title: Spread in model climate sensitivity traced to atmospheric convective mixing.
abstract: Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.
authors.affiliation: Climate Change Research Centre and ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney 2052, Australia.
authors.forename: Steven C
authors.initials: SC
authors.lastname: Sherwood
authors.affiliation: Laboratoire de Météorologie Dynamique and Institut Pierre Simon Laplace (LMD/IPSL), CNRS, Université Pierre et Marie Curie, Paris 75252, France.
authors.forename: Sandrine
authors.initials: S
authors.lastname: Bony
authors.affiliation: Laboratoire de Météorologie Dynamique and Institut Pierre Simon Laplace (LMD/IPSL), CNRS, Université Pierre et Marie Curie, Paris 75252, France.
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Dufresne
mesh_terms: D000392:Air Movements; D001272:Atmosphere / Q000737:chemistry*; D002245:Carbon Dioxide / Q000378:metabolism; D002980:Climate; D005246:Feedback; D057232:Global Warming / Q000706:statistics & numerical data*; D008962:Models, Theoretical; D013696:Temperature; D035501:Uncertainty
chemical_list: D002245:Carbon Dioxide