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CO 2 forcing induces semi‐direct effects with consequences for climate feedback interpretations
Author(s) -
Andrews Timothy,
Forster Piers M.
Publication year - 2008
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2007gl032273
Subject(s) - forcing (mathematics) , cloud feedback , environmental science , cloud forcing , climatology , radiative forcing , climate model , atmospheric sciences , cloud computing , climate change , climate sensitivity , geology , computer science , oceanography , operating system
Climate forcing and feedbacks are diagnosed from seven slab‐ocean GCMs for 2 × CO 2 using a regression method. Results are compared to those using conventional methodologies to derive a semi‐direct forcing due to tropospheric adjustment, analogous to the semi‐direct effect of absorbing aerosols. All models show a cloud semi‐direct effect, indicating a rapid cloud response to CO 2 ; cloud typically decreases, enhancing the warming. Similarly there is evidence of semi‐direct effects from water‐vapour, lapse‐rate, ice and snow. Previous estimates of climate feedbacks are unlikely to have taken these semi‐direct effects into account and so misinterpret processes as feedbacks that depend only on the forcing, but not the global surface temperature. We show that the actual cloud feedback is smaller than what previous methods suggest and that a significant part of the cloud response and the large spread between previous model estimates of cloud feedback is due to the semi‐direct forcing.