Three-dimensional Climate Simulations of Moist Greenhouse Atmospheres.

Friday, 19 December 2014
Eric T Wolf, University of Colorado at Boulder, Boulder, CO, United States and O. Brian Toon, University of Colorado at Boulder, Department of Atmospheric and Oceanic Sciences, Boulder, CO, United States
Here we use a modified version of the Community Earth System Model (CESM) from the National Center of Atmospheric Research to simulate stable moist greenhouse atmospheres and subsequent thresholds for triggering a thermal runaway for an Earth-analog planet. The model utilizes an improved convection scheme and a correlated-k radiative transfer scheme, capable of accurately modeling both water-rich and CO2-rich atmospheres while maintaining adequate computational efficiency. We explore hot climates caused both by the inevitable increase in the solar constant over time and by the addition of large amounts of CO2 into our present atmosphere. With the current atmospheric CO2 inventory, a runaway greenhouse is not triggered until the Sun becomes greater than 17.5% brighter than today. However, near a 12.5% increase in the solar constant, the climate system experiences an abrupt transition from a warm state (Ts < 305 K) to a moist greenhouse state (Ts > 330 K). Projections for anthropogenic CO2 emissions predict that in an extreme scenario humans could elevate CO2 ~10 times beyond the present atmospheric level. We find that such a case would not trigger a runaway greenhouse. Excessively large CO2 partial pressures (>0.2 bar) may trigger a runaway under the current solar insolation, however, such high levels of CO2 are unlikely to occur.