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WUN Videoconference: Prof. Jim Kasting, Penn State University
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WUN Videoconference: Prof. Jim Kasting, Penn State University
"Atmospheric composition and climate on the early earth" Abstract: Earth's atmosphere and climate have both undergone marked changes during the course of Earth's history. Our climate has remained warm most of the time even though the Sun was considerably fainter in the distant past. This warmth was almost certainly caused by enhanced greenhouse gas concentrations. CO2 and H2O were probably the dominant greenhouse gases initially, but CH4 may have become an important contributor soon after life evolved, with atmospheric concentrations exceeding 1000 ppmv. Revised calculations show that the greenhouse effect of methane is smaller than previously estimated. However, C2H6 (ethane), formed photochemically from CH4, may also have been an important greenhouse gas. If the atmospheric CH4/CO2 was greater than a few tenths, organic haze should have formed, and this should have cooled the surface by creating an anti-greenhouse effect. Stability was achieved, in Daisyworld fashion, when an optically thin haze was present. Indirect evidence for the existence of such a haze comes from studies of mass independent fractionation (MIF) in S isotopes from ancient rocks. The rise in atmospheric O2 at ~2.4 Ga, marked by the disappearance of the MIF signal, caused a decrease in methane, probably triggering the Paleoproterozoic glaciations. An earlier, less well documented glaciation at 2.8-2.9 Ga corresponds to a second anomaly in the MIF record and may also have been triggered by biological innovations. February 20, 2008 12:00 pm - 1:00 pm 2422 SRB - Donahue Room |
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