by Nic Lewis
Official estimates of future global warming may be overstated.
A brief press release summary of my new article (in the third person)
One of the most important conclusions of 6order The Intergovernmental Panel on Climate Change (IPCC AR6) Assessment Report aims to reduce uncertainty in climate sensitivity estimates for a doubling of atmospheric carbon dioxide. Since 1979, likely range (66% chance) of climate sensitivity is between 1.5°C and 4.5°C. This range remains wide, until IPCC AR6 narrows likely ranges from 2.5°C to 4.0°C.
A new paper by independent scientist Nic Lewis is published in the journal Climate dynamics challenges the IPCC AR6 conclusions on climate susceptibility. Lewis’ analysis reduces climate sensitivity by a third, compared with the range provided by IPCC AR6. These results suggest that future global warming due to fossil fuel emissions could be significantly less than policymakers predicted.
In 2015, the World Climate Research Program organized a Workshop to reduce uncertainty in estimates of climate sensitivity to increased carbon dioxide. Ultimately, the Workshop resulted in the publication of a report (92 pages) by multiple participants that thoroughly evaluated all lines of evidence (Sherwood et al., 2020). The main result of this paper is to reduce likely climate sensitive value range to 2.6 oC to 3.9 oC. While Lewis was an invitee to the 2015 Workshop, he is not a co-author of this paper. Sherwood et al. The paper strongly influenced IPCC AR6’s assessment of climate sensitivity.
Lewis’ paper critiqued the methods used in Sherwood et al. paper, finding significant errors, inconsistencies, and other omissions. Lewis corrected these shortcomings and also revised the primary input data, almost entirely to reflect more recent evidence. The results of Lewis’ analysis identified a likely range from 1.75 to 2.7oC for climate sensitivity. Center estimate from Lewis analysis is 2.16 oC, much lower than IPCC AR6 likely range. This large reduction compared to Sherwood et al. shows how sensitive climate estimates are to input assumptions. Lewis analysis implies that climate sensitivity is more likely below 2 oC than it is above 2.5 oC.
The lower estimates of climate sensitivity determined by Nic Lewis have profound implications for climate models and projections of 21-year warmingst century. The climate models used in IPCC AR6 have climate sensitivity values ranging from 1.8oC to 5.6oC. IPCC AR6 assesses that some climate models have too high climate sensitivity values. Therefore, AR6 selects only climate models with reasonable values of climate sensitivity for use in the predictions of 21st century climate change. Lewis analysis indicates that the majority of climate models used in IPCC AR6 have higher values likely range.
Nic Lewis is the author of ten peer-reviewed articles on climate susceptibility. Lewis’ latest paper is titled ‘Objectively matching climate-sensitive evidence’. It can be downloaded for free here. A detailed explanation of the article is available here.