Sensitivity of a general circulation model to changes in northern hemisphere ice sheets

Abstract

Sensitivity experiments with a general circulation model demonstrate the role of ice sheet size on the local, regional, and global climate. Model experiments isolate the effects of albedo, height, and area of the ice sheets and show how the National Center for Atmospheric Research Community Climate Model 1 responds to changes in the size of northern hemisphere ice sheets. A flat ice sheet with full glacial areal extent but no elevation is used to study albedo effects. A full ice sheet with full glacial areal extent and elevation is used to represent height effects. An ice sheet with half the glacial area of the others but the full glacial elevation is used to represent area effects. All of the sensitivity experiments have (1) interactive sea surface temperatures calculated by a slab ocean and (2) modern boundary conditions except for the ice sheets. The experiments show that both the full and flat ice sheets lower the global mean surface temperatures (GMT) by 2.5°C and that the GMT is dependent upon the area, rather than the height, of the ice sheets. High ice sheets maintain colder temperatures than lower ice sheets over the ice sheets themselves, but compensating warmer temperatures occur downstream from the high ice sheets. The downstream warmer temperatures are the result of (1) glacial anticyclones that cause subsidence and reduced cloud cover during summer as well as reduced soil moisture and (2) increased southwesterly flow across the Atlantic Ocean that results in increased southerly advection of warm air during winter. A dynamical effect of the high ice sheets during summer is to change the wave number of the planetary waves in the midlatitudes, whereas a thermodynamic effect of the flat ice sheets during summer is to lower the geopotential heights throughout the northern hemisphere. In general, northern hemisphere ice sheets induce both a local response over the ice sheets and a regional response downstream from the ice sheets but have little impact on the southern hemisphere except where sea ice expands.