By Sid Perkins
Extensive agricultural irrigation can significantly affect local climate and may be masking the effects of global warming in some areas, a new study suggests.
In arid and semiarid regions, irrigation enables farmers to grow crops that wouldn’t naturally thrive. But adding large volumes of water to normally dry ground changes the climate in many ways, says Lara M. Kueppers, an ecosystem scientist at the University of California, Merced. For example, irrigation typically darkens the soil, which then absorbs more heat. The resulting increase in evaporation cools the air and boosts humidity, she notes.
Kueppers and her colleagues used computer models to estimate the climatic effects of irrigation in California, where farmers provide extra water to around one-twelfth of the state’s land. One set of simulations depicted modern land use, and another set simulated only natural vegetation.
During a simulated 20-year period, August daytime-high temperatures at irrigated sites were on average 7.5°C lower than they would have been if they’d been covered with natural vegetation, says Kueppers. In those irrigation-cooled areas, humidity increased an average of 25 percentage points.
Applied to California as a whole, these results imply that irrigation keeps the average August temperature about 0.38°C lower than it would otherwise be. That’s about the same amount of greenhouse gas–triggered warming that some climate models suggest will occur in coming decades, the researchers note in the Feb. 16 Geophysical Research Letters. If the extent of irrigated farmland declines in the future because of water shortages or urban sprawl, irrigation’s evaporative cooling effect probably will decrease as well.
