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Simpson's Law and the Spectral Cancellation of Climate Feedbacks
  • Nadir Jeevanjee,
  • Nicholas Lutsko,
  • Daniel Koll
Nadir Jeevanjee
Geophysical Fluid Dynamics Laboratory, Geophysical Fluid Dynamics Laboratory

Corresponding Author:nadir.jeevanjee@noaa.gov

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Nicholas Lutsko
Scripps Institution of Oceanography., Scripps Institution of Oceanography.
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Daniel Koll
Massachusetts Institute of Technology, Massachusetts Institute of Technology
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We spectrally resolve the conventional clear-sky temperature and water vapor feedbacks in an idealized single-column framework, and show that the well-known partial compensation of these feedbacks is actually due to an almost perfect cancellation of the spectral feedbacks at wavenumbers where H2O is optically thick. This cancellation is a natural consequence of ‘Simpson’s Law’, which says that H2O emission temperatures do not change with surface warming if RH is fixed. This cancellation is eliminated for the alternative RH-based feedbacks proposed by Held and Shell (2012). These results bolster the case for switching from conventional to RH-based feedbacks. We also find that the RH-based clear-sky lapse rate feedback is negligible, so the impact of changing lapse rates depends crucially on whether relative or specific humidity is held fixed.