State dependence of CO2 forcing and its implications for climate sensitivity

Haozhe He; Ryan J Kramer; Brian J Soden; Nadir Jeevanjee

doi:10.1126/science.abq6872

State dependence of CO₂ forcing and its implications for climate sensitivity

Science. 2023 Dec;382(6674):1051-1056. doi: 10.1126/science.abq6872. Epub 2023 Nov 30.

Authors

Haozhe He¹, Ryan J Kramer^{2

3}, Brian J Soden¹, Nadir Jeevanjee⁴

Affiliations

¹ Rosenstiel School of Marine, Atmospheric and Earth Science, University of Miami, Miami, FL, USA.
² Goddard Earth Science Technology and Research II, University of Maryland at Baltimore County, Baltimore, MD, USA.
³ Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
⁴ Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA.

PMID: 38033059
DOI: 10.1126/science.abq6872

Abstract

When evaluating the effect of carbon dioxide (CO₂) changes on Earth's climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO₂ concentration (IRF_2×CO2) is constant and that variances in climate sensitivity arise from differences in radiative feedbacks or dependence of these feedbacks on the climatological base state. Here, we show that the IRF_2×CO2 is not constant, but rather depends on the climatological base state, increasing by about 25% for every doubling of CO₂, and has increased by about 10% since the preindustrial era primarily due to the cooling within the upper stratosphere, implying a proportionate increase in climate sensitivity. This base-state dependence also explains about half of the intermodel spread in IRF_2×CO2, a problem that has persisted among climate models for nearly three decades.