par Gavin Schmidt (traduit par Pierre Allemand)
Un grand nombre d’articles de presse et de commentaires sont sortis cette semaine concernant une présentation et un communiqué de presse de Tim Barnett, Scripps et collègues au congrès de l’AAAS (American Society for the Advancement of Science), (The Independent, John Fleck ,(et de nouveau ici) David Appell…etc. (NdT : sites en anglais). Pourquoi cela a-t-il donné lieu à autant d’attention, alors qu’aucune publication n’est encore disponible ?
Fondamentalement, c’est parce qu’il s’agit véritablement d’une bonne idée. L’idée sous-jacente est celle-ci : quels que soient les changements dans les forçages, il y a un certain délai dans la réponse climatique en raison de la grande inertie thermique des océans – Les réchauffer ou les refroidir prend beaucoup de temps. Pendant ce changement, il y aura un “déséquilibre radiatif” dans la couche supérieure de l’atmosphère. Dans le cas d’un accroissement des gaz à effet de serre (qui causent un réchauffement), cela implique que la planète absorbera plus de radiations solaires qu’elle n’en émet sous la forme de radiations à de grandes longueurs d’onde. Ce déséquilibre persistera jusqu’à ce que la planète retrouve son quasi-équilibre « normal ». (La planète n’est, évidemment, jamais en équilibre parfait, il y a de petites fluctuations dans les moyennes annuelles qui se produisent en l’absence de tout forçage, mais ces fluctuations sont petites comparées aux forçages anthropogéniques actuels, et au déséquilibre net correspondant).
Ce déséquilibre est vraiment une grandeur importante – les estimations du potentiel de réchauffement restant dans le ‘tuyau’, l’importance de l’effet de réchauffement par les aérosols, etc. dépendent tous de la connaissance de cette valeur. Néanmoins, il est très difficile d’effectuer des mesures depuis l’espace. Obtenir la valeur globale exacte de toute l’énergie rayonnée à des longueurs d’onde élevées en même temps qu’une estimation correctement calibrée de l’énergie solaire reçue sous forme de rayonnement et estimer la différence entre les deux avec une précision suffisante (fractions de W/m2) se situe au-delà de nos possibilités actuelles.
Bien, dans ces conditions, comment l’estime-t-on ? Toute cette énergie doit aller quelque part, et il est facile de montrer que ni la surface des terres émergées, ni les glaciers ne peuvent stocker entièrement cette énergie. En conséquence, celle-ci va réchauffer les océans, et en fait, l’analyse historique des températures des océans, par Levitus et collaborateurs en 2000, récemment mise à jour ( dans les Geophysical Research Letters) montre un réchauffement qui est en adéquation avec le déséquilibre radiatif suggéré par les modèles climatiques (environ 0,5 W/m2 de 1955 à 1995 et probablement jusqu’à 0,7 W/m2 au cours de la dernière décennie, Hansen et al (2002)).
Prendre en compte la chaleur de l’océan pour détecter les changements climatiques permet de limiter le bruit de fond présent dans les mesures de températures de surface, en raison des événements météorologiques qui affectent beaucoup plus les mesures atmosphériques que les couches océaniques, constamment mélangées. L’inconvénient est que les mesures océaniques ne permettent pas de remonter très loin dans le temps.
Un travail antérieur de l’équipe de Barnett a démontré que les modèles couplés forcés avec des gaz à effet de serre montrent un réchauffement océanique similaire à celui donné par les mesures instrumentales. Mais certaines questions restaient sans réponse, en particulier le degré de variation décennale, la durée des observations, et l’importance relative, dans les modèles du forçage des aérosols sur la sensibilité climatique (qui ne peuvent pas être réellement dissociés l’un de l’autre en utilisant ces mesures). (un rapport récent des National Academies présente cela plus en détail). La modélisation de la série la plus récente de résultats ayant été effectuée, et les chiffres mis en ligne pour le 4ème Rapport d’Evaluation du GIEC, il est maintenant temps de revoir la question avec des modèles et des observations remis à jour.
Quand cela sera fait, est-ce que cela fournira vraiment la ‘preuve finale’ du réchauffement climatique d’origine humaine ? Comme je l’ai indiqué plus haut, les travaux préliminaires de Levitus, Barnett, Hansen et al. ont déjà démontré que c’était une bonne approche. Par conséquent, il s’agit plus de peaufiner les détails, que de ‘prouver’ ex-abrupto le réchauffement global. Si la nouvelle série de modèles concorde mieux avec les données que précédemment, (comme communiqué à l’AAAS), et si le résultat apparaît suffisamment probant vis-à-vis de certaines incertitudes encore présentes (forçage par les aérosols, composantes du modèle océanique, etc…), alors, celui-ci contribuera à renforcer le ‘faisceau d’évidences’ montrant que le réchauffement global d’origine humaine est déjà effectif.
One week previous to the Barnett press release Scripps issued a press release describing research by Koppers and Staudigel on undersea volcanic activity.
It is my understanding that many oceanographers believe that volcanic activity, including phenomena such as deep-sea thermal vents, can affect ocean currents. Additionally, if a single above-sea volcanic eruption can effect global temperatures, is it not reasonable to assume that the thermal energy released below the sea does so also? How much is known about such effects, if any? Is Barnett’s seeming certainty perhaps a bit premature?
Thanks.
For those of us without direct access to the AGU’s publications, a free-access version of the 2005 Geophysical review Letters paper by Levitus et al. is available here… (Link via the publications page of the NODC Ocean Climate Laboratory, which is full of links to interesting stuff.)
While on the topic of the oceans’ response to warming, I would very much like to see a RealClimate posting on the effects on sea levels of GW. If I understand correctly, thermal expansion is an important factor, not just the melting of ice caps. (Just to remind Stefan of his response to my previous request a while back ;-)…)
Regarding global warming shouldnt science be “looking for it” anywhere they can find it?
Great! I mentioned talking about the warming of the oceans and Levitus, et. al. in comment #19 in this post and now you’re covering it. I was unaware of this new information, which is quite welcome.
A correction: your link for “Levitus and colleagues in 2000” actually points to the abstract for another paper by Levitus et. al. Anthropogenic Warming of Earth’s Climate System in Science 2001 292: 267-270. You can find this paper here. The 2000 paper Warming of the World Ocean can be found here Science 2000 287: 2225-2229.
Good topic! Thanks.
[Response: Thanks. I’ve corrected the links – gavin]
I am new to all this so bear with me. Surely with all the attention to global warming by so many people, there does not need to be any more proof that global warming is taking place. I agree that we should be watching what the planet is doing and the need for research to be ongoing, but surely we should be acting on what we now have as evidence on global warming. The fact is that the oceans are getting warmer. This could take some time to work out by how much and by what causes. Time is something you should not play with due to the fact before you know it, it’s to late to doing anything about it.
Gary is indeed new to all this if he hasn’t spent time at the several IndyFunded sites that disseminate cherry-picked information stating the opposite of what you say. Welcome, Gary. Welcome to RC.
Best,
D
It really doesn’t seem reasonable to attribute ocean imbalances selectively to a particular prominent forcing (GHG’s) when “comprehensive ocean measurements do not go back very far”.
Given the short length of the trustworthy oceanic observational record, particularly in the deep ocean, the ocean response to multi-decadal and longer natural forcings is virtually unknown. And any potential longer-time-scale internal oceanic oscillations are virtually unobserved. Are ocean models so robustly based on first principles that they can be trusted without validation against sound observations over the time scales of interest? Are the observations of initial and boundary forcings of ocean models adequate to provide good simulations? Given that the answer to this for atmospheric models is a resounding “NO” (particularly because of sub-grid scale processes which need to be effectively pre-ordained through parameterizations), and given that oceanic circulations have much longer adjustment time scales, yet also have much more intense small scale (gyre) circulations than the atmosphere, my instinct is that we are not even close to being able to trust ocean models without long term validation data. There is so little understanding about how the ocean parses its response to forcings by 1) suppressing (local convective scale) deep water formation where excessive warming patterns are changed, 2) enhancing (local convective scale) deep water formation where the changed excessive warming patterns are co-located with increased evaporation and increased salinity, and 3) shifting favored deep water formation locations as a result of a) shifted patterns of enhanced warming, b) shifted patterns of enhanced salinity and c) shifted patterns of circulation which transport these enhanced ocean features to critically altered destinations.
Why are any changes in (imbalances of) deep water formation so critical? Because deep water formation is a clear positive feedback mechanism. Most deep water formation (in today’s climate) occurs where fairly saline water is chilled to near freezing. Thus cold water is selectively sequestered into a slow, deep millenial scale circulation, leaving more surface area to be dominated by warm, stable surface water. And in turn this warm surface water is left in greater control of the shorter time-scale climate which we have been able to observe during the instrumental period.
Before jumping to conclusions, there needs to be much better justification of the basic assumption that we just happen to be sitting at a time when GHG forcing is larger than all other potential long term (longer than the observational record) oceanic imbalances.
[Response: One thing to remember is that our expectation (admittedly based on models) is that the planaetary radiation imbalance has only been greater than about 0.5 W/m2 on a sustatined basis since the late 70s (i.e. Fig 14c in Hansen et al (2004)) though with a notable dip due to Pinatubo in the early 1990s. Thus, the ocean records do go back far enough to determine the magnitude of this. Our estimates of the changes in forcings over that period are reasonable, and allow us to say pretty much unequivocally that the biggest change over that period has been due to GHGs. It is therefore not jumping to conclusions to make an attribution.
With respect to your comments about ocean models, you are completely correct. They haven’t reached the point of development when we are sure that they are doing everything right for the right reasons, and so my instinct is to be very cautious in interpreting ocean model results that rely on shifts in the location of deep water production etc. However, the physics of the change of heat storage is much more related to how heat anomalies diffuse through the thermocline in the bulk of the oceans (not at the poles). You get very similar results using slab oceans (no changes in transports etc.), and similar results using very different conceptual models – in this sense it is a robust result. We aren’t talking about deep water here – most of the action is in the top 700m i.e. below the mixed layer, but not in the really slow deep circulations. Features related to the decadal variability, particularly in the Pacific, are probably tied to the interactions of ENSO variability and the sub-tropical gyres, and are clearly more difficult to model. They therefore remain a challenge to explain.- gavin ]
Re: Barnett’s paper, decadal variability?
Here’s the full Barnett et. al. paper Detection of Anthropogenic Climate Change in the World’s Oceans.
I am interested in the statement
The length of the record is about 45 years. Decadal variability? In the world ocean heat content in the surface/mid layers? Here’s a quote (from Fleck quoting Warming of the world ocean, 1955-2003 – abstract linked in as “recently in GRL” in the post).
Aside from model uncertainty questions, which are understandable, what natural process could possibly cause significant temperature variability in the ocean system, which has such a huge inertia, on timescales as small as a decade? What am I missing here? It seems quite incredible.
[Response: As mentioned above, this appears to be related to variability in the central Pacific, and is indeed larger than people may have initially anticipated. My guess is that heat content changes related to ENSO events and their interaction with decadal variability in the gyres are probably responsible, but I’m basically just handwaving here. If I find a paper that addresses it more clearly, I’ll let you know. – gavin]
Gavin, thank you very much for this excellent post. RealClimate is managed by serious scientists.
Gavin: “…The disadvantage is that comprehensive ocean measurements do not go back very far. …” “….But questions remained concerning the degree of decadal variability, the length of the record and the balance in the models between aerosol forcing and climate sensitivity (which can’t really be disentangled using this measure). …..”
“…..When this is done, will it really provide the ‘final proof’ of man-made global warming? As I indicated above, the preliminary work by Levitus, Barnett, Hansen and others has already demonstrated that this is a good approach. Thus it is more a question of refining the details, rather than suddenly ‘proving’ global warming….”
Wetzel: “… the ocean response to multi-decadal and longer natural forcings is virtually unknown. …” “…my instinct is that we are not even close to being able to trust ocean models without long term validation data…”
Dave: “…what natural process could possibly cause significant temperature variability in the ocean system, which has such a huge inertia, on timescales as small as a decade? What am I missing here? It seems quite incredible…”
What are we missing?… It is scaring
‘Surely with all the attention to global warming by so many people, there does not need to be any more proof that global warming is taking place.’
–Gary
I wish this were true, but I live in the US and know that it is not. I’ve met many people who think global warming is junk science and even more that just don’t seem to care at all.
Its quite amazing, actually. Virtually every developed or developing country takes it seriously, yet we seem to have our hands over our ears shouting ‘No, its too costly to worry about it! It probably doesn’t exist anyways. La la la la… I can’t hear you.’
Ok, I exaggerate somewhat. But that’s the prevailing attitude throughout much of the country and in most areas of government. I hope its changing, but I just don’t know.
Is there a good primer or recent reference on modeling ocean response to changes in surface insolation?
In particular it seems important to look at changes in deep water or intermediate water formation (rates and temperature) — for example: will anthropogenic warming increase the rate of Mediterranean Intermediate Water formation or change the temperature of the resulting water mass? Will it change the rate of saline outflow from the Red Sea? It seems like a lot of these processes won’t reach equilibrium for thousands of years.
There is at least one other robust indicator of planetary warming that has not been discussed in this posting – warming of the continents. For those interested in other indicies of continental heat gain the paper by Beltrami and others http://envscilab.stfx.ca/2001GL014310final.pdf would be a useful resource. In comparison with heat gain by the oceans, these other terms are substantially smaller, but they illustrate how widespread the observed warming is.
Without dipping too much into the politics, the White House officially acknowledges global warming and anthropogenic sources. The “I can’t hear you” response is from the popular media.
The George Bush record on climate change policy:
September 2000 George Bush made a campaign promise to require power plants to reduce CO2 emissions. March 2001 after election the President George Bush in a letter to republican senators stated that no reduction of CO2 would be required. When asked about the change, the White House replied that the President never intended to regulate CO2 emissions and the campaign promise was put in a speech by a speech writer without the presidents knowledge.
As widely reported in the media in June 2003, the White House suppressed references to climate change in an EPA report. Dropped from the report were references to a 2001 National Research Council report’s conclusions that greenhouse gases were the only likely explanation for the global warming. The President had commissioned and praised the report earlier. Also omitted was a 1999 study documenting an increase in global temperature, but the EPA report included a study partially paid for by the American Petroleum Institute that refuted global temperature increase.
In August 2004 the administration sent a letter to congress stating that human-released greenhouse gases were the only likely explanation for the global warming that occurred over the past thirty years. When questioned about the why he changed the climate change position George Bush replied, “Ah we did? I don’t think so.”
The conservative Heritage Foundation asked “why the about face?”
The change in policy is only cosmetic. This is again another rhetoric technique. You make a small change to change your public image, but the change is only a surface change. You make yourself look good by seemingly embracing a position or policy that you disagree with but it is only a token effort, and you really don’t accept the position or policy. The goal is to calm down or stop your opponents by making them think they have won, without actually giving in to them.
The President has a negative public image in environmental policy. By apparently accepting climate change science he is trying to improve his image as an environmental president. But there is no substantive change in the climate change policy. The environmentalist community uses the word “greenwashing” to describe such a tactic.
Look at the current climate change materials on the EPA website. The main theme is that there is climate change but there is so much uncertainty about the science. Conservative think tanks take a very similar position.
For example, on the EPA site:
“Figuring out to WHAT EXTENT the human-induced accumulation of greenhouse gases since pre-industrial times is responsible for the global warming trend IS NOT EASY. This is Because OF OTHER FACTORS, both NATURAL and human, affect our planet’s temperature. SCIENTIFIC UNDERSTANDING of these OTHER FACTORS â�� MOST NOTABLY NATURAL climatic variations, changes in the sun’s energy, and the cooling effects of pollutant aerosols â�� REMAINS INCOMPLETE.”
The language used emphasizes uncertainty.
The EPA continues:
“Nevertheless, the Intergovernmental Panel on Climate Change (IPCC) stated there was a “discernible” human influence on climate; and that the observed warming trend is “unlikely to be entirely natural in origin.” In the most recent Third Assessment Report (2001), IPCC wrote “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities”
This section is trying to cast doubt on the IPCC report, one of the most comprehensive climate change studies. The term “nevertheless” is used to imply that the IPCC conclusions are going beyond or contrary to what is scientifically known, and this section uses quotation marks around “discernible”,
“unlikely to be entirely natural in origin” and “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities” to make key IPCC conclusions seem questionable.
The current White House climate change policy is continued research, but not any action. Regardless of what the White House proclaims in public sound bites, it still questions the basic climate science.
Re # 12: It is amazing, for me, to learn about the overwhelmingly high heat gain of oceans compared to the other components.
This paper helps very much to understand how important is to think well before jumping with any conclusion
Thank you!
Re: Warming in the Pipeline
From Hansen et. al. (2002)
I have always been a bit baffled by the physical processes involved here. This is an important subject which is not widely and clearly understood by non-scientists like me. Yet it is a crucial finding and I believe that many people, including environmentalists and policy advocates, do not understand its implications.
Is there some simple intuitive explanation of how this pipeline warming is estimated with respect to an equilibrium climate sensitivity at a doubling of CO2 equivalent (thus including methane, ozone, aerosols, CFCs…)? How and at what point do the oceans come into heat balance? Don’t the oceans have to absorb heat not only in the surface/mid layers but right down to the bottom for radiative heat balance to occur? And isn’t that a process that takes hundreds to thousands of years?
If I’ve asked a poorly formulated question and missed some basic assumptions, that’s OK. Still, the gist of what I’m trying to find out remains. And I think others could benefit from understanding this important result.
[Response: A big part of the response to any radiative forcing is what happens to sea surface temperatures (since they control most of the feedbacks that are important in the equilibirium response). The presence of the radiaitve imbalance at the top of the atmosphere is an indication that the sea surface has not warmed up sufficiently to match the forcing. This is because (as you note) deeper layers of the ocean are also heating up. However, this is a slow process, and the sea surface will come close to equilibrium with the forcing in around 30 to 50 years. It is because the deep ocean adjusts so slowly, that the long term remaining imbalance will be small and barely noticeable in the surface temperature data. Given an actual imbalance, you can estimate how much further the sea surface will need to warm to remove it – this is the warming ‘in the pipeline’. The number comes from using the climate sensitivity (say 3+/-1 C) so if the imbalance is 0.7 W/m2, you might expect ~0.5 C of additional warming eventually. Most of that however will come within a few decades. – gavin]
I have a question for the experts at RealClimate: From Levitus et al. 2005 GRL (cited in the initial posting above) Figure 1 there appears to be a loss of about 4 X 10^22 J in ocean heat content from about 1979 to 1987. Where did this vast amount of heat go? Looking at Levitus et al. 2005 Figure 3, no other heat sink can account for such a large flux. Did it go back out into space through the top of the atmosphere? If a significant fraction of this heat lost from the ocean went into the atmosphere one might have expected the surface air temperature to have increased faster during this period than during the subsequent period of the 1990s when the ocean heat content gained >5 X 10^22 J, but this is not what was observed (see reference Figure 2.7c in the IPCC TAR Working group I).
In addition to #17, there is a discrepancy between the modelled inflow of extra heat into the oceans in the 1955-2000+ period and the TOA balance, which shows a large loss of heat in the 1985-2000 period.
Based on ocean data and models, the extra inflow of heat was calculated to be between 0.2 W/m2 (Levitus) via 0.5 to 0.7 W/m2 (Hansen for the last decade).
But at the top of the atmosphere, there is an increasing loss of energy in the tropics of 3 W/m2 since 1985, according to Chen e.a. (see http://pubs.giss.nasa.gov/docs/2002/2002_ChenCarlsonD.pdf )
As that is in the 30N-30S band, that represents 50% of the earth’s surface, of which a large part is ocean…
The loss of heat was due to increased Hadley cell circulation, resulting in decreased cloud cover. The increase in insolation of 2 W/m2 from the cloud cover decrease was accompanied by an extra outgoing 5 W/m2 heat flow…
How does that fit in the ocean models?
[Response: And yet it warms! Two things can be going on here, firstly, the loss in the tropics may be more than compensated for in the rest of the world (which is consistent with the extensive Southern Ocean warming observed by Gille (2002) ), and secondly, the data from the tropics may be less complete or accurate than claimed, although I am not aware of any specific reasons why that might be. I would therefore go with my first answer. – gavin]
Re: Latest Levitus et.al.
Finally, a link for the full Levitus et. al. (2005) paper Warming of the world ocean, 1955 – 2003, which I had not been able to read at AGU (agu.org).
A noteworthy quote:
Also, if you look at Table T2 in this paper, you will see that ocean sea surface heat storage 0-700m from 1955-2003 (in W/m2) is always higher at northern latitudes than the corresponding southern latitudes in every case, even with the extensive Southern Ocean warming as noted by Gavin responding to #18.
Here’s a little something I wrote up on a train from Paris to London on a vacation (I have a B.Sc. in physical geography, so I am not an expert to the extent of Drs. Mann, Bradley, etc.):
Skeptics often cite volcanism as a cause for climate change and Greenhouse Gas (GHG) increases. It is a cause or a trigger for “catastrophic” changes over a short term period. However, it leads primarily to the onset of ice ages or cool periods rather than a warmer period (due to volcanic ash that blocks out the sun). Since volcanism in long-term periods (i.e. decadal and century timespans) is relatively constant, one would expect GHG fluctuations to be minimal. In other words, fluctuations in global GHG concentrations are not dependent (generally) on volcanic events.
Another aspect of climate change which is often cited (and is dealt with in this portion of RealClimate) is the temperature of the oceans. As the atmosphere warms, the ocean must as well, since there is a coupling effect between the two ecosystems.
Skeptics say that submarine volcanism explains the current warming of the oceans. However, this is completely untrue, since over decadal or century-long periods, such volcanism (caused by sea floor spreading and undersea earthquakes) remains fairly constant, since the rate of sea floor spreading (or tectonic activity) remains fairly constant. This, in an unchanged situation (i.e. with no other variables present, such as atmospheric temperature fluctuation) would result in a relatively constant mean ocean temperature.
Now, since there are variables present (GHG concentration, atmospheric temperature changes, etc.), the ocean temperature cannot remain constant if it seeks to find equilibrium (like all things in nature). Therefore, this argument by the skeptics is inaccurate and leads to the confusion of the general public (due to the disinformation done in the media today).
While the distribution of heat over the oceans is a complicated matter, due to radiative in/output (modulated by cloud cover), ocean and air currents, some more discrepancies arise.
Greenhouse gases are rather well mixed over both hemispheres, but Levitus 2005 points to differences in aerosols between the NH and the SH. The influence of aerosols is mostly in the NH, were a significant lower warming (than over the SH) should be visible. But that is not the case, it is the opposite which happens. That is even more striking for the Indian Ocean, where the northern part is heavily loaded with aerosols, causing 13 +/- 2 W/m2 less insolation, compared to the southern part. But both parts are warming at near equal speed.
[Response: You are confusing heat content gain with surface air temperature rise. They are in fact opposing phenomena. SAT in zones of deep ocean mixed layers is expected to warm more slowly than average, precisely because the energy is warming the deeper ocean layers instead of the surface. While aerosols do play a part in hemispheric differences, the biggest difference in the SAT trends is because of the greater expanse of ocean in the SH. ]
Moreover, if we compare the 9-13 year sun cycle with the global sea surface temperatures (SST), the variation is app. 0.2-0.3 K within a cycle (see the bottom graph at NASA). That a variation of only 0.1% (app. 1.3 W/m2 TOA) in direct sunlight may give such a large variation at the ocean’s surface is probably a question of modulation of cloud cover. How that translates into real W/m2 variation at the sea surface is something the physicist may calculate… See further the discussion about sun cycle – cloud cover within the comments at https://www.realclimate.org/index.php?p=42
[Response: A student activity worksheet? I have no idea where those numbers come from or what they are based upon. The best evidence (so far) is that there is about a 0.1K cycle in surface tempertures associated with sunspot cycles, and that is only detectable once all volcanic and ENSO-related activity is substracted out (White et al, 1997).]
While there is no recent increase in solar activity, the average activity still is (much) higher now than in the beginning of the 20th century. According to different solar reconstructions, the average increase in solar activity since the Maunder minimum may have been three times the variation within one cycle…
[Response: … or as little as one times the solar max/min variation. It is the quantification of ‘much higher’ that is at issue here. Estimates of the magnitude long term solar changes remain mostly speculative (Foukal et al, 2004). ]
That solar activity is largely underestimated was a topic at the SORCE meeting last October:
“Widespread empirical evidence from the extensive Earth climate datasets suggests the presence of an 11-year solar signal of order 0.1K in surface, atmospheric, and ocean temperatures. But general circulation models (GCMs) underestimate this response by as much as a factor of five.”
[Response: Find a proper reference for the quotation about the GCM results – as far as I can tell none of the presentations at that meeting made that claim. ]
Thus that greenhouse gases and aerosols are the only/dominant factors in recent ocean temperature increases seems to be a little premature…
[Response: No-one is claiming that GHG + aerosols are the only mechanism, the claim is that they are the dominant forcings. Well mixed GHG are exerting a forcing of around 2.4W/m2 more than during the pre-industrial. At the largest supportable value for long term solar (0.3%), it has only contributed 0.6W/m2 since the Maunder Minimum. It would need to be 8 times that to even equal GHGs. Plus, over the last 25 years, there has been no siginficant change in solar. The ocean heat content analysis by Barnett (and in other groups) show that the changes are most consistent with the GHGs becoming increasingly dominant over this time. – gavin]
Sorry, I was comparing heat content (not SST, neither SAT) of different parts of the oceans down to 300 m depth (where most of the variation is visible), based on the data of Levitus e.a. which can be downloaded from the NOAA web site. If you plot the NH data against the SH data, the heat content of both is varying more or less simultaneous (with larger amplitude in the SH) and the trend is near equal. That means that the NH parts of all oceans, which have a smaller volume, are heating faster than the SH parts. As the area/volume ratio for the NH parts of the oceans is practically the same as for the SH, the surface heating (W/m2) must be larger in the NH parts, within the constraints of heat exchange via ocean and air currents (and partly by the difference in warming area in the tropics vs. the cooling areas in the higher latitudes)…
While the general heat increase due to GHGs is app. 2.4 W/m2 since the industrial revolution, the global reduction (direct and indirect) by sulfate aerosols is estimated to be over 1 W/m2 (see Hansen. As most of this is over the NH (be it mostly over land), this should have a discernable influence on the difference of heat inflow between NH and SH parts of the oceans, the other way out than observed…
I suppose that the NASA doesn’t give students any wrong data. But while the graph was only for SST (sea surface temperature, something different of SAT – surface air temperature, even at sea), the influence of the solar cycle and volcanic episodes (El Chicon and Pinatubo) is visible globally in the oceans until a depth of 300 m in the Levitus data. The global amplitude (down to 300 m) is between 1E+22 and 3E+22 J, compared to ~4E+22 J for the increase in heat content in the period 1955-2003 [note: there seems to be a discrepancy in units between the story of Levitus and the data]. The ENSO and other recurrent phenomenon shouldn’t have much influence on the ocean’s global heat balance. The variation in heat of average 2E+22 J in only one decade for the upper 300 m needs a lot of heat inflow variation at the surface…
About solar, there is certainly an underestimate, depending on the model, of at least a factor 2. See the “optimal coefficient” experiments of Physicsweb
While the exact mechanism for the influence of the solar cycle on cloud cover still is not known, the influence nevertheless is observed, see Kristjánsson. A change of 2% in cloud cover over a sun cycle, good for a change of 3 W/m2 in the tropics (see Chen e.a. and cooler (0.5 K) and dryer climate in Boston to give one known example…
Thus the “only 0.6 W/m2” in insolation since the Maunder Minimum, in reality may have been fortified to a difference of several W/m2… While there is no change in solar strength in the past 25 years, the level still is high and the oceans still may not be in equilibrium with the heat inflow…
And models don’t reflect SST/outgoing IR/cloud cover variations over the last decades, even not over the 60N-60S latitude band (leaving not much area to be checked…). See Allan & Slingo 2002
Without imputing motives to someone I have observed to be earnest, post 22 is practically the optimal throw the spaghetti against the wall missive. It is almost impossible to untangle, has many unidentified assumptions, some of which are at best questionable but stated as accepted, and finds the trees but ignores the forest. Answering it completely would require a long deconstruction during which the back and forth would include repeated, but you didn’t answer point 54 (well yes we did, back in our 36th post).
One can only start, perhaps with the first paragraph. My question would be what is the characteristic mixings time for surface waters between, say the south and north Atlantic/Pacific.
Posts #22 and #23 seem to me to sum up a lot of the honest scientific contrarianism responses and the better replys.
Profuse obscure detail, that takes ages to for those who fully understand the details to work through (and longer, if at all, for those like me who don’t), is presented and, somehow ‘O.J. Simpson is not guilty’ because we all get lost in debate about it and can’t see the wood for the trees?
Nope, climate isn’t a technicality? If current ideas are wrong it will be obvious? You don’t need to be Einstein to know e=mc2, broadly, works, nor do you need to be Isacc Newton to know the accelleration due to the gravity of Earth is, broadly, 9.81m/s/2 (OK, in a vacuum).
No, only if the broad measures change will my thinking (mid prediction warming) change. Getting lost in detail ,x, y, z, let alone ‘he said’ ‘you said’ ‘they say’ ‘you’re dishonest’ ‘no you’re dishonest’ ‘stop ad homming’ ‘no you stop ad homming’ are just distractions (which, sadly, I’ve fallen for several times).
So, contrarians, where are the obvious problems with the consensus view? The huge errors, the huge faults?
[Response: We broadly endorse this view, and will not allow the kinds of discussion here that often swamp unmoderated message boards. Please stick to the issues raised in particular posts, and try and pick one issue at a time – only confusion is served by bringing in twenty unrelated issues whose only link is that they use the word ‘radiation’. – gavin]
I realize that I have combined too many points in one comment (by my search to find the answers). But if someone can give an answer to the only one question which matters: why the relative large variability over the global oceans? That such a large amount of heat can escape or can be added in such short time intervals, are either increase or decrease of ice volume or heat input from space / escape to space.
For the upper 300 m, where the variation is most visible, there are peaks in heat content of the global oceans around 1962 (but the data before 1961 are highly variable), 1979, 1990 (small) and 2002.
While GHGs/aerosols may be the dominant factor in the average increase, they are emitted in rather continuous increasing amounts for GHGs and increasing + constant (after 1975) amounts for sulfate aerosols.
In comparison: the sunspot maxima where in 1957 (large), 1969 (small), 1980, 1989/1991 (double peak) and 2000/2002 (double peak).
The large volcanic eruptions in the past half century (reaching the stratosphere) were Agung (1963), El Chichon (1982) and the Pinatubo (1992). Smaller one’s were Fernandina (1968 ) and Fuego (1974). See Hofmann e.a.
A fingerprinting study of the ocean data, compared to GHG/aerosols, ice volumes, solar variance and volcanic influences may give some more insight…
In response to #23,
There seems to be little exchange in surface waters between the NH and SH (as good as is the case for air currents), see the main sea currents. The world wide deep water exchange (the “conveyor belt”) has an overturn of app. 1500 years…
I’ve been intrigued to see how the issue of decadal variability has played out in regards to our 2001 paper (Barnett, Pierce, and Schnur 2001, Science v. 292, p. 270; ). Some people looked at parts of that work (for example, the lower right panel of Figure 1) and point out how the climate model oceans show a smooth and pretty much unbroken increase in heat content over the historical period. As we all know, the real world doesn’t show a smooth and unbroken increase in heat, so the model is obviously crazy, right?
Um, no. What we do (by “we” I mean the people who ran the model — Warren Washington’s group at NCAR) is run the climate model repeatedly; in this case, it was run five times. Each repetition has somewhat different starting conditions, so the details of exactly how it evolves is different in each of those five “ensemble members”. The differences between these ensemble members represent the unpredictable parts of daily weather variability that we can’t forecast more than a couple weeks in advance. But when you average all the ensemble members together, you average out the unpredictable short timescale variability, and are left with the slow, predictable part that is consistent across *all* the ensemble members. In this case, it comes from human factors — greenhouse gases and sulfate aerosols released into the atmosphere by cars, smokestacks, and so on.
So my point is that yes, the actual climate model runs we used *do* show a lot of decadal and higher frequency decadal variability. But, we went to some effort to remove the unpredictable part of that variability. The remainder is the predictable part, which is the smooth, unbroken increase in temperature over the years. Since it takes some effort to get at this predictable part of the signal, and yet that’s been one of the notably misunderstood parts of our work, it does give one the feeling that in the science of climate change, no good deed goes unpunished. In any event, we will show the unpredictable part in the upcoming work (i.e., the paper that is the origin of the press conference that this topic is about), so people can see it and directly compare for themselves.
As a side note, this is related to a common misperception I frequently see in the “letters to the editor” column of our local paper. Every time there is a snowstorm somewhere, somebody writes in asserting that this disproves predictions of a warming climate. Actually, the future evolution of temperature, both regionally and globally, will have the complex ups and downs on yearly to decadal time periods that it has always had. But, taken over many decades (say, 30-60 years) the overall trend will, on average, be towards generally increasing temperatures.
In response to #27, some questions remain:
The increase of ocean heat content in Barnett, Pierce and Schnur 2001 is only compared to GHGs and aerosols, all other human and natural influences excluded. But there are offsets between GHGs/aerosol combinations and solar activity (especially as derived by Hoyt and Schatten), which may have been underestimated (see Stott e.a. 2003)
If one simply should compare only the influence of solar (by H&S or even LBB) with the increase in heat content of the oceans, one can get a similar conclusion: that solar is the main driving force in ocean heat content. Which also explains much of the observed large variations. One only needs to increase the direct insolation variance with a factor 3-5 (e.g. by observed variations in cloud cover, see also my comment #18)…
Secondly, and even more important, the increase in observed heat content (based on NOAA data) is near equal for the NH and the SH (see: World oceans), while the area’s/volumes are different. If one corrects for area (NH 43%, SH 57%), the upper 700 m NH parts of all oceans are heating over 40% faster than the SH parts (see World oceans corrected).
This is completely opposite of what can be expected by models, as the cooling influence of aerosols must be much higher in the NH than in the SH. And the melting of Arctic ice in recent decades (less in Antarctica) adds to this discrepancy…
[Response: Instead of expecting what the models should do, one needs to actually look at the models – which is what Barnett et al are doing. Remember too that reflecting aerosols are not the only additional forcing – tropospheric ozone and black carbon are both more prevelant in the NH, and are both warming factors. I would also question the point that one ‘only’ needs to increase solar by a factor of 3-5 to get it to match. This is huge, and there is no good evidence that it is that strong (for instance there should be a stronger response to the 11 year cycle than observed). The fact that the known physics of greenhouse gases (which are only uncertain to about 10%) do explain the trends, is then much more likely than an additional (mysterious) solar effect, the evidence for which is much more wishful thinking than actual facts. – gavin]
Gavin,
I liked to have a response of the Barnett team, but here are some remarks on your response:
I haven’t seen the new work of Barnett e.a., as it is not published (yet). But I am surprised by your reaction. The (GCM) models don’t reflect reality anywhere if the direct and indirect cooling by (mostly) sulfate aerosols are not taken into account (see the IPCC graph. That is the largest human induced cooling factor, some 1 W/m2 against the 2.4 W/m2 warming by GHGs since the start of the industrial revolution.
But most of the aerosols are emitted in the NH (see: IPCC graphs d)-h)). Thus the radiation heat balance, according to the IPCC, in the NH must be far less positive than in the SH (as example: a loss of 5 W/m2 TOA due to aerosols in the NH Indian Ocean). If the models don’t reflect such differences in radiation balance between the hemispheres, then there is something wrong with the models…
But globally, the oceans are warming (much) faster in the NH than in the SH…
The solar cycle is visible in the upper 150 m of the oceans, with a variation in SST of ~0.1 K, from a TOA solar change of 0.5-1.5 W/m2. See White e.a.. From the same source:
“Rind e.a. [1999]… …identified internal positive feedbacks that nearly doubled the [note: modeled!] global average temperature response to solar forcing expected from the Stefan Boltzmann radiation law”.
That was due to increased global moisture content, decreased global average cloud cover and decreased sea ice extent at high latitudes. Not (yet) a factor 3-5, but not a mystery either.
The physics of GHG heating are known within rather strict limits, but the response of climate on these physics, like evaporation, convection, the lapse rate discrepancy in the tropics, and especially cloud cover are far from settled…
[Response: Why do you think aerosols are not included in the GCMs? They are in all the simulations currently being analysed for the IPCC AR4, and one presumes that these are the runs Barnett is working with. As I said before, other warming factors are also more prevelant in the NH (specifically tropospheric O3 and black carbon – including its effect on snow albedo). Our own (as yet unpublished – so you don’t need to believe me) simulations also show NH oceans warming at a slightly faster rate than the SH including all of the forcings we have discussed.
With respect to solar you are confusing a forcing with the feedbacks. Feedbacks as discussed by Rind are applicable to all forcings, not just solar, and the idea that climate sensitivity is greater than S-B would suggest is well known. – gavin]
Gavin,
The GCM’s I know of (and as described by the IPCC 2001) do include a large cooling effect from aerosols. See Fig 2D in Crowley 2000
As most aerosols, especially sulfate aerosols, are emitted in the NH, the cooling effect should be largest there. Which is contradicted by the observations.
Now you say that (current?) GCM’s show a larger warming effect in the NH. As GHGs are well mixed over the whole globe, that means that there is a net warming effect from aerosols?
[Response: Crowley (2000) was not a GCM. Current GCMs (i.e. experiments that have been done for the upcoming IPCC AR4) include many different forcings (up to 14 different things in our latest runs – see the link I gave previously). In particular, they include tropopsheric O3, black carbon (a warming aerosol) and the indirect effect of black carbon on snow albedo (again a warming effect). ]
Please explain the (difference between the) aerosol amounts/composition/effects (especially for ocean heat content) over the NH and SH in the 2001 GCMs vs. current GCMs for the calculations of the 1945-1975 period and the 1975-200x period.
[Response: Too much detail, you’ll need to wait for the relevant publications to come out (sorry!). ]
That all forcings have similar feedbacks can’t be right. Solar has its largest influence in the lower stratosphere (including changes in jet stream position), CO2 in the lower troposphere. See KNMI: “Solar forcing of climate change: evidence from the past”. Although the cosmic rays-cloud connection weakened in the past decade, the solar luminosity-cloud connection still holds.
Short waves (visible light) are trapped in the upper 100 m of the oceans, leading to warming of the whole layer. Long waves (infrared) light from the sun, GHGs, clouds, are trapped at the surface of the oceans, directly leading to increased “skin” temperature, more water vapor (a very effective GHG), faster convection (with more loss of heat to space in the tropics),… How each of them converts to real regional/global temperature increases/decreases is another point of discussion…
[Response: You are strictly correct, but the feedbacks highlighted by Rind et al where simply feedbacks from surface temperature changes, and therefore would be similar for any process that changed surface temperatures. The details of the physics of different forcings (i.e. ozone effects due to solar, snow albedo and cloud effects due to aerosols etc.) do vary the feedbacks slightly differently though. – gavin]