Climatic impact of spectrally resolved irradiances during the late Archean as modeled with EMAC-FUB

Details Climatic impact of spectrally resolved irradiances during the late Archean as modeled with EMAC-FUB Climatic impact of spectrally resolved irradiances during the late Archean as modeled with EMAC-FUB

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21b1659k&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P21B-1659

Biology

[0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [3359] Atmospheric Processes / Radiative Processes, [5225] Planetary Sciences: Astrobiology / Early Environment Of Earth

Scientific paper

During the Archean eon the surface temperatures of the Earth are assumed to have been high enough to support liquid water, despite a lower luminosity of the young Sun. This fact, known as the faint young Sun paradox, can be explained by assuming higher concentrations of greenhouse gases during the early stages of the Earth. But there is still an ongoing debate about the possible range of greenhouse gas concentrations that are consistent with the geologic evidence. We present a study in which we investigate this problem using the Chemistry Climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) in a resolution of T42/L39 with the high-resolution shortwave radiation scheme FUBRad (EMAC-FUB). We are using a constructed, spectrally resolved irradiance dataset valid for the Archean Sun, and analyze the climatic impact of the reduced solar luminosity, an anoxic environment, an increased CO2 concentration, and the different land mass. In total six simulations have been performed, where two simulations only differ by the O2 and O3 content and otherwise have present day conditions. Four simulations use a global ocean, as the distribution and fraction of the continents are highly uncertain during the Archean, and anoxic conditions. Three simulations use a reduced solar luminosity, where two CO2 scenarios are tested (3 ± PAL and 10 ± PAL). As proxy for the early Sun during the late Archean at 2.5 Ga (109 years ago) we take the dwarf star β Com. The spectrally resolved irradiances are compiled from measurements and modeled data, and scaled to a total solar irradiance (TSI) of 82 % the present TSI (i.e. 1121 W m-2). We show that in an anoxic environment with reduced solar luminosity at 2.5 Ga, a global ocean, and present day greenhouse gases, it is still possible to have liquid water in tropical latitudes, even though the global, annual mean surface temperature is below the freezing point of water. When the CO2 concentration is increased, the region of open water widens. The removal of oxygen and ozone has a large impact on the thermal structure of the upper atmosphere (cooling by 140 K) and the dynamics, i.e. the polar night jet vanishes and the Hadley circulation intensifies.

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