Tag Archives: James Annan

The Big Bet on Climate Sensitivity

On the front page of this blog is my candidate for the most important risk indicator in the world: the atmospheric concentration of CO2 (currently at around 398 parts per million, or 42% above the pre-industrial level of approximately 280 ppm).

The degree to which the world’s temperature responds to a rise in atmospheric CO2 is captured in a metric called ‘equilibrium climate sensitivity’. This sensitivity number is an estimate of how much global mean temperature will rise should atmospheric CO2 concentration double (in other worlds rise to 560 ppm) such that the heat going into the earth system is back in balance with the heat emitted from the earth system; i.e, the attainment of a new equilibrium.

To put this number is context, the global community has (somewhat arbitrarily) taken a rise of 2 degree Celsius in global mean surface temperature to be regarded as the threshold beyond which the world will experience dangerous climate change. So the critical question then becomes: at what level of CO2 in the atmosphere will we become committed to 2 degrees Celsius plus of warming? By extension, should our best estimate of equilibrium climate sensitivity be 2, then we will cross the dangerous climate threshold of 2 degrees of warming only if we double atmospheric CO2. If the equilibrium sensitivity number were 3, we would cross the dangerous climate change threshold at a far lower level of atmospheric CO2. In short, a low sensitivity number is good, a high one bad.

In 2007, the Intergovernmental Panel on Climate Change (IPCC) published its latest estimate for equilibrium climate sensitivity in its Fourth Assessment Report (AR4). This report is taken by policy makers to be the consensus view of climate scientists at a particular time. And here is the best estimate as of 2007:

Equilibrium climate sensitivity is likely to be in the range 2°C to 4.5°C with a most likely value of about 3°C, based upon multiple observational and modelling constraints. It is very unlikely to be less than 1.5°C.

Surprisingly, despite a plethora of papers and the advancement of computer modelling, the climate sensitivity number has hardly moved over the years. But we now appear to have the makings of a new consensus that the climate sensitivity number may be somewhat lower than the 3 degree best estimate agreed upon in 2007.

If true, this is certainly good news and should be applauded. The latest paper supporting a slightly lower climate sensitivity number is that of Otto et al., which was published in Nature Geoscience. Unfortunately, the original paper is behind a paywall, but, realising the importance of the paper, Nature has published an open access synopsis (which they term Supplementary Information) that can be found here.

One of the authors of the study is Nic Lewis, who has previously published work suggesting a much lower equilibrium climate sensitivity number than in the IPCC’s 2007 report. Lewis stresses (here) the credentials of the authors in the new Nature Geoscience paper, including the fact that many of them are deeply involved in the creation of the IPCC’s Fifth Assessment Report (AR5), to be published in 2014:

The authors include fourteen climate scientists, well known in their fields, who are lead or coordinating lead authors of IPCC AR5 WG1 chapters that are relevant to estimating climate sensitivity. Two of them, professors Myles Allen and Gabi Hegerl, are lead authors for Chapter 10, which deals with estimates of ECS and TCR constrained by observational evidence. The study was principally carried out by a researcher, Alex Otto, who works in Myles Allen’s group.

In sum, this is a legitimate paper and doesn’t emanate from a closet libertarian fruitcake or some embittered contrarian loon who was passed over for tenure.

Lewis has also helpfully produced a graphic showing the climate sensitivity estimates based on various observational periods (as to why he is putting this onto the climate skeptic blogs Bishop Hill and Watts Up With That I can’t quite fathom):

ECS Estimates jpeg

Helpfully, the graphic also contains box and whisper plots, which Lewis describes thus: Continue reading