Climate Stability (and Instability) on Long Time Scales

Tuesday, October 30, 2001 - 9:30am - 10:30am
Keller 3-180
James Kasting (The Pennsylvania State University)
On billion-year time scales, the classic question for climatologists is the so-called faint young Sun problem: How did Earth's climate remain warm despite a solar luminosity decrease of 20-30 percent compared to today? The likely answer is that the atmospheric greenhouse effect was much larger as a consequence of higher concentrations of volcanogenic CO2 and biogenic CH4. Both of these gases are involved in negative feedback loops that contribute to long-term climate stability. That said, the controls appear to have broken down several times in Earth's history, leading to periods of global, or near-global, glaciation. The first of these Snowball Earth episodes occurred at ~2.3 Ga. It coincides precisely with the initial rise of atmospheric O2 and was probably triggered by the loss of the methane component of the atmospheric greenhouse. After an ice-free interval of more than 1.5 billion years, global glaciation recurred near the end of the Proterozoic at 750 Ma and 600 Ma. The cause of these latter two Snowball Earth episodes is less clear. Possible triggering mechanisms include drawdown of atmospheric CO2 by weathering of equatorially-situated continents, or further drawdown of CH4 caused by rising O2 and sulfate levels and corresponding increases in methanotrophic and sulfate-reducing bacteria. All Snowball Earth glaciations were presumably ended within ~10 million years by the buildup of volcanic CO2. The question of how Earth's biota managed to survive these climate catastrophes will be discussed.