The Changes in Permafrost Induced by Greenhouse Warming: A Numerical Study Applying Multiple-Layer Ground Model

Abstract

Several previous studies indicate that an increase in atmospheric CO₂ could increase summer dryness on the land surface over the continental region in mid- to high-latitudes. However, the numerical models used in these studies did not sufliciently reproduce permafrost at high latitudes in the Northern Hemisphere, because of their simplified representation of ground hydrological processes; most of them consist of only one layer, and/or exclude freezing and thawing of soil moisture. In order to investigate the mechanisms associated with permafrost and the changes it undergoes, four experiments are performed using a coupled atmosphere-ocean GCM (MRI-GCM1), which includes a four-layer ground model, with a bottom at 10 m depth: the runs with freezing/thawing of soil moisture under normal and doubling concentration of atmospheric CO₂ (F1 and F2) and their counterparts without the freezing/thawing (NF1 and NF2). The result with soil freezing (F2-F1) predicts a substantial increase of surface soil moisture over northern high latitudes during summer, while the result without soil freezing (NF2-NF1) indicates enhanced summer dryness in the same area. It is suggested, by means of a comparison study, that the CO₂-induced wet summer due to the inclusion of soil freezing is mostly attributable to the thawing of a part of the permafrost in deep layers caused by the warming, augmenting liquid water available to upper layers, which moderates summer dryness at the surface. The thawing effect may be reduced to some extent if a gravitational term, which contributes to reducing the upward diffusion, is included in the formulation of soil water migration.