2005 Annual Science Report
Pennsylvania State University Reporting | JUL 2004 – JUN 2005
Modeling Early Atmospheric Composition and Climate
Pushker Kharecha, James Kasting, and Janet Siefert have written a paper, currently submitted to Geobiology, that describes a coupled model of the anaerobic, early Archean biosphere, prior to the origin of oxygenic photosynthesis. The model includes organisms that metabolize using H2, H2S, and Fe++ as reductants. A primary goal was to estimate the production rate of methane. We find that for reasonable assumptions about hydrogen outgassing rates and the hydrogen escape rate to space, CH4 production should have been within a factor of 3 of the modern value, ~500 Tg CH4/yr. In the low-O2 Archean atmosphere, this would have been enough to produce CH4 concentrations of 1000 ppmv, or higher if the escape rate was slower than assumed. Such CH4 concentrations could have made a major contribution to Earth’s greenhouse effect, helping to keep the climate warm despite reduced solar luminosity at that time. Prior to the origin of anoxygenic photosynthesis, primary production by H2-using methanogens could have produced extremely high CH4 concentrations, several percent or more. Greenhouse warming might then have produced a hot early Earth, ~70°C, perhaps explaining the observed predominance of hyperthermophiles near the base of the rRNA tree.
More recently, a graduate student, Irene Schneider, and an undergraduate, Patrick Kasting, have derived a new set of near-IR absorption coefficients for CH4. This should allow us to improve our estimates of the greenhouse effect of CH4-rich atmospheres.
Also, climate modeler David Pollard and I have a paper in press in JGR concerning Snowball Earth. In it, we argue that “thin-ice” solutions like the ones proposed by Chris McKay in GRL (2000) are plausible, even likely, in the tropics. This may explain how photosynthetic algae and other light-dependent organisms survived the Neoproterozoic glaciations.
PROJECT INVESTIGATORS:James Kasting
PROJECT MEMBERS:Dave Pollard
RELATED OBJECTIVES:Objective 1.1
Models of formation and evolution of habitable planets
Earth's early biosphere