9 September 2010

Is irrigation masking our warming climate?

by Kate Melville

Irrigation has made it possible to feed the world's population, and it may also be temporarily counteracting the effects of climate change in some regions, say scientists in the Journal of Geophysical Research.

"Irrigation can have a significant cooling effect on regional temperatures, where people live," said the study's lead author, Michael Puma, a hydrologist from Columbia University. "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?"

Scientists generally agree that in the last century, humans have warmed the planet about 0.7 degrees Celsius. How much warmer Earth will get depends not only on future carbon emissions but an array of other variables. For instance, earth's oceans and vegetation have been absorbing a growing share of emissions, but recent studies suggest this uptake may be slowing. This could lead to more carbon dioxide in the air, and accelerated warming. On the other hand, 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. The pumping of vast amounts of heat-absorbing water onto crops is also lowering temperatures in some regions, say the researchers.

In the last hundred years, the amount of irrigated farmland has grown four-fold. The new study is 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. The team 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 Celsius. But regionally, the cooling can match or exceed the impacts of greenhouse gases, suggests the study. 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 Celsius. The study also found as much as 0.5 degree Celsius cooling in heavily watered regions of Europe, Asia and North America during the summer.

The study suggests also that irrigation may be shaping the climate in other ways, by adding up to a millimeter per day of extra rain downwind of irrigated areas in Europe and parts of Asia. It also suggests that irrigation may be altering the pattern of the Asian monsoon, the rains that feed nearly half of the world's population. However, these findings are more uncertain, the authors caution.

Irrigation has increased because it boosts crop yields, but concern is growing that groundwater supplies in India and China may not keep up. "Near term and future climate predictions are essential for anticipating climate shocks and improving food security," the researchers note. "The study points to the importance of including irrigation in regional and global climate models so that we can anticipate precipitation and temperature impacts."

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Source: Journal of Geophysical Research