The effect of surface pressure on snowball earth deglaciation

Abstract

A 1D radiative-convective model was built to investigate the effect of surface pressure on Snowball Earth deglaciation. This type of model was chosen because it is accurate enough to replicate both the modern climate and the extreme Snowball state, but simple enough that the consequences of a large structural change can be readily understood. Most models of the Snowball Earth keep the surface pressure fixed at its present value of 1 bar. However, deglaciation requires a CO2 inventory in the range 0.1{0:4 bar. This substantially increases the surface pressure, which increases the surface temperature. Therefore, it would require less CO2 to escape a Snowball state than these models would suggest. A previous study used the correct surface pressure of 1 bar + PI;CO2 , but the warming caused by the increased pressure was attributed to pressure broadening of the CO2 absorption lines. Here it is shown that pressure broadening is not the primary effect; instead, the increase in surface pressure allows convection to extend down to higher pressures, while the energy balance at the top of the atmosphere remains unchanged. This process, which is new to the literature, is termed 'convective deepening'. At the deglaciation threshold of 0:25 bar, compared to an atmosphere with surface pressure fixed at 1 bar, it was found that the surface is 3K warmer if pressure broadening is included, and 13K warmer if both pressure broadening and convective deepening are included. Therefore, convective deepening is the major source of warming when the surface pressure increases in Snowball Earth conditions. With 0:25 bar of CO2, Cp decreases by 3%, while Rspecific decreases by 5%, which means the moist adiabatic lapse rate increases, causing a 2K increase in surface temperature. This effect, while smaller than convective deepening and pressure broadening, is not negligible.