HR: 09:15h
AN: U51B-06    [Abstracts]
TI: Low Climate Sensitivity to Atmospheric CO2 during the Ice Ages
AU: * Toggweiler, J
EM: Robbie.Toggweiler@noaa.gov
AF: Geophysical Fluid Dynamics Laboratory/NOAA, PO Box 308, Princeton, NJ 08542, United States
AB: Ice core records from Antarctica show that atmospheric CO2 has varied between 180 and 280 ppm during the ice ages of the last 500,000 years. The ice cores also show that there is a strong correlation between the temperature of the air over Antarctica and atmospheric CO2 in which the air temperature jumps up by 8-10 deg C as the pCO2 increases at the end of each cold glacial stage. So, what are the ice cores trying to tell us about the climate system's sensitivity to CO2? The point of this presentation is that the ice cores are telling us more about local changes around Antarctica than global sensitivity. In this regard, the two hemispheres seem to be doing different things: the Southern Hemisphere is responding primarily to an internal mechanism with a 100,000-yr period that is focused on Antarctica; the ice sheets in the Northern Hemisphere are responding to the Milankovitch forcing at 23,000 and 41,000 years. d18O records derived from foraminiferal tests incorporate both signals. If the combined record is viewed simply as "ice volume" then one would erroneously conclude that the global climate is varying more strongly with CO2 than it really is. The sawtooth-shaped cycles in the ice cores are a response to an internal feedback that alters the stratification and circulation of the ocean and the winds around Antarctica. CO2 comes out of the ocean when the stratification breaks down and the surface temperature warms (Toggweiler et al., Paleoceanography, 21(2), 2005PA001154, 2006). Thus, there are two components to the warming recorded in the ice cores, one associated with local SST and wind changes and one associated with the greenhouse effect from CO2. It would seem that most of the 8-10 deg. warming, perhaps 6 or 7 deg C, is due to the SST and wind changes. The remainder, 3 deg C or less, is the greenhouse response. The small remainder puts an upper limit on the greenhouse response for the planet as a whole.
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4930 Greenhouse gases
DE: 4932 Ice cores (0724)
SC: Union [U]
MN: 2007 Fall Meeting