Expanded irrigation might mitigate the effects of climate change in some areas, a new study has revealed.
But some major groundwater aquifers, a source of irrigation water, are projected to dry up in coming decades from continuing overuse, and when they do, people may face the double whammy of food shortages and higher temperatures.
"An important question for the future is what happens to the climate if the water goes dry and the cooling disappears? How much warming is being hidden by irrigation?" asked Michael Puma at Columbia University's Earth Institute and its affiliated NASA Goddard Institute for Space Studies.
On one hand, earth's oceans and vegetation have been absorbing a growing share of emissions with declining rates of absorption. But humans are also cooling the planet to some degree, by releasing air-polluting particles that lower temperatures by reflecting the sun's energy back into space.
Puma and his coauthor, Benjamin Cook found that irrigation-linked cooling grew noticeably in the 1950s as irrigation rates exploded, and that more rain is now falling downstream of these heavily watered regions.
In warm, dry regions, irrigation increases the amount of water available for plants to release into the air through a process called evapotranspiration. When the soil is wet, part of the sun's energy is diverted from warming the soil to vaporising its moisture, creating a cooling effect.
The study suggests that irrigation may be altering the pattern of the Asian monsoon, the rains that feed nearly half of the world's population.
"The study points to the importance of including irrigation in regional and global climate models so that we can anticipate precipitation and temperature impacts, and better manage our land, water and food in stressed environments," Lall said.
The study is published in the Journal of Geophysical Research.