A Closer Look at Moderating Views of Climate Sensitivity

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“ Worse than we thought” has been one of the most durable phrases lately among those pushing for urgent action to stem the buildup of greenhouse gases linked to global warming.

But on one critically important metric — how hot the planet will get from a doubling of the pre-industrial concentration of greenhouse gases, a k a “climate sensitivity” — some climate researchers with substantial publication records are shifting toward the lower end of the warming spectrum.

There’s still plenty of global warming and centuries of coastal retreats in the pipeline, so this is hardly a “benign” situation, as some have cast it.

But while plenty of other climate scientists hold firm to the idea that the full range of possible outcomes, including a disruptively dangerous warming of more than 4.5 degrees C. (8 degrees F.), remain in play, it’s getting harder to see why the high-end projections are given much weight.

This is also not a “single-study syndrome” situation, where one outlier research paper is used to cast doubt on a bigger body of work — as Skeptical Science asserted over the weekend. That post focused on the as-yet-unpublished paper finding lower sensitivity that was inadvisedly promoted recently by the Research Council of Norway.

In fact, there is an accumulating body of reviewed, published research shaving away the high end of the range of possible warming estimates from doubled carbon dioxide levels. Chief among climate scientists critical of the high-sensitivity holdouts is James Annan, an experienced climate modeler based in Japan who contributed to the 2007 science report from the Intergovernmental Panel on Climate Change. By 2006, he was already diverging from his colleagues a bit. That’s when he wrote this:

Climate sensitivity is 3C…. Plus or minus a little bit, of course. But not plus or minus as much as some people have been claiming in recent years :-)

The 3C, of course, is 3 degrees C. (5.4 degrees F.). The piece described the findings in his 2006 Geophysical Research Letters paper with Julia Hargreaves, “ Using multiple observationally-based constraints to estimate climate sensitivity.”

He’s reinforced his view in light of the latest research and temperature patterns. On Jan. 27, he posted a comment on Dot Earth that in the last few days has resurfaced in many places around the Web. Here’s the most important line from Annan’s Dot Earth comment, in which he notes how recent events point to less warming from a given buildup of carbon dioxide: 

[T]here have now been several recent papers showing much the same – numerous factors including: the increase in positive forcing (CO2 and the recent work on black carbon), decrease in estimated negative forcing (aerosols), combined with the stubborn refusal of the planet to warm as had been predicted over the last decade, all makes a high climate sensitivity increasingly untenable. A value (slightly) under 2 is certainly looking a whole lot more plausible than anything above 4.5.

And here’s an excerpt from “A Sensitive Matter,” a must-read post on his blog on Feb. 1:

So, sensitivity has been in the climate blogosphere a bit recently. Just a few days ago, that odd Norwegian press release got some people excited, but it’s not clear what it really means. There is an Aldrin et al paper, published some time ago – which gave a decent constraint on climate sensitivity, though nothing particularly surprising or interesting IMO. We thought we had sorted out the  sensitivity kerfuffle several years ago, but it seems that the rest of the world still hasn’t yet caught up. As I said to Andy Revkin (and he published on his blog), the additional decade of temperature data from 2000 onwards (even the AR4 estimates typically ignored the post-2000 years) can only work to reduce estimates of sensitivity, and that’s before we even consider the reduction in estimates of negative aerosol forcing, and additional forcing from black carbon (the latter being very new, is not included in any calculations AIUI). It’s increasingly difficult to reconcile a high climate sensitivity (say over 4C) with the observational evidence for the planetary energy balance over the industrial era.

If you care about this heated, consequential question, I encourage you to read Annan’s full post, which includes a section on a kind of tribalism that takes hold in situations like this and that, he says, could affect the conclusions of the next Intergovernmental Panel on Climate Change report on the basics of greenhouse heating.

Of course, I may still be exhibiting “reverse tribalism” even by digging in here, but at least I’m stating that up front.

The reason it’s worth working to clarify what’s going on is that a lower climate sensitivity could substantially expand the timescale on which decarbonization of humanity’s energy menu would need to take place to blunt climate change. This could raise the odds of a Thornton Wilder ending to our “ large-scale geophysical experiment.”

Over the weekend, William Connolley, the prickly and provocative author of the climate-focused blog Stoat, warned against overinterpretation of Annan’s comments by climate “septics” (his spelling). By e-mail I asked Connolley to offer his view of whether the high climate sensitivity estimates (often referred to as the “long tail” of the range of warming possibilities) still had weight. He replied:

Thanks for asking. My personal opinion is that the “long tail” should be given little weight. However, I should immeadiately say that I’m heavily influenced by James Annan — I’ve known him (online) for years and have come to trust his opinions. I’d also be strongly influenced by Gavin [Gavin Schmidt of NASA and the Realclimate.org blog], too, but I don’t think they greatly differ — but James can be rather more outspoken.

My feeling, on the “policy” side, is that the “long tail” remains rather convenient and people are reluctant to let it go.

James also identifies a possible problem in the way IPCC subgroups can come to “own” a particular area, and find outside opinions — even those clearly from within Science rather than the wackosphere — unwelcome. I don’t know how serious that is: again, I’d be inclined to trust James Annan on this, but that’s all I’d be doing. Perhaps an investigative journalist might take an interest.

For the record, I am interested and have various queries out on how “consensus” is determined in the climate panel’s writing process (something that’s been explored here before).

And, once again, don’t get me wrong. Even with almost no chance of the high end of climate sensitivity estimates being right, the odds of substantial, prolonged and disruptive climate change (and changes in ocean chemistry) are still plenty high enough to justify a sustained push toward an energy menu that works for the long haul.

And given the inherent wide range of people’s feelings of risk, I also don’t expect the evolving science to eliminate debate over how fast to push and how much to spend.

Finally, it’s especially important to keep pushing toward new energy norms given how little humanity has done so far to shift from unfettered fuel burning.

Anyone who’s ever gotten an extension on a tough homework assignment or paper knows how that can work out — simply with a later all-nighter. (See a 2005 discussion of climate policy in the context of homework here.)

The stakes here are far higher than in potentially flunking out of a course.

For these reasons, I can understand why some climate campaigners, writers and scientists don’t want to focus on any science hinting that there might be a bit more time to make this profound energy transition. (There’s also reluctance, I’m sure, because the recent work is trending toward the published low sensitivity findings from a decade ago from climate scientists best known for their relationships with libertarian groups.)

Nonetheless, the science is what the science is.

Thus, this post.

Gavin Schmidt of NASA and the Realcimate blog sent this note as part of a group e-mail exchange:

Andy, I think you may be slightly misrepresenting where the ‘consensus’ on this issue has been. While there have been occasional papers that have shown a large tail, and some arguments that this is stubborn – particular from constraints based on the modern tranisent changes – there has always been substantial evidence to rule these out. Even going back to the 2-11deg C range found in the initial cpdn results in 2005, many people said immediately that the high end was untenable (for instance).

Indeed, the consensus statements in the IPCC reports have remained within the 1.5 – 4.5 range first set by Charney in 1979. James’ work has helped improve the quantifications of the paleo constraints (particular for the LGM), but these have been supported by work from Lorius et al (1991), Kohler et al (2010), etc. and therefore are not particularly radical.

By not reflecting that, you are implying that the wishful thinking of people like Ridley and Lindzen for a climate sensitivity of around 1 deg C is tenable. It is not, and James’ statement was simply alluding to that. For reference, James stated that his favored number was around 2.5 deg C, Jim Hansen in a recent letter to the WSJ quote 2.5-3.5 (based on the recent Palaeosens paper), and for what it’s worth the CMIP5 GISS models have sensitivities of 2.4 to 2.7 deg C. None of this is out of the mainstream.

I sent Schmidt and the group this reply:

In policy circles, including popular calculations of emissions trajectories necessary to avoid a high chance of exceeding 2 degrees C. of warming, the hot tail has not been trimmed (unless I’m missing something?).

To me, that says the climate science community — including the Intergovernmental Panel on Climate Change science working group — has not adequately conveyed the reality you state here.

Dana Nuccitelli has posted a long useful piece at Skeptical Science exploring the issues raised by Annan and others on the sensitivity question. Here’s an excerpt:

It’s something of a mystery why the climate contrarian blogosphere lit up in response to Annan’s fairly mainstream, uncontroversial comments, other than the fact that he was rather critical of the IPCC.  Annan said equilibrium climate sensitivity is unlikely to be higher than 4.5°C – there are few if any mainstream climate scientists who would disagree with this.  He also said that sensitivity is unlikely to be much less than 2°C.  This rules out the beliefs of many prominent climate scientist contrarians, like Roy Spencer (who believes equilibrium sensitivity is around 1.3°C) and Richard Lindzen (who believes it’s less than 1°C).

In short, the comments Annan made which were celebrated by climate contrarians are incompatible with the beliefs of the most prominent climate contrarian scientists, but entirely compatible with mainstream climate scientists like James Hansen.

April 24, 12:09 p.m. | Update |

A new paper in the Journal of Climate by Nic Lewis is in line with other work pointing to a lower climate sensitivity. (The Skeptical Science and Bishop Hill blogs are disputing whether Lewis’s new paper is an outlier or signaling an emerging consensus.)

Here’s the title and link: “An objective Bayesian, improved approach for applying optimal fingerprint techniques to estimate climate sensitivity.”

From the abstract:

A detailed reanalysis is presented of a ‘Bayesian’ climate parameter study (Forest et al., 2006) that estimates climate sensitivity (ECS) jointly with effective ocean diffusivity and aerosol forcing, using optimal fingerprints to compare multi-decadal observations with simulations by the MIT 2D climate model at varying settings of the three climate parameters. Use of improved methodology primarily accounts for the 90% confidence bounds for ECS reducing from 2.1–8.9 K to 2.0–3.6 K….