Three-dimensional Climate Simulations of Moist Greenhouse Atmospheres.

Abstract:

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 (T_s < 305 K) to a moist greenhouse state (T_s > 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.