Evidence for Changing Climate Sensitivity in the Geologic Past
Abstract
The response of climate to past changes in atmospheric greenhouse gas composition can be used to assess Earth System sensitivity. Unlike Charney climate sensitivity, which is related to the strength of feedbacks involving short timescale climate processes such as those involving clouds and water vapor, Earth System sensitivity also integrates feedbacks involving long timescale changes in the cryosphere, terrestrial vegetation, and deep ocean circulation. We show that paleoclimate data from the Miocene, Pliocene and Pleistocene epochs provide evidence for changing sensitivity through time, probably due to changing boundary conditions due to tectonics. In the middle Miocene (~18-12 Ma), major climate change trends appear to be coupled to pCO2 changes; however, in the late Miocene (~12-5 Ma), climate was warmer than today even while pCO2 was similar to today, indicating a decoupling between long-term climate evolution and pCO2 change. In the Pliocene and Pleistocene, there appears to have been strong coupling between climate and pCO2 changes; there is a wide range of Earth System sensitivity values, all of which exceed or are at the high end of Charney and Earth System sensitivity estimates derived from climate models. In the early Pliocene warm period, pCO2 was 350-400 ppm, implying an Earth System sensitivity (temperature change for a doubling of pCO2) of 7-9C. Subsequently, large Northern Hemisphere glaciations began to occur when pCO2 dropped below about 300 ppm in the middle Pliocene. For the Late Pleistocene ice ages, estimates of tropical sensitivity are approximately 3-4C for a doubling of pCO2, which we suggest represents a minimum value for global sensitivity during the last ~500,000 years. Overall, paleoclimate data have important implications. First, tectonic changes in basin shape impacted ocean circulation and climate sensitivity through the Miocene and possibly the earliest Pliocene, indicating that the initial conditions of the mean oceanic state plays a role in the climate response to pCO2 change. Second, to explain why early Pliocene estimates of Earth System sensitivity are so high, it may be important to improve how the large observed changes in upper ocean circulation are simulated by models. Third, in a period when ocean basins were similar to modern, ice age climate sensitivity to pCO2 changes is underestimated by climate models even when long term changes in solar forcing and ice sheet size and distribution are taken into account, implying that internal positive feedbacks are stronger than previously thought.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A14C..01R
- Keywords:
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- 3344 ATMOSPHERIC PROCESSES / Paleoclimatology;
- 4999 PALEOCEANOGRAPHY / General or miscellaneous