Drought And Global Warming

Stuart Staniford has a couple of posts on studies looking at the potential drought effects of climate change.  The second deals with a 1999 study which gets into the potential physical links between rising CO2 and drought.  Here's Stuart:

The paper's discussion of the physical mechanisms at work is interesting. There are separate discussions of the more northern interior of the US ("CNA1") and the more southerly portions of North America (eg Mexico, Texas - which they call "CNA2").

For CNA1:

In general, the increase of both CO2 and water vapor in the model atmosphere increases the downward flux of longwave radiation absorbed by the continental surface. This results in an early disappearance of snow cover (with a large surface albedo), thereby increasing the solar energy absorbed by the continental surface. Because of the increase in the surface absorption of both longwave and solar radiation, evaporation is enhanced during spring and early summer (Figure 10a), reducing the soil moisture in the CNA1 region. By mid-summer, the soil moisture is reduced to the point where evaporation can no longer increase. Thus, evaporation is decreased and sensible heat increased, reducing the near-surface relative humidity and cloud cover, which increases insolation absorbed by the continental surface and makes more energy available for evaporation. On the other hand, the rate of precipitation hardly increases over the continents in summer because of the low relative humidity in the lower troposphere (Figure 10a). As a matter of fact, the rate of precipitation even decreases slightly after mid-summer when the soil becomes very dry. Therefore, the soil moisture anomaly remains negative throughout the rest of the summer and early fall in ‘CO2 + SUL’

In sharp contrast to the summer situation discussed above, soil moisture in CNA1 increases during winter in ‘CO2 + SUL’ (Figure 9a). Despite the increase in the downward flux of longwave radiation, the rate of evaporation hardly increases over the continental surface in middle and high latitudes where the increase in the downward flux is compensated mainly by the upward fluxes of sensible heat and longwave radiation rather than latent heat flux in winter. On the other hand, the rate of precipitation increases significantly in the CNA1 region where the relative humidity in the lower troposphere is much higher in winter than in summer (Figure 10a). This is quite different from the situation in summer when the relative humidity in the lower troposphere is very low and precipitation hardly increases in this region despite the increase in the rate of evaporation in the surrounding oceans.

The increase in precipitation, together with the failure of evaporation to increase,
accounts for the increase of soil moisture in the CNA1 region during winter in
‘CO2 + SUL’.

This is one of my biggest concerns personally with climate change.  How will it affect agricutlure in the Corn Belt?  A little self-interested, I know, but still an important issue.