Continental Moisture Availability and Planetary Temperature in an Idealized GCM

In CMIP-class GCMs, land tends to "dry out" with greenhouse warming outside of the high latitudes, as evaporative demand consistently increases but mean precipitation does not. Soil moisture, relative humidity, and common water-availability indices tend to decline. Yet, paleoclimate evidence is usually interpreted to imply that past greenhouse climates were generally well-watered on land, while glacial periods were generally arid - just the opposite.

Motivated by this apparent discrepancy, we perform greenhouse warming experiments over a wide range of planetary temperatures with a slab-ocean atmospheric GCM coupled to a simple land-surface water/energy balance model in idealized continental geometry. We assess the results using several nondimensional measures.

The mean-state terrestrial aridity strongly depends on the extent of subtropical seaways and on the prescribed ocean heat transport. Unexpectedly, the aridity responses to warming can dramatically differ from the canonical CMIP story. In very wet terrestrial settings, including much of the mid-latitudes and sometimes the tropics as well, precipitation can increase enough to stop evaporative demand from increasing at all. This is a tantalizing analog to the paleo-evidence. However, when the terrestrial tropics are drier, precipitation there declines very strongly with greenhouse warming even as tropical ocean precipitation increases, causing strong aridification. Future work will seek to understand these and other surprising results, and to explain why they do not generally occur in the full GCMs.