HR: 09:45h
AN: U51B-08 INVITED    [Abstracts]
TI: Geologic Constraints on Climate Sensitivity
AU: * Royer, D L
EM: droyer@wesleyan.edu
AF: Dept. of Earth & Environmental Sciences, Wesleyan University, Middletown, CT 06459, United States
AU: Berner, R A
EM: robert.berner@yale.edu
AF: Dept. of Geology & Geophysics, Yale University, New Haven, CT 06520, United States
AU: Park, J
EM: jeffrey.park@yale.edu
AF: Dept. of Geology & Geophysics, Yale University, New Haven, CT 06520, United States
AB: A firm understanding of the relationship between CO₂ and temperature is critical for interpreting past climate change and for making predictions of future climate change. A recent synthesis of data spanning the Last Glacial Maximum to present-day suggests a 5-95 % likelihood range of 1.5 to 6.2 °C warming for each doubling of CO₂ (ΔT(2x)). However, some evidence is not consistent with this range; further, CO₂ concentrations and global temperatures have been similar to or lower than the present-day for most of the last several hundred thousand years, and so this past time interval also may not represent the best analog for predicting globally-warm future climates. In an effort to reduce the uncertainty of ΔT(2x), we turned to the Phanerozoic record, an interval that includes times when the Earth was both colder and substantially warmer than the present-day. The GEOCARB and GEOCARBSULF long-term carbon cycle models have been used to calculate multi-million-year patterns of Phanerozoic CO₂. A critical factor in this approach is the effect of atmospheric CO₂ level on the rate of CO₂ uptake by calcium and magnesium silicate mineral weathering. A rise in temperature, accompanying a rise in CO₂, increases the rate of silicate weathering, which in turn accelerates atmospheric CO₂ consumption, forming a negative feedback loop. Using the logarithmic relation between temperature change and CO₂, we examined how different values of ΔT(2x) affect calculated Phanerozoic CO₂ levels for best estimates, and physically reasonable ranges, of all other factors affecting CO₂ in the long-term carbon cycle. We then compared the model calculations against an independent proxy dataset for atmospheric CO₂ that spans the past 420 Myr. Our results that best fit the proxy data are broadly consistent with most climate model calculations and greatly restrict the possibility of a weak radiative forcing by CO₂ (ΔT(2x) < 1.5 °C) over multi-million year timescales. A ΔT(2x) of at least 1.5 °C has likely been a robust feature of the Earth system for the last 420 Myr, independent of temporal scaling.
DE: 0428 Carbon cycling (4806)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1620 Climate dynamics (0429, 3309)
SC: Union [U]
MN: 2007 Fall Meeting