POLAR STRATOSPHERIC CLOUDS - A HIGH-LATITUDE WARMING MECHANISM IN AN ANCIENT GREENHOUSE WORLD

The presence of water vapor clouds in the stratosphere produces warmin g in excess of tropospheric greenhouse warming, via radiative warming in the lower stratosphere. The stratospheric clouds form only in regio ns of very low temperature and so the warming produced by the clouds i s concentrated in polar winter regions. Results from a paleoclimate mo deling study that includes idealized, prescribed polar stratospheric c louds (PSCs) show that the clouds cause up to 20 degrees C of warming at high latitude surfaces of the winter hemisphere, with greatest impa ct in oceanic regions where sea ice is reduced. The modeled temperatur e response suggests that PSCs may have been a significant climate forc ing factor for past time intervals associated with high concentrations of atmospheric methane. The clouds and associated warming may help to explain long-standing discrepancies between model-produced paleotempe ratures and geologic proxy temperature interpretations at high latitud es, a persistent problem in studies of ancient greenhouse climates.