Official IPCC estimates of future global warming may be overestimated
London, September 20 – A new paper reduces the estimate of climate sensitivity – the amount of warming expected for a doubling of carbon dioxide concentrations – by one third. The results therefore suggest that future global warming will be much less than expected.
The paper, by independent scientist Nic Lewis, has just appeared in the journal Climate dynamics. This is an important challenge to the official view of the Intergovernmental Panel on Climate Change (IPCC).
Lewis criticized a 2020 assessment of climate sensitivity by Sherwood et al., which heavily influenced the IPCC’s Sixth Assessment Report, in 2021. Lewis commented:
“It is unfortunate that Sherwood et al. ‘s assessment of climate sensitivity, which supported the UN Framework Convention on Climate Change, contained such serious errors, inconsistencies and shortcomings in its methods”.
After the Sherwood et al. methods and revising key input data to mainly reflect more recent evidence, the central estimate for climate sensitivity comes down from 3.1°C per doubling of CO2 concentration in the original study to 2.16°C in the new paper.
This large reduction shows how sensitive climate sensitivity estimates still are to input assumptions, and that values between 1.5°C and 2°C remain quite plausible.
- Climate sensitivity represents the long-term global temperature increase caused by a doubling of atmospheric CO2 concentration. There are different measures of climate sensitivity. Both the Sherwood and Lewis papers estimate the so-called ‘effective’ climate sensitivity, which reflects a new equilibrium state projected from century-long changes to a doubling of the CO2 concentration. This measure is considered the most relevant one for predicting climate change in the coming two centuries.
- Climate sensitivity has always been a very important, but also highly uncertain parameter in the climate change discourse. Earlier IPCC reports estimated that its value was likely to be somewhere between 1.5 °C and 4.5 °C, with a best estimate of 3 °C. Following the Sherwood paper, however, the 2021 Sixth Assessment Report shifted that range upwards, to 2.5 to 4 °C. While this may sound boring to outsiders, to insiders it was a revolutionary change.
- Lewis’ corrections and revisions lead to a likely range of 1.75 to 2.7 °C, which is not only lower but also much less uncertain than either the 2021 official IPCC assessment or the very similar Sherwood et al . estimate (2.6 to 3.9°C).
- Nic Lewis is the lead or sole author of ten peer-reviewed papers on climate sensitivity. He was a participant in the 2015 workshop that kicked off the World Climate Research Program project that led to the Sherwood et al. 2020 paper, but he was not a co-author of that paper.
“The significant reduction in assessed climate sensitivity when updating key input data suggests that the increase in the lower end of the climate sensitivity range in the IPCC Sixth Assessment Report was unwarranted.”
Lewis’s paper is entitled ‘Objective combination of climate sensitivity evidence’. It can be downloaded freely here. A detailed explanatory article on the question paper is available here.