HR: 15:10h
AN: PP22B-07    [PDF]
TI: Equilibrium Tropical Climate Sensitivity Determination from Paleoclimatic Data
AU: * Lea, D W
EM: lea@geol.ucsb.edu
AF: University of California, Department of Geological Sciences, Santa Barbara, CA 93106
AB: The key scientific uncertainty in the global warming debate is the equilibrium climate response to a doubling of atmospheric carbon dioxide. Coupled atmosphere-ocean general circulation models predict a wide range of equilibrium climate sensitivities (2.0 to 5.1 deg C), with a consequently large spread of societal implications. Comparison of models with instrumental data have not been able to reduce the uncertainty in climate sensitivity. An alternative way to gauge equilibrium climate sensitivity is to use paleoclimatic data. Two recent advances, the development and application of proxy recorders of tropical sea surface temperature (SST) and the synchronization of the deep-sea and Antarctic ice-core time scales, make it possible to directly relate past changes in tropical SST to atmospheric carbon dioxide levels. A detailed 360 ky record of past tropical SST variations in the warm (modern SST = 26.2 deg C) eastern equatorial Pacific waters north of the Galapagos Islands was previously obtained from core TR163-19 (2 deg N, 91 deg W: 2348m) (Lea et al., 2000). A new age model has been developed for this record by aligning the benthic O18 record of TR163-19 to nearby core V19-30, which itself has been aligned to Vostok air O18 (Shackleton, 2000). Using this age model, the coherency between the TR163-19 SST record and Vostok CO2 is 0.98 at 100k and 0.94 at 41k (0 phase lag). The strong correspondence of tropical Pacific SST and Vostok CO2 leads to the hypothesis that atmospheric carbon dioxide variations are the dominant control on tropical climate on orbital time scales (100k, 41k). Calibration of the influence of past greenhouse gas variability (CO2 and methane) on tropical SST indicates a tropical climate sensitivity equivalent to 4.4 plus/minus 1 deg C for a 4 W per sq. m radiative increase from doubling of atmospheric pCO2. This result suggests that the equilibrium response of tropical climate to atmospheric CO2 changes might be similar to the upper end of available predictions from coupled models. The two key uncertainties in this study are: 1) the degree to which factors other than greenhouse gases (e.g., ice volume) affect tropical SSTs, which could be tested by GCMs; and, 2) the reliance on a single site with potential regional sensitivity, which could be improved by using multiple tropical SST records.
UR: http://www.geol.ucsb.edu/faculty/lea/
DE: 0325 Evolution of the atmosphere
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting