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

WGI_AR5_FigBox12.2-1 - sensitivity estimates ECS

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.

schmidt 2014 - Reconc warming trends

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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7 Responses to “ClimateDialogue on Climate Sensitivity”

  1. Victor Venema Says:

    Let me emphasis that I did not complain about false balance. Before we get another discussion of Climate Etc. quality. My complaint about the CCNF was about the format of its “fact checker”. My reservations were apparently shared by the CCNF community, as the problem was largely resolved.

    I do not like term false balance that much, it suggests that the problem is numerical, while I feel the problem is more on of quality, of the inappropriate formats used to present climate science in the mass media.

    One remaining complaint about the CCNF is that you need a Facebook account to be able to comment and I prefer not to have one.

  2. Bart Verheggen Says:

    Thanks for the clarification, Victor. Of course the issue not numerical per se, but the weight of evidence probably translates in a certain numerical division of scientific opinions. After all, it’s not a coincidence that most experts agree that smoking increases the risk of cancer; that’s a direct result of the weight of evidence leaning heavily to that side. The societal relevance of properly communicating the existence of an expert consensus are underscored by e.g. Maibach et al.

  3. Victor Venema Says:

    Bart, I would guess that I have more posts the last year about climate pseudo-sceptics and their ideas as I have posts about science. It could be similar here. Not? I do not see that as false balance.

    The problem is the format. The fake debates, which are not suited to improve scientific understanding, and the presentation of fake experts as credible ones.

    If a quarter of the American public does not accept climate change, why not spend a quarter of the time on that? Possibly even more because it is interesting and these people are a political liability for the USA.

    When I just write about science, I have a lot less readers as when I write about climate sceptics and then explain how it conflicts with the scientific method or why it does not fit with our understanding of the climate system. Why shouldn’t media use the same trick to get people interested in science? People like controversy. That is something we cannot change.

    The problem is that you should not just leave a climate sceptic run his Gish Gallop or go unchallenged. But a program that picks up one of their talking points and gives space to discuss the evidence for and against it in sufficient detail would be fine.

  4. gaia.sailboat Says:

    I;m the Founder of the Flat Earth vs the Round Earth Society. We will debate calmly and intelligently.

    Be the first to sign up here.

    Bill – a Flat Earther

  5. Rick Says:

    I remember, even clearer now 20yrs later, when I was studying Electrical Engineering at University doing Philosophy 101. We were the first year that had philosophy added to the course with the intention to opening the eyes of potential engineers to a frame of reference for the decisions we may one day make as engineers.

    The topics I remember were:
    • Value judgments, how our personal values influence what’s right
    • The energy crisis – over reliance of fossil fuels
    • Global warming, The Greenhouse effect, man-made CO2 emissions
    I very much enjoyed the Value judgments topic. Why do we make bridges only 2.5 times stronger than their maximum loading? What makes your decisions and values more important than others? Does it take into account the potential of natural disasters? Excellent stuff!
    The Energy Crisis topic didn’t make as much sense to me. So many loaded learnings. We weren’t philosophizing, we were being brain washed. I could understand that fossil fuel is a finite resource but I also knew, at the same time I was being brain washed, the constant discovery of more and larger deposits that technology was helping us find were being discovered. We were being told to use Nuclear energy, solar, wind, tidal, etc. etc. This only had me think, what is the environmental cost of those resources? Why aren’t we discussing those in philosophy rather than being brain washed? I was sure, even without evidence, the environmental cost of making solar panels was likely to be expensive. Not only were fossil fuels required to make them but how much processing and environmental damage? I knew we weren’t being encouraged to think, but to agree. Anyway, so what if I don’t agree?
    Then the topic of Global Warming. I can’t tell you why, it must have been instinct but the whole topic did not sit right with me. Maybe because it was so accusational? It was our fault! And therefore it was our responsibility to fix it. Nope! I didn’t have anything to do with our current position at 20yrs of age. I knew it wasn’t me, and I was feeling uncomfortable about the whole delivery of this brain washing. I immediately agreed, we probably should stop polluting the planet and reduce our use of fossil fuels but the rest was rubbish.
    I was not happy and there was a lack of scientific evidence. And then, the evidence that was produced? Well it was a chart of the earth’s temperature related to the suns radiation. I don’t have a copy any longer, all these years later and I can’t find it online. What I saw, at least in my mind was a direct correlation between the suns output compared to the earth’s temperature. It was as clear as day for me. I wanted to find evidence to support my gut feeling and the only piece of evidence that seemed to matter, but there was none. Keep in mind the internet wasn’t what it is today. The best thing about the internet at that moment in time was the release of Netscape, so I got to see boobs on the computer. Yes indeed, remember the very first steps into the World Wide Web? I do.
    Needless to say, I failed philosophy. I would not spew their lies. I still say, I have never learned more than I did when I failed philosophy.
    All these years later I have found a growing movement of educated and intelligent people who share my suspicion towards the global warming lie. Ok, I better clarify that comment. The lie that global warming is due to man-made CO2 emissions.
    I encourage everyone to research this for themselves. It shouldn’t be a surprise that I refer you to a community I am involved with SuspiciousObservers.
    Here is Ben’s latest conference which is a great start and overview. Watch this if nothing else. Ben Davidson: The Variable Sun and Its Effects on Earth | EU2014
    Their website contains a wide variety of brilliant information including:
    • Starwater – water comes from stars and every planet has water
    • C(lie)mate – the global warming lie
    • Agenda 21
    Check out the daily SO news on YouTube at
    See weather presented from a space perspective.
    It’s bigger than you think.

  6. Hank Roberts Says:

    > it’s bigger than you think
    You think it’s bigger than the measurements?
    You can look this stuff up, but youtube isn’t the place to go.

  7. Hank Roberts Says:

    Question for Bart and other scientists — is there an assessment of the paleo climate sensitivity vs. contemporary looking at what difference the contemporary rate of change may make in the sequence and kind of events?

    I’m thinking of ocean pH — which is changing fast because CO2 is changing fast, vs. paleo times when a much slower rate allowed the slow step in the ocean’s chemistry to convert dissolved CO2 into other compounds about as fast as it was forming (so pH didn’t go up rapidly). I think that’s established chemistry.

    I’m now moved to wonder about wind patterns driving warm ocean water under ice shelves and glaciers — with a much slower rate of change, would there have been less change in wind, so less change in ocean currents, so less overall melting under the polar and Greenland ice edge? (pure speculation on my part)

    Wondering what other sequences of events might take different — significantly different — pathways, with the contemporary rate of change vs. the paleo.

    That would I think make climate sensitivity different this time?

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