Causes of Higher Climate Sensitivity in CMIP6 Models

TLDR: In this article, the authors show that the global surface temperature response to CO2 doubling has increased substantially in the Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8-5.6k across 27 GCMs and exceeding 4.5K in 10 of them.

Abstract: 15 Equilibrium climate sensitivity, the global surface temperature response to CO2 16 doubling, has been persistently uncertain. Recent consensus places it likely within 1.517 4.5K. Global climate models (GCMs), which attempt to represent all relevant physical 18 processes, provide the most direct means of estimating climate sensitivity via CO2 qua19 drupling experiments. Here we show that the closely related effective climate sensitiv20 ity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), 21 with values spanning 1.8-5.6K across 27 GCMs and exceeding 4.5K in 10 of them. This 22 (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks 23 from decreasing extratropical low cloud coverage and albedo. Both of these are tied to 24 the physical representation of clouds which in CMIP6 models lead to weaker responses 25 of extratropical low cloud cover and water content to unforced variations in surface tem26 perature. Establishing the plausibility of these higher sensitivity models is imperative 27 given their implied societal ramifications. 28 Plain Language Summary 29 The severity of climate change is closely related to how much the Earth warms in 30 response to greenhouse gas increases. Here we find that the temperature response to an 31 abrupt quadrupling of atmospheric carbon dioxide has increased substantially in the lat32 est generation of global climate models. This is primarily because low cloud water con33 tent and coverage decrease more strongly with global warming, causing enhanced plan34 etary absorption of sunlight – an amplifying feedback that ultimately results in more warm35 ing. Differences in the physical representation of clouds in models drive this enhanced 36 sensitivity relative to the previous generation of models. It is crucial to establish whether 37 the latest models, which presumably represent the climate system better than their pre38 decessors, are also providing a more realistic picture of future climate warming. 39