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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

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