Climate Letter #1161

A new study challenges the last IPCC determination of climate sensitivity.  The IPCC accepts a wide range of global temperature increase estimates based on a potential doubling of the CO2 level, with a median of 3.0C, taken from an assortment of many different models.  The new work basically finds fault with the conventional methods of model construction, which result in unreasonable estimates on the low side.  A different method is proposed and defended which narrows the range of estimates and raises the median to 3.3C.  The authors have solid reputations and their work is likely to receive serious consideration in the next IPCC report.  If widely accepted there is a practical implication because the carbon budget numbers used for staying within the Paris targets would need to be considerably tightened.  This first link is to the abstract of the study, which has a pdf link to the full study, with open access.
https://www.atmos-chem-phys.net/18/5147/2018/acp-18-5147-2018.html

–This next link takes you to a preprint of a follow-up Dessler report that has a clearer expression of what the revised sensitivity numbers look like under the new method.  Scroll down to line 230 for the best statement of justification for the conclusion and how the numbers could be tightened even more.
https://eartharxiv.org/4et67/

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Researchers have a new explanation for large-scale changes in rainfall over tropical forests involving a direct result coming from higher CO2 levels.  The effect reinforces projections that the Amazon forests will become drier in the future while those in Africa and Indonesia will otherwise get wetter.  The lead author of the study “highlighted the fact that the resulting droughts and forest mortality in the Amazon and a potential increase in flooding in other rainforests may have an impact on biodiversity, freshwater availability and food supplies for economically vulnerable populations.”