Interesting paper in GRL by Lobell et al. suggesting that temperature under doubled CO2 could be cooler in irrigated regions than the climate models suggest. Because of the relatively small area of the planet under irrigation, the overall effect is small, but the potential effect on food-production seems worthy of better understanding:

The reduction of warming is less than 25% of the temperature increase modeled for doubled CO2 in most regions; thus greenhouse warming will still be dominant. However, the results indicate that land use interactions may be an important component of climate change uncertainty in some agricultural regions. While irrigated lands comprise only ~2% of the land surface, they contribute over 40% of global food production. Climate changes in these regions are therefore particularly important to society despite their relatively small contribution to average global climate.