Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21c1676p&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P21C-1676
Physics
[3310] Atmospheric Processes / Clouds And Cloud Feedbacks, [3359] Atmospheric Processes / Radiative Processes
Scientific paper
Several studies have shown that for certain simple atmospheric models consisting of at least one greenhouse gas, the outgoing longwave radiation has an upper limit. If the incoming shortwave radiation exceeds this upper limit, a Runaway Greenhouse occurs, since there is a steady gain in energy by the atmosphere. Therefore the maximum distance between a planet and a star at which a Runaway Greenhouse occurs, may define the inner limit to the habitable zone of solar systems. Furthermore a Runaway Greenhouse may have occurred in the early history of Venus as well as on the extra-solar planet GJ 1214b and may occur on Earth in the distant future. Most previous studies on the Runaway Greenhouse were conducted with conceptual equilibrium models with either a very simple representation of clouds or with no cloud representation at all. Therefore the influence of clouds and especially of the cloud-albedo feedback on the onset of a Runaway Greenhouse is still poorly understood. The cloud-albedo feedback may, however, change the value of the critical incoming shortwave radiation as well as give rise to multiple equilibria. In the present study we use a model with an interactive cloud representation, namely a modified single-column version of the general circulation model ECHAM6. Our atmospheric configuration corresponds to Earth-like conditions, but with increased incoming shortwave radiation. We consider only the early phase of a Runaway Greenhouse, that is, before the surface temperature reaches the boiling temperature of water. In our first experiments we changed the incoming shortwave radiation by shifting the geographical latitude of the column. This allowed us to increase the average incoming shortwave radiation to a maximum value of 416 Wm-2 by placing the column at the equator. For this value of the incoming shortwave radiation, we find a stable equilibrium at temperatures as high as in the 360's K. Cloud albedo is markedly higher for these high temperatures, which stabilizes the climate. If, by contrast, the clouds are set to be transparent to radiation and the surface albedo is correspondingly higher to obtain the same control climate, an incoming shortwave radiation of 420 Wm-2 leads to a Runaway Greenhouse. Our results thus indicate that the critical incoming shortwave radiation is crucially influenced by cloud feedbacks.
Marotzke Johannes
Popp Markus
Schmidt Harry
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