor: hang in there dude, and have some faith that scientists are generally keeping open minds. After all, finding something that nobody else has found would be very rewarding (wrt publication and otherwise). We’re doing what we’re trained to do and what we do best. When there are enough data suggesting that our working hypotheses are incorrect, a new paradigm will emerge that is hopefully closer to The Truth. But without serendipity, the only way these data emerge are through the normal work of scientists. You and other members of the public may not have much confidence in our work (and the oil industry and their hired guns play on that), but I think you’d have to admit that we’ve done quite well in the past. Scientists are making good progress doing science, maybe better progress than others are making in other disciplines (e.g., policy), and almost certainly more progress than if we tried to do that other stuff.

  35. 35
    Lynn Vincentnathan says:

    I think I read somewhere that if the THC slows or closes down, then the ocean’s ability to take up atmospheric CO2 will diminished — I think because less nutrients are churned up, causing phytoplankton to diminish. I’m not sure about this, but if it’s true then it would be another positive feedback loop in the GW process. I don’t suppose that’s been added to the models.

  36. 36
    Don Strong says:

    You guys should do a piece on the conveyor and Heinrich events. Those of us not among the climatocogniscienti need some grounding.

  37. 37
    David Wojick says:

    Re the Response to #30. Flow is important for at least two reasons. First, if some of the northbound return flow of the THC is actually carried by the wind driven GS then that fraction will not stop if the THC shuts down. The much greater wind driven GS velocity means that the THC pressure gradient is not driving the embedded THC flow at this point. From your numbers this could include all of the THC flow. Second, if there is northbound THC flow that is not part of the GS it probably has to be below it, otherwise the GS would cut it off. At that depth there is very little heat transport. The uphot of all this is that most of the ocean heat transport that warms Europe is probably due to the wind driven GS and would not stop if the THC shut down.

    [Response: Let me try again. Assuming we can think of these things as quasi-indpendent, you can consider the THC to be the flow pattern you get by integrating the flow across the basin, and the wind-driven part as what you get by integrating through depth. These are independent of each other, yet if I look at the upper-level Gulf Stream transport it will contains a bit of both - some of the mass flux is balanced by the deep western return flow, and some through the shallow eastern return flow. Each part contributes to the heat transport. If the deep return flow slows, then the mass transported by the Gulf Stream decreases, and the associated heat transport decreases. That this does effect European temperatures is seen in the model simulations above, as well as from evidence in the paleo-record. Does this imply that heat transports go to zero? No, because the wind driven part is still there (as is the part related to planetary waves), but it does mean that they may be noticeably less. - gavin]

  38. 38
    Michael Jankowski says:

    Re#27, Thanks Gavin (I’ll smack myself on the forehead and give myself a nice big “duh!”)

    Re#28, My perception is actually the opposite – each new iteration of modeling and analysis seems to result in a downward projection of both the magnitude and pace of global warming.

  39. 39
    Timothy says:

    Re #28 Stephen Prosser. Assuming all other things are equal then I would expect that a shutdown in the THC would lead to more heat transport by the atmospheric circulation as the temperature gradient would be greater. Consequently I would expect a more active storm track and more intense cyclones. Therefore the equilibrium temperature change would be somewhat less than that approximated by simply removing the heat trasported by the THC. Also, climate models generally show much greater warming at higher latitudes [due to the ice-albedo feedback I guess] so with a THC shutdown in a global warming scenario the impact would again be lessened. [The greater high latitude warming produces a weakening of the storm track in somee models global warming experiments].

    What I’m more interested in is: Where does all this extra heat go that is no longer being brought to the North Atlantic? If you look at the figs at the top you can see some red in the SH. Due to the THC the Atlantic has a net SH -> NH heat transport so a collapse of the THC would see more heat left in the South Atlantic. My main worry would be what impact this would have on:

    *1 – Antarctic ice melt. [Presumably increased -> ocean sea level rise]
    *2 – Tropical convection patterns. [Tropical convection being very sensitive to SST]

    Basically having London become more like Vancouver is not a major concern. It’ll cost in terms of having to make the infrastructure more resilient to colder winters, but it’s not what I would call ‘dangerous’ climate change in terms of what the UNFCCC commits us to avoid. Sea level rise that overwhelms the Thames Barrier and floods London, or changes to tropical convection which affect agriculture for millions of African subsitence farmers, however I would consider dangerous.

    Clearly these types of questions are second-order questions compared to the first-order question of whether or not the THC will collapse/weaken appreciably, and therefore harder to address, but they are more important from an impacts point of view.

  40. 40
    stephan harrison says:

    This has been a fascinating discussion. However, I haven’t yet seen anything on the contribution made to changes in salinity of the North Atlantic by the melting of ice-rich permafrost. About 50% of Canada and Russia are underlain by permafrost and this is, in places, more than 1km deep. Not only is melting likely to release significant quantities of GHG, there will also be an increase in the discharge of Arctic rivers. Has this been modelled?

  41. 41
    Timothy says:

    #40 – GCMs certainly deal with rivers, although how they deal with catchment basins vary.

    When I’ve heard people talking about increased river flow in Arcitc rivers it’s normally been on the basis of greater precipitation rather than due to permafrost melt. I’m not sure whether this is because it’s thought that the increase due to greater precipitation is greater than the increase due to melt or whether because there is more confidence in an increase in precipitation [and its magnitude] compared to that of melting permafrost.

  42. 42
    Bruce the Canuck says:

    Re #39: “Basically having London become more like Vancouver is not a major concern. It’ll cost in terms of having to make the infrastructure more resilient to colder winters…”

    How many times do Canadians have to state we don’t live in igloos! Vancouver’s summer and winter temperates are very similar to London’s, to within a degree or two. The main difference is we have sunnier dryer summers, and almost twice as much rain in the winter. Snow is rare.

  43. 43

    The increase of Arctic river discharge was 7% in the period 1936-1999, or 128 km3 on a total of app. 1800 km3/yr. But total precipitation (including rain/snow on the Arctic ocean) is estimated to have increased with 500 km3/yr.

    One should compare that with the amounts of water which melt and refreeze in the Arctic. In the last decades, the maximum summer melt is around 21,000 km3, but practically the same anount refreezes in winter (the Arctic winter ice volume has a smaller decline than summer ice). Thus even if the THC should shut down or reduce in summer due to an increased melt (of which I have not found any evidence), it should restart in winter, when ice forms and salt concentrates in the cold remaining water, giving it a higher density.

    Thus (the increase in) precipitation over the last decades is far lower than what melts and refreezes as Arctic floating ice. The amounts which slowed down the THC at the 8.2 kyr event (115,000-230,000 km3) to the contrary where higher than what could be refreezed in winter, thus preventing/reducing the densification of the polar waters.

    A rough calculation says that one need a 300-fold increase in precipitation in the Arctic to have a similar problem as with the 8.2 kyr event.

    [Response: There's two errors in this reasoning.
    (1) You compare a seasonal cycle (which over the year integrates out to zero) with a mean flux. To drive a mean deep water formation, you need a mean surface flux. The annual cycle of melting and refreezing does nothing, but a net sea ice export does (as it continually exports freshwater and thus provides a mean salinity enhancement). Of course convection occurs only during a few days in the year when the highest surface water densities can be reached, but the THC operates on longer time scales, it responds to the mean fluxes sustained over a number of years.
    (2) You compare a volume of freshwater in km3 (the catastrophic drainage of Lake Agassiz which triggered the 8k event) with a freshwater flux in km3/year (enhanced precipitation, river and meltwater runoff). - stefan]

  44. 44
    Gil says:

    Gosh .. DaVinci must be rolling over in his grave He lived at a time when the greatest minds were engaged in argument over how many angels could fit on the head of a needle. Leonardo never stopped observing .. and always asked why. Why as important to him as how. Though he may wonder if anything has actually changed, for the good of us all, I wish he were here now. ‘Some’ of what he would observe would include the following:

    Up to 80% of the krill are gone in southern oceans.
    Southern ocean temperatures are down.
    Northern ocean temperatures are up.
    Alaska (and much of northern Canada) has increased in temperature to the point that it is greatly affecting the permafrost.
    Some plants are blooming way too early, some way too late.
    Birds are disappearing in huge numbers in England .. and all over the world.
    There are droughts all over the planet.
    Seems that the West Antarctic Ice Sheet isn’t as stable as we thought just 3 years ago.
    Warm-water phytoplankton has moved north many hundreds of miles.
    Rice and grain harvests are down, planet wide.
    The Gulf Stream is slowing.
    By some estimates, up to 40% of the artic ice is gone.
    Ocean dead zones are happening earlier and are getting bigger .. much bigger.
    There are more polar stratospheric clouds observed now than ever before.
    Storms are forming earlier, are greater in number and greater in strength.
    It’s cold where it should be warm, and warm where it should be cold.
    8 of the last 10 years have been the warmest on record since we started keeping records!

    This is only a smattering of what he would see today.

    Surely you all know that one has a cascading effect on the other and that the whole is MUCH greater then the sum of its parts.

    I don’t want to sound alarmist. That is not my aim as I know some of you will immediately conclude. That being said, I would much rather be a bit freaked out and proven wrong then to walk blindly off a cliff to my doom. If ANYONE of you had a lump on your arm, you would go see your doctor right away. You would not wait till you had 20 lumps before you went to see him. The world .. our world .. over which we are stewards HAS MANY LUMPS!!! Stop arguing about who’s theory is right or wrong, about the need for more data and better forecasting models. The planet is telling you something .. and its not whispering. It’s yelling it out. Its time for all of us to stop thinking in absolutes and start ACTING on what we are witnessing!

    [Response: Dear Gil, as scientists we are the doctors who need to make the diagnosis - thus we have to discuss all those "lumps" in every detail. Like, is the Gulf Steam actually slowing? I'm not sure it is.
    Is what we already know enough to act and start reducing our emissions? There I agree with you - in my judgement it is. But that involves a value judgement. Our role as scientists is not to make this judgement for you; it is to investigate the facts and lay out what we know and what we don't know as clearly as we can, so that everyone can make their own judgement. And that's why we have to discuss all those details... - stefan]

  45. 45
    Timothy says:

    #42 – I’ve had a look around the place [Met Office website and Canadian Weather service website] and, although I’ve not been able to find anything that shows things very clearly, it looks as though winter mean and minimum temperatures are about 5 degrees lower in Vancouver than in London. That’s about the same difference as between London and the Scottish Highlands. As I said, I don’t think it is a major concern, but… it would have implications for transport infrastructure [for example], which grinds to halt in the South East pretty much whenever there is any snowfall. [Because at the moment snowfall is very rare so it isn't worth spending the money in the equipment to be able to deal with it...].

    I did find one page though that said that the climate in Vancouver was a good reason to move there, so I guess it can’t be that cold…;-).

  46. 46
    Gil says:

    Hi Stefan,

    I do understand this. Please know that I have the highest degree of respect for scientists as a whole and to the commitment, dedication and effort it takes to become one. Many of my close friends are scientists; some of them work for me. I get into this with them as well.

    My concern is that there is too much analysis. Analysis leads to paralysis. While I understand that you need to discuss the ‘lumps’ in details, I offer to you for your consideration that my doctor, while he would indeed seek out additional information, would nonetheless based on the observed phenomena (the lumps) attempt some remedial action. If he found that course of treatment to be ineffective, he would make the required changes. Stated otherwise, you can’t steer a parked car!

    I will grant that no one knows for sure if the Gulf Stream is slowing, but, it does appear to be. I know that it is impossible to predict exactly what the weather will do on a global level due to the enormous complexity of the equation and the huge numbers of variables. It is however possible to make some guesses. (I know this is hard to do in academia as one can be ostracized for this type of thinking. Carl Sagan did not receive much support form the scientific community at large when he suggested there must be extra solar planets. Turns out they are the rule, not the exception.) That being said, we can learn from what we see. Just before writing this, I note that it has snowed in Somalia for perhaps the first time in recorded history. I see that the droughts in Australia are getting worse as the rains have failed and that they are experiencing some of the highest temperatures on record. I live in Victoria BC and the temperature here hit 30 Celsius last Friday (way above normal) and was close to 40 inland.

    I appreciate that it is not the scientists that will make the changes we need. That falls into the hands of our governments (God help us) and indeed, into our own hands as individuals. I am VERY grateful that you do what you do Stefan. I just encourage you all to be a bit less concerned with absolute data and more concerned with the obvious changes that are occurring across this planet. I agree with Trevor when he says: it is better to be roughly right than precisely wrong! We need loud voices now. The time for sitting on the fence has past. Speak and the world WILL listen.

    Cheers,

    Gil

    [Response: Gil, I think we agree on one point: by debating every symptom and every piece of evidence in great detail (important as this is to science), we risk losing sight of the "big picture". We don't know for sure what the Gulf Stream is doing - but there are some basic facts about global warming that science is sure about. Scientists like to discuss the new and not yet universally agreed results, and the media like to focus on the controversies - repeating what we have known for over a decade is simply not "news". So perhaps it is important for us scientists to remind people every now and then of the basic uncontroversial facts, which are not disputed in the scientific community any more (only by a few industry lobbyists). (I've tried that e.g. in this article.) -stefan]

  47. 47
    grundt says:

    #45, Regarding temperature, it could seem there is not much to worry, if things to be taken in account are only the ones you point.
    But regarding ecology, crops, all the issues which imply adaptation, migration, etc., it could be (and is already being) disastrous.
    It takes a long time to adapt to all the changes.

    #44 Gil, all you include in your list is, as you say, only a fragment.
    All this reminds us we are no more than one more animal species on Earth.

  48. 48
    Hank Roberts says:

    http://news.xinhuanet.com/english/2002-05/20/content_401041.htm
    Yangtze floods, one month early.

    I know in medical statistics, if you have a whole lot of weak evidence, from a lot of different measures or theories, all suggesting there’s something going on not yet well understood, it’s more convincing that if you have only one piece of weak evidence that’s easy to dismiss as being that one-in-twenty accidental blip.

  49. 49
    Eli Rabett says:

    Did some googling. Two differences appear to be that Vancouver BC is wetter (1117 mm/yr) and sunnier (1900 hrs/yr) than London UK (750 mm/yr rain and 1613 hrs sun/yr)

  50. 50
    Michael Jankowski says:

    Re#48, as stated in the article, they’ve been anticipating the possibility of severe and early flooding for some time as the result of a recurrence of El Nino. They’ve been on flood alert since the beginning of the year. One could certainly argue that it’s worse (or that El Nino is made more frequent) due to GW.

    I invite you to take a look at this article from June 1st’s USA Today http://www.usatoday.com/printedition/news/20050602/a_tornado02.art.htm . Tornado numbers in 2005 are well below normal in the US. Would that be evidence of natural variability, a positive by-product of GW, or simply one of “more extreme variability” brought on by GW?

    It didn’t seem to get a lot of press, IMHO. However, if the opposite were true – if we in 2005 were experiencing a much higher than normal level of tornadoes – do you think you and/or a lot of other people would also consider this a sign of the negative impacts of GW? I’ve already seen a number of articles where people were worried that 2005 would have wacky summer weather and either a lot of hurricanes or very strong hurricanes (or both) due to GW. I can only imagine where the finger-pointing would be if tornadoes were well above average so far in 2005. But since they’re well-below average? Nothing but silence.

    If the Yangtze floods arrived a month late, would that also potentially be evidence of GW?

    For a slightly difference perspective, here’s an article about the flooding back in 1998 edition.cnn.com/WORLD/asiapcf/9808/25/china.floods.01/
    with a lot of the blame being put on land-use changes. The same goes for this one in 1999, which provides some more specific details about some of the changes in the 50′s and 60′s http://www.wsws.org/articles/1999/sep1999/chin-s09.shtml . This report on 1998 flooding does include the GW argument but mainly discusses land-use changes http://www.usembassy-china.org.cn/sandt/fldrpt.htm . Even solar variability is mentioned along with other “freak” weather events (some of which may or may not be linked to GW).


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