Today – after a long pause – a new Climate Dialogue has started about climate sensitivity and transient climate response. This of course is a highly relevant topic. Equilibrium climate sensitivity (ECS) and especially the transient climate response (TCR) largely determine how much warming we can expect this century.
Climate Dialogue is a platform for discussions between invited climate scientists on controversial climate topics. They represent a range of views. Discussions are strictly moderated and should be on topic. Last year we did four Climate Dialogues, about arctic sea ice, long-term persistence, regional climate models and the hot spot in the tropics.
We are very happy that a few key players in the debates surrounding climate sensitivity have agreed to participate in this Climate Dialogue. John Fasullo of NCAR is well-known for his publications with Kevin Trenberth about the role of the natural variability and the oceans in the recent hiatus. James Annan, who recently moved back to the UK after working for many years in Japan, has also published a lot about climate sensitivity and is also frequently blogging about it. Finally Nic Lewis is a relative newcomer in climate science. After a career in the financial sector he recently became a climate scientist, focusing on climate sensitivity. He published a few papers about climate sensitivity, one with a group of IPCC lead authors.
A Sensitive Matter
Some readers will remember that Nic Lewis and I published a GWPF report in March of this year with the title A Sensitive Matter: How the IPCC hid the good news on global warming. One of the main conclusions of our report was that observationally based estimates for ECS and TCR (based on the so-called instrumental period, say 1850-2014) are “superior” at the moment and are to be preferred over estimates from GCMs or paleorecords.
We claim that these “observational” estimates indicate best estimates for ECS that are close to the lower boundary of the likely range in several IPCC reports which is 1.5–4.5°C. We give an “observational” likely range of 1.25-3°C with a best estimate of 1.75°C. This is substantially lower than the average ECS value of the CMIP5 models which is more than 3°C.
IPCC gave a best estimate of 3°C in AR4 but none in AR5. This lack of a best estimate was only mentioned in a footnote in the SPM. Lewis and I wrote in our report that the discrepancy between the observationally based best estimate and the model based best estimate was likely the main reason why IPCC didn’t give a best estimate this time. So IPCC on the one hand says climate sensitivity will likely be anywhere between 1.5–4.5°C and no best value can be given. But when doing projecetions AR5 relies (also in the Working Group II and III reports) on climate models that have an average climate sensitivity of 3.2°C.
So it is really important to find out where the discrepancy between the observationally based and model based estimates comes from. This will certainly be a crucial question in this Climate Dialogue.
Lewis in his guest blog repeated his likely range for ECS being lower than that of AR5:
The soundest observational evidence seems to point to a best estimate for ECS of about 1.7°C, with a ‘likely’ (17-83%) range of circa 1.2–3.0°C.
Annan agrees that the observational record points towards lower values for ECS and TCR. But nevertheless he comes up with slightly larger value for the best estimate than Lewis:
The recent transient warming (combined with ocean heat uptake and our knowledge of climate forcings) points towards a “moderate” value for the equilibrium sensitivity, and this is consistent with what we know from other analyses. Overall, I would find it hard to put a best estimate outside the range of 2-3°C.
Fasullo did not yet give his preferred likely range but he emphasized in his guest blog that lowering the lower boundary of the likely range in AR5 to 1.5°C (was 2°C in AR4) maybe was justified at the time but not any longer:
In short, I argue that although IPCC’s conservative and inclusive nature may have justified such a reduction at the time of their report, the evidence accumulated in recent years argues increasingly against such a change. Lees verder…