Urban heat island, meet rural cool island

Here's something I hadn't heard much about before. Global warming skeptics go on a lot about urban heat islands causing a significant part of temperature increases over the 20th century.

But there is another effect - increased irrigation causing an artificial cooling:

Scientists are just beginning to get a handle on irrigation's impact. In a hundred years, the amount of irrigated farmland has grown four-fold, now covering an area four times the size of Texas. Puma and his coauthor, Benjamin Cook, a climatologist at Goddard and Columbia's Lamont-Doherty Earth Observatory, are the first to look at the historic effects of mass watering on climate globally by analyzing temperature, precipitation and irrigation trends in a series of model simulations for the last century. They 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. ..

Globally, irrigation's effect on climate is small—one-tenth of one degree C (about 0.2 degree F). But regionally, the cooling can match or exceed the impacts of greenhouse gases, say the scientists. For example, the study found some of the largest effects in India's arid Indus River Basin, where irrigation may be cooling the climate up to 3 degrees C, (5.4 degrees F) and up to 1-2 degrees C in other heavily irrigated regions such as California's Central Valley and parts of China. The study also found as much as .5 degree C cooling in heavily watered regions of Europe, Asia and North America during the summer.

Irrigation has increased because it boosts crop yields, supporting many millions of small farmers, said Upmanu Lall, head of the Columbia Water Center at the Earth Institute. But concern is growing that groundwater supplies in India and China may not keep up.

So, use up your groundwater, and suddenly you may find your local area quite a bit hotter than before.

I trust Watts up With That will post on this soon. They had better, after spending so much time on urban heat islands.