by Nic Lewis reposted from Climate Etc.

Official estimates of future global warming may be overstated.

A brief summary in press release style of my new paper (written in the third person)

One of the most important conclusions of the recent 6th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6) was to reduce the uncertainty in estimates of climate sensitivity to doubling the amount of carbon dioxide in the atmosphere.  Since 1979, the likely range (66% chance) of climate sensitivity has been between 1.5°C and 4.5°C. This range has remained stubbornly wide, until the IPCC AR6 narrowed the likely range to be between 2.5°C and 4.0°C.

A new paper by independent scientist Nic Lewis published in the journal Climate Dynamics challenges the conclusions of the IPCC AR6 about climate sensitivity.  Lewis’ analysis reduces the magnitude of climate sensitivity by one third, relative to the range provided by the IPCC AR6. These results suggest that future global warming in response to fossil fuel emissions could be significantly less than has been assumed by policy makers.

In 2015, the World Climate Research Programme convened a Workshop aimed at reducing the uncertainty in estimates of climate sensitivity to increasing carbon dioxide.  The Workshop ultimately resulted in publication of a report (a 92 page paper) by many of the participants that thoroughly assessed all lines of evidence (Sherwood et al, 2020).  A key result of this paper was to reduce the likely range of climate sensitivity values to 2.6 oC to 3.9 oC.  While Lewis was an invited participant to the 2015 Workshop, he was not a coauthor on this paper.  The Sherwood et al. paper strongly influenced the IPCC AR6’s assessment of climate sensitivity.

Lewis’ paper critiqued the methods used in the Sherwood et al. paper, finding significant errors, inconsistencies and other shortcomings. Lewis remedied these shortcomings and also revised key input data, almost entirely to reflect more recent evidence.  The results of Lewis’ analysis determined a likely range of 1.75 to 2.7oC for climate sensitivity.  The central estimate from Lewis’ analysis is 2.16 oC, which is well below the IPCC AR6 likely range.  This large reduction relative to Sherwood et al. shows how sensitive climate sensitivity estimates are to input assumptions.  Lewis’ analysis implies that climate sensitivity is more likely to be below 2 oC than it is to be above 2.5 oC.

The lower estimates of climate sensitivity determined by Nic Lewis have profound implications for climate models and projections of warming for the 21st century.  Climate models used in the IPCC AR6 had values of climate sensitivity ranging from 1.8oC to 5.6oC.  The IPCC AR6 judged that some of the climate models had values of climate sensitivity that were too high.  Hence the AR6 selected only the climate models with reasonable values of climate sensitivity to be used in projections of 21st century climate change.  Lewis’ analysis indicates that a majority of climate models used in the IPCC AR6 have values higher than the likely range.

Nic Lewis has authored ten peer-reviewed papers on climate sensitivity. Lewis’ latest paper is entitled ‘Objectively combining climate sensitivity evidence’. It can be freely downloaded here. A detailed explanatory article about the paper is available here.