ClimateDialogue on Climate Sensitivity

After a bit of a “hiatus”, ClimateDialogue (CD) has re-opened again with a discussion on climate sensitivity. On the one hand this site is unique in bringing together ‘mainstreamers’ and ‘contrarians’ (both in the organization and in the discussions), hopefully leading to both enhanced clarity on what the (dis)agreements are really about and to decreased polarization. On the other hand it’s controversial because a ‘false balance’ is embedded in its structure (by purposefully inviting contrarian scientists to the discussion, rather than e.g. randomly inviting experts).

Whether the positives or negatives dominate is in the eye of the beholder (opinions about that vary wildly), but also depends very strongly on the participation of the mainstream (both as invited experts and as contributing to the public discussion). See also my initial reflections at the time of the first launch. Discussions on ClimateDialogue will be facilitated and moderated by Bart Strengers (NL Environmental Assessment Agency, PBL) and Marcel Crok (freelance journalist), where the former has a mainstream view of climate science and the latter a contrarian view. I am still involved in the background, as is KNMI (NL Meteorological Institute). ClimateDialogue is funded by the Dutch Ministry of Infrastructure and Environment.

In the current ‘dialogue’ James Annan, John Fasullo and Nic Lewis are discussing their views about climate sensitivity (the equilibrium warming after a doubling of CO2 concentrations, ECS). In the latest IPCC report (AR5) the different and partly independent lines of evidence are combined to conclude that ECS is likely in the range 1.5°C to 4.5°C with high confidence. The figure below shows the ranges and best estimates of ECS in AR5 based on different types of studies, namely:

– the observed or instrumental surface, ocean and/or atmospheric temperature trends since pre-industrial time

– short-term perturbations of the energy balance such as those caused by volcanic eruptions, included under “instrumental” in the figure

– climatological constraints by comparing patterns of mean climate and variability in models to observations

– ECS as emergent property of global climate models

– temperature fluctuations as reconstructed from palaeoclimate archives

– studies that combine two or more lines of evidence

The pros and cons of these different lines of evidence are discussed, as well as the weighing of the evidence and the resulting range and best estimate.

Fasullo argues for “combined consideration of the individual approaches” and for an ECS range between 2 and 4.5 degrees C (based on recent evidence since the publication of AR5 pointing to the middle of this range).

Annan writes: “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.”

Lewis prefers a subset of studies in the “instrumental” category over model-based and paleo-climate approaches and conclude that “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’s first response to Lewis hits the nail on the head imho:

Nic Lewis appears to be arguing primarily on the basis that all work on climate sensitivity is wrong, except his own, and one other team who gets similar results. In reality, all research has limitations, uncertainties and assumptions built in.

A few recent studies (Shindell, 2014; Kummer and Dessler, 2014) suggest that the “efficacy” of cooling aerosols may be larger than expected, because they are largely confined to the landmasses of the Northern hemisphere. This could explain the difference in ECS as deduced from the instrumental period vs from climate models and paleo-climate. After all, such differences need explaining. If correct, this would mean that ECS is indeed somewhere in the middle of the IPCC range (~3 degrees C). In turn, this would mean that the warming should pick up speed again in the near future, James Annan argues, since the current rate of warming is close to the lower end of the model range:

If these results are correct, then the current moderate warming rate is a bit of an aberration, and so a substantial acceleration in the warming rate can be expected to occur in the near future, sufficient not only to match the modelled warming rate, but even to catch up the recent lost ground.

However, Schmidt et al (2014) show that when accounting for the timing of ENSO events, for the actual evolution of climate forcings (e.g. volcanic aerosols) and for inaccuracies in global temperature datasets, observations and the model ensemble mean are very similar. That weakens Annan’s conclusion about the future warming trend, as cited just above.

Figure (Schmidt et al., 2014): Observed and modeled warming trends corrected for observed ENSO timing, updated forcing estimates, and improved spatial coverage.

Climate sensitivity is a policy relevant metric, since a more sensitive climate would warrant stronger emission reductions in order to remain below the same target of maximum allowable warming. Since global emissions are still rising, this is however merely relevant to future policymaking. Even if ECS is on the low side of the likely range, emissions would have to be reduced (though less drastically so) in order to remain below the two degree target, for example. Any realistic change in our scientific understanding is not going to make a big difference for the required policy response, at least not in the short to medium term (~decades).

Like at CCNF, there is a public discussion alongside the ‘invitee-only’ discussion, so feel free to chime in (registration required). You may also contact one of the editors (Bart S, Marcel C) or me to bring something into the invitee’s discussion.

There are similarities between ClimateDialogue and ClimateChangeNationalForum (CCNF, with which I’m also affiliated) e.g. in trying to make scientific discussions visible to the public, though there are also important differences. E.g. CCNF has a more open discussion structure, whereas CD has moderated discussions between a smaller set of invited participants. At CCNF the set of science columnists is more reflective of the scientific spectrum of opinion than at CD (though CCNF has also been criticized for “amplifying nonsense”; see also the recent discussion at CCNF itself). This post has also been published on CCNF.

As a tongue-in-cheek epilogue, let me end with this hilarious clip from “Last week tonight” with John Oliver, in which he contrasts the common ‘false balance’ with what a ‘statistically representative climate debate’ would look like:

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Tags: climate sensitivity, ClimateDialogue, James Annan, John Fasullo, Nic Lewis