Graphic showing a thermometer rising, with a 2 written at the top to imply a 2 degree temperature rise

The lower the climate sensitivity the better – but what we need is zero carbon

Graphic showing a thermometer rising, with a 2 written at the top to imply a 2 degree temperature rise

Following the publication of a paper presenting a new narrower estimate of “equilibrium climate sensitivity” – a measure of how future greenhouse gas emissions could alter the climate – Professor Joanna Haigh, co-director of the Grantham Institute, explains the implications of climate sensitivity and why it should be interpreted carefully.

What concerns me about a recent paper published in Nature is the interpretation of its results by some commentators. The findings have been pounced on by some as an indication that climate scientists have been exaggerating the risk associated with greenhouse gas increases. Even some climate scientists have concluded that “the risk of very high surface temperature changes occurring in the future will decrease”. But this can only be the case if carbon dioxide (CO2) emissions cease.

When we think about the future of the Earth’s climate the first thing to consider is how concentrations of atmospheric greenhouse gases will change; the next is how the climate will respond – aka the climate sensitivity. Estimating climate sensitivity is difficult – we need to know not only the direct impact of greenhouse gases trapping heat radiation, but also the impact of knock-on effects such as changes in humidity, cloud, ice, and the broader carbon cycle, including plant species and cover.

‘Equilibrium Climate Sensitivity’ is an estimate of the increase in average global temperatures that would occur when the Earth has fully adjusted to atmospheric CO2 doubling in concentration from pre-industrial levels. A range of different methods have been employed to calculate ECS, using observational records of CO2 concentration and temperature. The models used range from simple energy balance considerations to complex computer simulations of the whole climate system, but all methods need to include assumptions of one type or another.

The 2013 report of the Intergovernmental Panel on Climate Change suggested that ECS lies between 1.5 and 4.5°C. However, the paper published last week suggests a narrowing of this range to between 2.2 and 3.4°C. It is not for me here to discuss the merits of that study, though I note its range still lies within that of the IPCC – and it is certainly not the last word on this issue.

What is important to note is that, while ECS gives an indication of climate sensitivity to increasing greenhouse gases, it is not very useful as a predictor of actual temperature. Firstly adjustment is very slow, and surface temperatures will continue to rise well after the date of the doubling. Secondly it assumes the concentrations of greenhouse gases have stabilised. An easier to visualise perspective is given by the idea of burnable carbon: there is a limit to the amount of CO2 that we can allow to accumulate in the atmosphere if we wish to avoid dangerous levels of warming. The greater the rate of CO2 emissions, the sooner that threshold will be reached.

Warming can only be halted if CO2 emissions cease. Of course, a lower ECS means that warming is slower, but it must not be interpreted as a maximum possible temperature increase. As long as we go on pumping greenhouse gases into the atmosphere, the temperature will rise and rise inexorably. We need to stop.

Find out more about Grantham Institute research on low-carbon pathways here.

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