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 (∆T(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 ∆T(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 ∆T(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 (∆T(2x) < 1.5 °C) over multi-million year timescales. A ∆T(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.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0428 Carbon cycling (4806);
- 0473 Paleoclimatology and paleoceanography (3344;
- 1616 Climate variability (1635;
- 1620 Climate dynamics (0429;