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Nobel Co-Winner Explains Megadrought

Posted by Christina B. Farnsworth

Jan 31, 2014 8:44:00 AM

Australia and the U.S. West are among the global areas in the midst of drought.

California Governor Jerry Brown declared a drought emergency last week and urged Californians to cut their water use by 20 percent. NPR covered the story. “California faces water shortfalls in the driest year in recorded state history," according to Brown’s office.

"We can't make it rain, but we can be much better prepared for the terrible consequences that California's drought now threatens, including dramatically less water for our farms and communities and increased fires in both urban and rural areas," Brown said. "I've declared this emergency, and I'm calling all Californians to conserve water in every way possible."

Trying to make rain did come up in the question and answer session of the CLIMAS Colloquium where Professor Jonathan Overpeck spoke. The Colloquium was held January 24 at the University of Arizona. But rain is unreliable in the face of drought Kelly Mott Lacroix said when speaking at a Tucson Science Cafe meeting on January 21st. In deserts “94 percent of precipitation evaporates.” Mott Lacroix, Research Analyst for the Water Resources Research Center, said.

 

Overpeck, whose presentation was titled “Emerging New Reality of Megadrought Risk,” views himself as a climate detective. He is the much published co-director of the university’s Institute of the Environment. He was also the coordinating lead author for the 2007 Nobel Peace Prize-winning United Nations Intergovernmental Panel on Climate Change (IPCC); his team won with former vice president Al Gore

Overpeck’s talk focused on how “Increased drought risk is (and will be) arguably one of the most certain and troubling aspects of anthropogenic (meaning “originating in human activity”) climate change for many parts of the world.”

Science is often about calculating probability. Sadly, as is often said, when the Colorado River was allocated among the seven Southwestern and Western states and Mexico, the river’s flow was high. Long term, Overpeck said, “The river flow is 25 percent lower than has been allocated.”  

Thus even without discussion of long-term drought, there are water issues. And recent satellite imagery has illustrated how much the Colorado river flow has been significantly reduced by groundwater pumping in the river basin. Think of it as the Colorado River basin groundwater pumping water loss as the equivalent of 1.3 Lake Meads. Mott LaCroix, in her talk, said pumped groundwater is “not renewable;” it is “mining water.” Surface water from rain and snow runoff feeding rivers, she says, is a renewable resource.

Overpeck spoke of black swans. He said that it is “emerging in the scientific literature that state-of-the-art climate and Earth system models are not able to simulate the full range of drought.” There is already decade-scale drought in both the the West and Southwestern United States, and Australia. (Australians call their drought the “Big Dry.”)

There is also evidence—in “paleoclimatic record of several continents, in both semi-arid and wetter regions”—of 25-to-50-year-long droughts interrupted by single precipitation events. Evidence from “annual laminations in soil” can be read like tree rings, Overpeck said. This lamination reading shows megadroughts occurred from 1000 to 1200 and from 1500 to 1700 in many parts of the globe. Overpeck spoke of the 2005 drought along South America’s Amazon River. Though the 2005 drought was labeled the river’s “drought of the century,” the Amazon’s 2010 drought was actually worse, Overpeck said. The biggest single uncertainty in predicting droughts and their duration is determining the effect of greenhouse gas emission increases on the probability of megadrought.

Denial, Overpeck says, is dangerous. His no regrets worst case drought plan is to plan for the 100-year African drought (much like highway builders elevate roadways in anticipation of 100-year floods). Global warming, Overpeck said, “will make megadroughts much worse.”

Mott Lacroix also spoke of the challenges faced with climate change. She, like Overpeck, said there would be more drought and wildfires and less snowpack. She also said that if crop yields were down that would mean a loss of not only food but jobs. Overpeck, when asked, thought there would be enough food but that prices would rise.

Overpeck, too, is particularly concerned about the ground-water pumping. “While no one is looking, groundwater is disappearing.” During the question and answer session he said that some of the lost groundwater is glacial—1,000 years old.

Because of the potential for population displacement and unrest, Overpeck says, “the U.S. military takes climate change seriously.”

Mott Lacroix, in her talk did say that the Southwest had already made significant water-use changes. She said people use much less water than they did in the 1970s and that use of ground water had decreased. She also praised rainwater harvesting from roofs as a way of capturing some of the rainwater that might otherwise evaporate. However, she said that attempts to harvest water on a grander scale interfered with replenishing rivers and would affect the water rights of others.

And, in a very interesting observation, Mott Lacroix said that Mexico had asked that some of their water be stored in Lake Mead while they dealt with some issues in their portion of Colorado River as it discharges into the Sea of Cortez. This makes one wonder how low Lake Mead would be if it was not holding some of the water to be returned to Mexico.

Potable water is life or death. Now or in the next few weeks, Overpeck’s lecture is online at www.climas.arizona.edu/media 

 


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