The AR5 Synthesis Report has been published with all the usual rhetorics such as that we have only so much years left to act. Readers here know that my interest with regard to AR5 has been climate sensitivity. So let’s just shortly review what happened in the field of climate sensitivity between the Synthesis Report (SYR) of AR4 (2007) and that of AR5 (2014). Let’s focus on the SPM because this is what is supposed to be the most policy relevant information.
The SYR SPM of AR4 mentions “climate sensitivity” seven times:
For GHG emissions scenarios that lead to stabilisation at levels comparable to SRES B1 and A1B by 2100 (600 and 850 ppm CO2-eq; category IV and V), assessed models project that about 65 to 70% of the estimated global equilibrium temperature increase, assuming a climate sensitivity of 3°C, would be realised at the time of stabilisation. [Figure SPM.8 on page 12]
The timing and level of mitigation to reach a given temperature stabilisation level is earlier and more stringent if climate sensitivity is high than if it is low. [page 20]
Global average temperature increase above pre-industrial at equilibrium, using ‘best estimate’ climate sensitivity [Table SPM.6 on page 20]
The best estimate of climate sensitivity is 3°C. [note d of table SPM.6 on page 20]
Equilibrium sea level rise is for the contribution from ocean thermal expansion only and does not reach equilibrium for at least many centuries. These values have been estimated using relatively simple climate models (one low-resolution AOGCM and several EMICs based on the best estimate of 3°C climate sensitivity) and do not include contributions from melting ice sheets, glaciers and ice caps. [note f of table SPM.6 on page 20]
The right-hand panel shows ranges of global average temperature change above pre-industrial, using (i) ‘best estimate’ climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). [Caption of figure SPM.11 on page 21]
Impacts of climate change are very likely to impose net annual costs, which will increase over time as global temperatures increase. Peer-reviewed estimates of the social cost of carbon23 in 2005 average US$12 per tonne of CO2, but the range from 100 estimates is large (-$3 to $95/tCO2). This is due in large part to differences in assumptions regarding climate sensitivity, response lags, the treatment of risk and equity, economic and non-economic impacts, the inclusion of potentially catastrophic losses and discount rates. [page 22]
Climate sensitivity is a key uncertainty for mitigation scenarios for specific temperature levels. [page 22]
Summarised: climate sensitivity and its uncertainties is highly relevant for the amount of future warming. The best estimate for climate sensitivity is 3°C, the lower bound is 2°C and the upper bound is 4.5°C.
The full AR4 Synthesis report mentions climate sensitivity 13 times. It for example said:
Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C.
Now straigth to the AR5 Synthesis Report SPM. It mentions this highly relevant parameter (according to AR4) … zero times! Not a word about it. The full Synthesis report does mention it four times. For example on page SYR-23 we read:
Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change. The equilibrium climate sensitivity (ECS) is likely in the range 1.5 °C–4.5 °C, extremely unlikely less than 1 °C, and very unlikely greater than 6 °C.
Now what has happened in the past seven years that climate sensitivity disappeared from the SPM of the Synthesis Report? Has it become irrelevant? Of course not. Climate sensitivity is all over the Synthesis Report because the models used to project the future climate have a climate sensitivity of about 3.5°C on average. So in all its projections IPCC assumes climate sensitivity is still >3°C. It’s there as some sort of hidden assumption.
Why not say so then? Well, exactly this assumption, that the model climate sensitivity is about 3.5°C, has been seriously challenged in the past few years in the scientific literature. The Lewis/Crok report A Sensitive Matter (published in March of this year) gave all the details about new observationally based studies that indicate the climate sensitivity is relatively low with best estimate values of between 1.5 and 2°C. Considerably lower than the 3.5°C climate sensitivity of the models.
Recently Lewis and Curry used all the relevant AR5 numbers and a very detailed uncertainty analysis to estimate the range and best estimate for climate sensitivity in a paper published in Climate Dynamics. Their preferred likely range is 1.25-2.45°C and the best estimate is 1.64°C. Again, these are not numbers invented by skeptics, those are the numbers of the IPCC itself. It assumes close to 100% of the warming since 1850 is due to humans, an assumption that goes much further than the iconic “it’s now extremely likely that most of the warmings since 1950 is due to humans” statement in AR5.
Now this specific paper of course came out after the IPCC deadline for the Synthesis Report. However as we document in the Lewis/Crok report, the IPCC was well aware of these recently published lower estimates of climate sensitivity. It lowered its lower boundary from 2°C back to 1.5°C (where it has been in most earlier IPCC reports).
The IPCC was saddled with a dilemma. A lot of conclusions in the report are based on the output of models and admitting that the models’ climate sensitivity is about 40% too high was apparently too…inconvenient. So IPCC decided not to mention climate sensitivity anymore in the SPM of the Synthesis Report. It decided to give the world a prognosis which it knows is overly pessimistic. One may wonder why. Did it want to hide the good news?
Do I understand it right that you see these “new observationally based studies” as strong evidence? I guess that is because you see some theoretical advantages and not just because the give lower values as the other methods.
These estimates are based on a still very small global warming we had up to now. The signal to noise ratio is thus still very small. Will you keep the opinion that these “new observationally based studies” are the strongest evidence when future estimates become higher?
In our report A Sensitive Matter Lewis and I explain in much detail why indeed we think that these observationally based estimates are “superior”. Have you read our report? If so and you disagree, please indicate what kind of estimates and which studies you would prefer? We also indicate in the report the many shortcomings of other estimates/studies.
VV, why not answer to the question raised? What constitutes ‘strong evidence’ is a different issue compared to how the IPCC weighed it between two reports. High resolution temperature data encompasses a short temporal period, lower resolution temperature proxy reconstructions are geospatially restricted and cannot resolve shorter time periods. Everybody knows this.
The question asked here is: why did the IPCC promote the sensitivity metric in the previous report and does not do it now? Why especially considering the ‘irreversible’ ‘4C by 2100’ etc catastrophic outcomes promoted by the synthesis report are utterly dependent on the same numbers?
Thanks Marcel! The problem with the downgraded climate sensitivity for the modeling community probably was that it was a) difficult to re-tune the models, and b) re-run the simulations again in time. Moreover, as a modeler that is supposed to contribute to a scientific consensus, you carefully avoid sticking out your head too far. If you are part of such a ‘model choir’, you will be left in peace as long as you swim mid-stream, but you’ll attract much closer scrutiny or even excommunication if you dare to add a dissonance to the choral. So there was and is the… Lees verder »
Marcel, I suspect this omission of any reference to climate sensitivity may be the work of Myles Allen who was one of the authors of the Synthesis Report.
You may remember that in one of the UK House of commons sessions on AR5 WG1, he said that climate sensitivity was a bit like Katie Price – a lot of people were talking about it but it wasn’t clear why. He seems to think climate sensitivity is not a very important parameter.
Myles Allan is paraphrased as saying:
“Myles Allen replied that climate sensitivity was not so important – rather like Katie Price, it wasn’t clear why people were talking about it so much. The climate scientists seem to be saying that “transient climate response” is more important.”
To assess the urgency to act on rising greenhouse gas emissions, TCR is of course more informative than ECS. But has that resulted in a more frequent use of TCR than ECS in the SPM?
TCR is not used in the AR4 and AR5 SYR SPM’s although it is now used in the full synthesis report.
From my own communication with Myles I know he is in favour of using TCR as the more policy relevant one. I agree with him on this although one probably has to keep an eye on both of course because if TCR is low but ECS is (very) high, lots of heat goes in the oceans which is relevant for sea level rise and probably for other things (changing weather patterns maybe?).
Shub Niggurath, trying reverse psychology, to make me not respond? There is no need for a conspiracy theory for a different emphasis on a specific term. These reports are written by different author teams and everyone has its own favorite way to explain stuff. I find it a pity that the previous IPCC report explicitly wrote that trends in extremes are likely more affected by non-climatic changes than trends in the means. While the new one did not. That is something that is important for my work. However, I see no need to assume a conspiracy, someone else wrote this… Lees verder »
“Shub Niggurath, trying reverse psychology, to make me not respond? There is no need for a conspiracy theory for a different emphasis on a specific term.” First, I don’t even know what you mean. I can no more make you ‘not respond’ than make you ‘respond’. I’ve frequently found that scientists such as you and theresphysics are newbies to online interaction and consequently have rather fanciful imaginations, blaming whatever thoughts that occur in their minds to other commenters. In reality, the situation is much simpler: each asks questions and others answer. To ask a question is not ‘conspiracy’. Again, you… Lees verder »
Keep up the good work Marcel
We all know why the results are being hidden. What we don’t know is how long they are willing to do it for …. AR10 anyone?
I think you are mischaracterizing Anthony Watts’ issue with BEST. As I understand it, they BEST authors were going to integrate Watts’ surface observation bias studies into their study. As I understand it, they released their results without doing so.
I do not know whether integrating Watts work into BEST would have mattered and do not care to venture a guess. However, I think you should contact Watts for comment before taking what appears to be an unwarranted slap at him.
From what I now understand about the relationship between IPCC and UNFCCC (IPCC provides the basis for Paris next year) it indeed can take a while before the “insights” about climate sensitivity will be accepted and more important – highlighted – by IPCC.
It also depends on other factors like emissions/concentrations and e.g. carbon cycle feedback. If concentrations start rising quicker due to positive carbon cycle feedback, then IPCC might be willing to lower ECS to say 2 degrees, because you can still claim there is big problem.
Thank you Shub – clear, concise and devastating.
Interesting to see this difference in AR4 and 5. It would have been much better if the known uncertainty in climate sensitivity had made it to the synthesis report as it is so crucial. But the uncertainty also gets lost in this post and comments, and the “good” in good news is a relative thing. My two cents: 1) While I agree with Marcel that the observational values are the most trustworthy, that does not mean other sources of information should be totally dismissed. The observational record is too short to include all processes. One example already given are climate-carbon… Lees verder »
Why not spread the good news? Because for the IPCC and the modelers it’s not good news. It’s the Kiss of Death.
BTW, what sensitivity is assumed in the 3% of models that are within a (wide) margin of error of observations? Why isn’t the IPCC praising and publicizing them?
I haven’t read Marcel’s paper but my (poor?) understanding of it is that he uses observational (presumably global temperature) data to derive at a climate sensitivity between 1.5-2 2°C. This method may be flawed if all the temperature fluctuations/increases over the last century were mainly due to natural causes. I am not saying that CO2 is not a greenhouse gas or that the theoretical greenhouse effect for a doubling is not around 1°C but a negative feedback (clouds) could have decimated the effect to an insignificant amount leaving only the natural causes, like ocean currents, as the main culprit for… Lees verder »