by Judith Curry

Climate Dialogue explores different views on climate sensitivity and transient climate response.

Climate Dialogue is a remarkable blogospheric experiment. From the About page:

[Climate Dialogue is] an international blog where invited scientists discuss controversial topics in climate science. There are several blogs that facilitate discussions between climate experts but since the climate debate is highly polarized and politicized, blog discussions between experts with opposing views are rare.

ClimateDialogue.org is the result of a request by the Dutch parliament to facilitate the scientific discussions between climate experts representing the full range of views on the subject. It is funded by the Dutch Ministry of Infrastructure and the Environment.

The aim of ClimateDialogue.org is to establish what the discussants agree on, where disagreements remain and what the possible or likely reasons behind these disagreements are. The project by no means aims to solve controversies nor give an objective, scientific final judgment on the topics under discussion.

In case you missed it, I participated in the inaugural discussion on Arctic sea ice.

Climate Dialogue has just posted on the topic of Climate Sensitivity and Transient Climate Response. Marcel Crok has given me permission to post his introductory essay that lays out the key topics of debate, some excerpts:

In AR5 it is indicated that the peer-reviewed literature provides no consensus on a formal statistical method to combine different lines of evidence. Therefore, in AR5 the range of ECS and TCR is expert-assessed, supported by, as indicated above, several different and partly independent lines of evidence, each based on multiple studies, models and data sets. Obviously, this expert judgement in AR5 has been performed deliberately, but it is not a straightforward procedure. The discussion on how to weigh the different lines of evidence is very old, not only in the scientific literature but also in the blogosphere and in reports and is still going on. For example, Nic Lewis, who takes part in this dialogue and was author/co-author of two studies mentioned in the instrumental category in figure 1, argues that instrumental or empirical approach studies with relatively low ECS values should be weighted much higher than IPCC did in AR5 (Lewis and Crok, 2014).

Others argue that the main limit on ECS is that it has to be consistent with palaeoclimatic data which point at ranges being consistent with the IPCC-range (Palaeosens, 2012, also mentioned in figure 1) and also in line with climate models likely range of about 2 to 4.5 0C (CMIP5). Some argue that palaeoclimatic data points to values in the upper part of the IPCC range (Hansen, 2013). In this dialogue we therefore want to focus first on the following two questions: 1) What are the pros and cons of the different lines of evidence? 2) What weight should be assigned to the different lines of evidence and their underlying studies? Best estimate With respect to the best estimate it was reported in AR5 that: “No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence.” Also, it was concluded that ECS is extremely unlikely less than 1°C (grey solid line in figure 1), and very unlikely greater than 6°C (grey dashed line).

So IPCC did not choose between the different lines of evidence with respect to the best estimate, but it was not discussed in much detail why. Therefore, the third question we will address is: 3) Why would a lack of agreement between the lines of evidence not allow for a best estimate for ECS? 4) What do you consider as a range and best estimate of ECS, if any? TCR range in AR5 AR5 concludes with high confidence that the TCR is likely in the range 1°C to 2.5°C, and extremely unlikely greater than 3°C (see figure 2).

The three invited experts are James Annan, John Fasullo, and Nic Lewis. All three essays are well written and clearly lay out their positions. Annan’s essay most closely follows the IPCC AR5 report and is arguably the ‘consensus’ position. Nic Lews argues for relatively low sensitivity values and Fasullo argues for relatively high sensitivity values. For my perspective, I have written numerous previous posts on the topic of climate sensitivity, including discussing papers by Annan and Lewis. My own views on this topic most closely correspond to Nic Lewis.

Check out the comments, this is the most interesting part. Each of the 3 authors has critiqued the other posts. Bart Strengers has summarized the posts, focusing on areas of agreement and disagreement.

Climate Dialogue has two sets of comments: one for the participants, and the other for the public. Interesting public comments will be pointed out to the participants.

Check out the Climate Dialogue site, and maybe even leave a comment.

Moderation note: This is a technical thread, keep your comments on topic – either the subject of sensitivity, or the Climate Dialogue approach.