By Dr. Pat Michaels at World Climate Report
A new, lower estimate of climate sensitivity
There is word circulating that a paper soon to appear in Science magazine concludes that the climate sensitivity—how much the earth’s average temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide—likely (that is, with a 66% probability) lies in the range 1.7°C to 2.6°C, with a median value of 2.3°C. This is a sizeable contraction and reduction from the estimates of the climate sensitivity given by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), in which the likely range is given as 2.0°C to 4.5°C, with a best estimate of 3.0°C.
Further, the results from the new analysis largely eliminate the “fat tail” of the distribution of possible values of the climate sensitivity (that the IPCC AR4 report was fond of) which included the possibility that very large climate sensitivities are a realistic possibility. In the new paper, the authors find only “vanishing probabilities” for a climate sensitivity value greater than 3.2°C and that values greater than 6.0°C are “implausible.”
Contrast that with the IPCC assessment of the literature (summarized in our Figure 1) which routinely includes studies concluding there is a greater than a 10% possibility that the true climate sensitivity exceeds 6°C and some which find that there is a greater than 5% possibility that it exceeds 10°C.
Figure 1. Climate sensitivity distributions retained (and in some cases recast) by the IPCC from their assessment of the literature. Note the “fat tail” towards the right which indicates the possibilities of the climate sensitivity having a very large positive value (that is, a huge degree of global temperature rise for a doubling of the atmospheric carbon dioxide concentration) (source: IPCC AR4).
The new paper, from a team of researchers led by Andreas Schmittner of Oregon State University, throws cold water on the IPCC’s tails. Here is its rather provocative abstract:
Assessing impacts of future anthropogenic carbon emissions is currently impeded by uncertainties in our knowledge of equilibrium climate sensitivity to atmospheric carbon dioxide doubling. Previous studies suggest 3 K as best estimate, 2–4.5 K as the 66% probability range, and non-zero probabilities for much higher values, the latter implying a small but significant chance of high-impact climate changes that would be difficult to avoid. Here, combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations we estimate a lower median (2.3 K) and reduced uncertainty (1.7–2.6 K 66% probability). Assuming paleoclimatic constraints apply to the future as predicted by our model, these results imply lower probability of imminent extreme climatic change than previously thought.
Figure 2 shows the distribution of the range of the earth’s probable climate sensitivity as determined by Schmittner et al. Note the rapid drop-off in the probability that the climate sensitivity is much greater than 3°C (the IPCC “best estimate” for the sensitivity), and that the distribution falls off less slowly towards the left (towards lower sensitivity) than towards the right (higher sensitivities). The “fat right-hand tail” of the distribution is gone and the possibility that the climate sensitivity is in the 1°C to 2°C range is not minimal.
Figure 2. Distribution of the land/ocean climate sensitivity as determined by Schmittner et al. (adapted from Schmittner et al., 2011).
The Schmittner et al. results join a growing number of papers published in recent years which, by employing investigations of the earth’s paleoclimate behavior (that is, how the earth’s temperature changes in the past when subject to changing climate forcings) have come to somewhat similar conclusions, especially regarding the (lack of) evidence to support the existence of the fat right-hand tail.
For example, researchers James Annan and Julia Hargreaves published a paper in 2009 that concluded many of the assumptions underlying the possibilities of very high climate sensitivities were unjustified. They wrote:
When instead reasonable assumptions are made, much greater confidence in a moderate value for [the climate sensitivity] is easily justified, with an upper 95% probability limit for [the sensitivity] easily shown to lie close to 4°C, and certainly well below 6°C. These results also impact strongly on projected economic losses due to climate change.
Annan made repeated comments during the IPCC AR4 review process that the IPCC’s handling of climate sensitivity and its probability distributions were incorrect. His complaints largely fell upon deaf ears.
However, as there are appearing more and more examples in the literature, of which Schmittner et al. is one of them, making a convincing case that the very high climate sensitivities are not defendable, there will be growing pressure on the IPCC in its Fifth Assessment Report to greatly shrink the fat tail of the probability distribution for the true climate sensitivity. However, the climate “realists” very bad experience with the last IPCC process makes them chary. James Annan, writing at his blog in reference to the new Schmittner et al. paper had this to say as to what may result from it:
That said, [the Schmittner et al. paper] is a useful antidote to the exaggerated uncertainty estimates that have been prevalent over recent years, and I certainly applaud the intentions and effort underlying this substantial piece of work. In any case, I expect the merchants of doubt to do their worst on it when they cite it in the IPCC report.
But, as the evidence mounts against a high value for the climate sensitivity, and evidence grows for a low value (recall that the observed rate of global warming for the past several decades has fallen well below IPCC best estimates), the IPCC is going to be hard-pressed to retain the status quo in its Fifth Assessment Report, especially in light of the enhanced scrutiny that its AR4 misdeeds brought upon the process.
But, as James alludes to, perhaps we ought not be holding our breath.
And, for those keeping score out there, about 10 years ago, a couple of us here at WCR were part of a team which published a paper in the journal Climate Research in which we employed a variety of techniques to derive empirical estimates of the amount of temperature rise that we could expect by the end of this century—a rise that could pretty well be considered to be in-line with the climate sensitivity. We concluded that the expected temperature rise between 1990 and 2100 would be in the range 1.0°C to 3.0°C with our best guess being 1.8°C (in contrast to the IPCC estimates, which, at the time, were for a rise of between 1.4°C and 5.8°C).
References:
Annan, J.D., and J.C. Hargreaves, 2009. On the generation and interpretation of probabilistic estimates of climate sensitivity. Climate Change, 104, 423-436, doi:10.1007/s10584-009-9715-y, http://www.jamstec.go.jp/frsgc/research/d5/jdannan/probrevised.pdf
Michaels, P.J., P.C. Knappenberger, O.W. Frauenfeld, and R.E. Davis, 2002. Revised 21st century temperature projections. Climate Research, 23, 1-9.
Schmittner, A., et al., 2011. Climate sensitivity estimated from temperature reconstructions of the Last Glacial Maximum, Science, in press*, http://www.princeton.edu/~nurban/pubs/lgm-cs-uvic.pdf
*According to the authors
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This, from the laest Ecologist rather complements the above…
“Durban climate summit: is it time to forget about 2 degrees of warming?”
http://www.theecologist.org/News/news_analysis/1117209/durban_climate_summit_is_it_time_to_forget_about_2_degrees_of_warming.html
I guess we’ll have to wait to see how they come to this number, but it doesn’t sound like there’s much in the way of negative ‘feedback’ from water/clouds. It’s a nod in the literature in the direction of something at least sane, I suppose, but I hope certain quarters won’t be expecting this to be some sort of compromise figure that all reasonable people can settle on as a basis for CO2 control policy.
Wow! If true this is real dynamite for us “luke-warmists”. We now have real amunition against the extreme alarmist views.
Superimposing natural oscillations on the 350 year rising trend in the CETs of 0.25C/century and keeping that upward trend going
http://www.vukcevic.talktalk.net/CET-NV.htm
for another 150 years, calculations show significant drop in temperatures:
http://www.vukcevic.talktalk.net/CET-NVa.htm
If the past rising trend isn’t kept going projected drop in temperatures due to natural oscillations could be much greater.
They left out the possibility that CO2 might have a cooling effect. Still working from the old assumptions using models that don’t work.
But thanks for your input fellas.
Five stages of death: Bargaining.
“It is plausible to be skeptical of a number of issues regarding the findings of IPCC WG1. However, whether atmospheric gases such as CO2 (and H20, CH4, and others) warm the planet is not an issue where skepticism is plausible” ….Judith Curry
The real debate is about ‘feedback;. Current predictions of climate catastrophe due to human influence all depend on the assumption of a positive feedback effect, such that adding a little more CO2 to the atmosphere will cause a runaway greenhouse effect. This is pure speculation. We have never witnessed such an effect in geological history although past temperatures have been much higher and CO2 levels have been much higher. This positive feedback, reflected in ‘climate sensitivity’, is not based on experiment nor observation. It remains an unfounded conjecture in conflict with all known observations.
Some scientists believe that water vapour also produces a negative feedback because the increased albedo of the consequent extra cloud cover will reflect more solar radiation back to space. This may or not be the case, but what we do know is that about 8000 years ago, when temperatures were much hotter than now (up to 7 deg. Celsius hotter according to the IPCC Arctic Impact Assessment Report – Section 2.7), we saw no positive feedback, no runaway greenhouse effect and temperatures eventually fell. Estimates of climate sensitivity are pure speculation in conflict with all known observations. We have never seen a runaway greenhouse effect on this planet even though temperatures have been much higher and CO2 levels have been much higher in the past. The relative stability of our climate over millions of years suggests that if there is a climate feedback effect, it is a negative one. Unfortunately, in the field of climate prediction, Science based on careful observation and experiment has been replaced by computer modelling and speculation.
Before you know it, they’ll be saying that whatever effect there is from greenhouse gases is too small to separate from the noise.
Post Normal Science?
In two years it will be reduced to maybe 0.5 to 1C, after that AGW will be dead and I hope half these clowns will be fired.
I have my doubts about all that. It has no sense to say we are 0.6 deg C warmer than in pre-industrial times, assuming the whole warming was caused by CO2, and then we project the curve up to 2100. 1980 ties were comparable or even colder than 1900s, 1940s were almost as warm as 2000s, MWP was warmer than present, even most of the interglacial was warmer than present. Recent warm peak was caused by natural oceanic variations, maybe combined somehow with the Sun activity. Since the most sensitive /polar/ areas show no relation with CO2 rise at all, all that sensitivity guesswork is nonsense.
Fired?
How about prosecuted?
Holy Thermometers Batman!
Considering climate sensitivity is the very crux of the CO2 CAGW argument, this is a huge step forward, in the enabling, of useful discussion. Could we be seeing a merging of skeptical Vs warmist opinion??
Now if only something could be published, which gives us some confidence in the + or – sign, or that it is even constant in any direction. GK
Andrew says:
November 9, 2011 at 12:17 pm
“In two years it will be reduced to maybe 0.5 to 1C, after that AGW will be dead and I hope half these clowns will be fired.”
Agree. But first it will be changed to varying dynamically between -1.0 to +1.0 (or so), dynamically, and then forgotten. The “clowns” you mention, will be science advisors for some president, or in the high echelons of the UN, governing some new scare.
“In the new paper, the authors find only “vanishing probabilities” for a climate sensitivity value greater than 3.2°C and that values greater than 6.0°C are “implausible.” ”
I am sure this has a lot to do with what has happened over the last several years.
I have two multiple choice questions that I would like answers for if anyone has them: (Thank you in advance!)
1. What period of time must elapse before the time is significant with respect to climate trends or even the lack of any trends?
A. 10 years
B. 13 years
C. 15 years
D. 17 years
E. 20 years
F. None of the above
2. What must the slope be in order for the warming/cooling to be considered NOT significant?
A. Less than +/- 0.002 C/year
B. Less than +/- 0.004 C/year
C. Less than +/- 0.006 C/year
D. Less than +/- 0.008 C/year
E. Less than +/- 0.010 C/year
F. None of the above
The sad thing is that its now way to late for any chance of ‘climate science ‘ to gain a reputation for being sensible , far to many lies , far to many chicken little calls and far to much religiosity for people to forget . Its not water under the bridge because there was so much water it washed the bridge away. Now its question of cleaning up the mess and building a new bridge .
An interesting issue will be the prior (if any) used in the new assessment. Annan and Hargreaves were critical of the nature of this (and some adjustments made) by IPCC in their 2009 paper.
“the IPCC is going to be hard-pressed to retain the status quo in its Fifth Assessment Report”
Not so. From IPCC Report AR5, July 2015, para 2.7.1 3: “There have been estimates of a lower climate sensitivity (Schmitter et al 2011, …) but there are some ambiguities and they remain controversial.”. There is no other reference to it in the report.
Sorry I can’t provide the link, I should have it in a few years’ time.
Determining the CO2 climate sensitivity from the conditions of the Last Glacial Maximum depends completely/100% on what your assumptions/calculations are for how much additional solar radiation is reflected away from the Earth by all that extra glacial ice, snow, desert and grassland and other changes in clouds and dust for example.
Climate science consistently downplays this impact and assumes something like just 1.0% more sunlight is reflected. (29.8% reflected today versus 30.8% at the Last Glacial Maximum). This 1.0% assumption magically gives one 3.0C per doubling from CO2. None of these papers actually tell you they are assuming 1.0% more sunlight is reflected (because it is built into a climate model simulation and because is so preposteriously low) but that is what they are using.
This study uses exactly the same assumption in the same kind of climate model simulation and, consequently, I put no stock into it.
Progress indeed.
Last year this would not have got by the “gatekeepers”.
The backpeddling by high-impact journals is underway.
So if a climate model simulation gives a lower value of climate sensitivity (here Schmittner, 2011) the results of GCMs are welcomed? If yes, the next question would be: Why is the GCM used by Schmittner more reliable than other GCMs predicting higher values?
I agree this is a step in the right direction, but…
This is still a paper based at least partially on model assumptions.
Observed data tells us this:
1. Since 1850 atmospheric CO2 has increased by about 40%.
2. Since 1850 the global temperature (insofar as it can be measured) has increased by about 0.9 K (HADCrut). (1850 is the date quoted by the IPCC as the start of accurate temperature data recording.)
There is no conclusive causation between the two statements 1. and 2.
Even if ALL the temperature increase was due to CO2, the linear progression would indicate a climate sensitivity of 2.25 K. This is essentially the same as the Schmittner et al median.
There is no observed acceleration in global warming, so a linear progression is at least fair.
However, we cannot attribute ALL the warming to CO2 because that would require us to deny that natural variability exists in any form.
So, all we can say is that SOME of the 0.9 K warming MAY have been caused by CO2.
This means the climate sensitivity is likely to be lower than 2 K, maybe significantly so.
.
Unfortunately, since no-one can say with any certainty how much of the Greenhouse Effect is contributed by CO2, any estimates, particularly those based on model assumptions, remain dressed-up guesses.
.
I agree with other posters that the near future will see further reductions in the estimates of CS.
Even 2.3 C per doubling is way out there. I think it’s more like 0.3 C. How does 2.3 C fit in with 0 warming since 1998 while CO2 increase is at the highest ever? There must be some very big negative effect exactly matching the CO2 effect.
Is the link to the original correct? I can’t find the paper.
M.A.Vukcevic says:
November 9, 2011 at 11:55 am
Vuk – I agree, although I came at it from the solar cycle length aspect, using Butler and Johnston’s trend line for Armagh, which is similar to the CET in climate and latitude.
I might add that using Butler & Johnston 1996 with the CET gives you a climate sensitivity by difference of roughly 0.7 C/doubling, ie similar to Lindzen & Choi 2011 and nowhere near Schmittner et al’s value.
In other words the world is not going to fry because of CO2, carbon tax or no carbon tax (sigh).