Hot off the press, in yesterday’s Journal of Climate, Nic Lewis and Judith Curry have re-calculated the equilibrium climate sensitivity (ECS) based upon the historical uptake of heat into the ocean and human emissions of greenhouse gases and aerosols. ECS is the net warming one expects for doubled atmospheric carbon dioxide. Their ECS ranges from 1.50 to 1.56 degrees Celsius.
Nic has kindly made the manuscript available here, so you don’t have to shell out $35 to the American Meteorological Society for a one-day view.
The paper is a follow-on to their 2015 publication that had a median ECS of 1.65⁰C. It was criticized for not using the latest-greatest “infilled” temperature history (in which less-than-global coverage becomes global using the same data) in order to derive the sensitivity. According to Lewis, writing yesterday on Curry’s blog, the new paper “addresses a range of concerns that have been raised about climate sensitivity estimates” like those in their 2015 paper.
The average ECS from the UN’s Intergovernmental Panel on Climate Change (IPCC) is 3.4⁰C, roughly twice the Lewis and Curry values. It somehow doesn’t seem surprising that the observed rate of warming is now running at about half of the rate in the UN’s models, does it?
Lewis and Curry’s paper appeared seven days after Andrew Dessler and colleagues showed that the mid-atmospheric temperature in the tropics is the best indicator of the earth’s energy balance. This means that any differences between observed and forecast midatmospheric temperatures there can be used to adjust the ECS.
Late last year, University of Alabama’s John Christy and Richard McNider showed that the observed rate of warming in the tropical mid-atmosphere is around 0.13⁰C/decade since 1979, while the model average forecast is 0.30⁰C/decade. This adjusts down the IPCC’s average ECS to the range of 1.5⁰C (actually 1.46⁰).
That’s three estimates of ECS all in the same range, and all approximately half of the UN’s average.
It seems the long-range temperature forecast most consistent with these findings would be about half of what the IPCC is forecasting. That would put total human warming to 2100 right around the top goal of the Paris Accord, or 2.0⁰C.
Stay tuned on this one, because that might be in the net benefit zone.