We know irrigation works- meaning, we know that we can turn a desert into farmland. But how much do we know about the side effects? A new report from two New York researchers entitled Irrigation and 20th Century Climate suggests that the worldwide increase in irrigation over the 20th century is actually changing weather patterns and masking some local effects of global warming. The report also raises the question that if current irrigation and resulting weather changes are hiding some localized effects of climate change, what will happen if the current water supplies in those areas used for irrigation runs out?
The report concludes that “generally have cooler temperatures and increased precipitation in the presence of irrigation,” which on one hand eerily echoes the U.S. manifest destiny slogan of “Water Follows the Plough,” but on the other hand makes perfect common sense. If agriculture pulls large amounts of water from underground sources, there will be more water evaporating, leading to the cooler temperatures and increased precipitation.
The report goes on to look at how that increased use of water results in more evaporation, higher cloud cover, and a reduction of temperatures in warmer seasons and even a reduction in traditionally heavy seasons of rainfall, like India’s monsoon. While they estimate that the worldwide cooling due to irrigation is just an average of two tenths of a degree Fahrenheit, they also claim that it could be much higher in concentrated localities- one degree in parts of North America or Europe, for example, or as much as 5 degrees in parts of India.
They point to the question of what will happen in places that are in danger of running out of water because of overuse for current irrigation projects. For example, if agriculture drains more than the Ogalalla aquifer can afford to lose, how much more will temperatures rise when the irrigation stops happening? What about a country like Yemen, where the danger of running out of water is immediate and real?
To read the report in full, visit Irrigation and 20th Century Climate