Tag Archives: Myles Allen

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

Siberian Permafrost Thaw and Risk Revisited (and Corrected)

Two weeks ago, I posted on a new paper by Dr Anton Vaks and colleagues looking at permafrost thaw in the context of overall climate risk. In that post, I talked about a 1.5 degrees Celsius rise in global mean temperature from today setting off significant permafrost thaw and carbon release.

After exchanging e-mails with Anton Vaks, the lead author of the report, I found that the correct number is a 1.5 degrees Celsius rise from pre-industrial levels. Given that we have already warmed by about 0.7 to 0.8 degrees Celsius from pre-industrial levels as of now, that puts the tipping point only around 0.8 degrees Celsius further away.

This is an important, and a very negative, correction—and it has massive risk implications. At the end of the post, I will explain why much of the media and blogosphere interpreted the paper incorrectly (including myself), but first I will look at the more important question of what a lower hurdle for permafrost thaw means.

Let’s start by reporting the relevant passages of the paper itself (note that the paper is behind a paywall):

We reconstruct the history of Siberian permafrost (and the aridity of the Gobi Desert) during the last ~500 kyr using U-Th dating of speleothems in six caves along a north- south transect in northern Asia from Eastern Siberia at 60.2°N to the Gobi Desert at 42.5°N.

Speleothems are mineral deposits formed when water seeps into a cave from surrounding bedrock and earth. If the surrounding bedrock and earth is frozen, you get no water seepage and no speleothem formation. So when an interglacial period reaches a sufficiently warm level, permafrost melts and speleothems form. U-Th dating refers to uranium-thorium dating that is accurate up to around 500,000 years.

The interglacials for the last 800,000 years can be seen in the following chart (not take from the paper, source here, click for larger image):

Interglacials jpeg

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The Trillionth Tonne

Communicating climate risk to non-specialists is not easy. Nonetheless, I think it is possible. In my own personal journey to understanding the risks my family and myself face, I have found that  getting to grips with the idea of a carbon budget has been vital. So I have a great deal of gratitude to those scientists who have thought long and hard about how to highlight the link between carbon and temperature change.

The carbon budget concept first found a wider audience in the journal Nature with the publication of two papers led by Myles Allen et al here, and Meinshausen et al here. A less technical commentary piece entitled “The Exit Strategy” also accompanied these two papers and is an absolute must-read for any thinking person.

The central tenet behind these papers is that only a limited amount of fossil-fuel carbon can burnt and turn into CO2 before we are committed to warming the earth by 2 degrees Celsius. Given our current state of knowledge, Myles Allen and his colleagues also suggest that our current carbon budget is one trillion tonnes (or rather this is their best estimate of what can be released). The time path over which that trillion tonnes of carbon is emitted has almost no bearing on the level of actual warming due to the lags of temperature change to CO2 and the fact that CO2 resides in the atmosphere for so long (click for larger image).

CO2 Emissions Paths jpg
Note that they tackle the question of climate sensitivity to CO2 somewhat differently from the approach taken by the Intergovernmental Panel on Climate Change (IPCC) . In short, the IPCC defines climate sensitivity as the rise in global mean temperature based on a doubling of atmospheric CO2 from pre-industrial levels. The preferred metric of Allen and his colleagues is how much global mean temperature rises per one trillion tonnes of carbon.

Helpfully, Oxford University hosts a web site based on this methodology telling us how far we are along the way to burning that trillionth tonne. The answer is here:

Trillionth Tonne jpg

Continue reading