Public Release: 

Global Change Research Should Focus On Rainfall, Not Temperature

University of Arizona

Professor Jim Shuttleworth says climate change researchers are concentrating too much on temperature change and should be more concerned with predicting variations in rainfall patterns.

After all, a couple of degrees change either way will not have much effect on development and management of agricultural systems, dams, roadways, or any of a hundred other things that concern humans.

But changes in rainfall - more severe floods or harsher and more prolonged droughts - can have huge effects on human activities and structures. Say you're designing a new road system, for instance. It doesn't matter much if the average temperature in the area will change from 55 degrees to 57 degrees during the next 30 years. What matters is the size of extreme storms. The same goes for agriculture. Extreme storms and droughts are what matter.

Shuttleworth, of the University of Arizona hydrology and water resources (HWR) department and an internationally recognized expert on global change research, advances these arguments in an article in Sept. 3 issue (vol. 77. no. 36, p. 347) of "EOS," a publication of the American Geophysical Union. And it's sure to lead to some heated discussion within the global change community.

Shuttleworth contends that work by the U.S. Global Change Research Program (USGCRP) now has made it possible to manage sustained development in a way that takes climate change into account.

He suggests that USGCRP should turn its attention to rainfall. The group should investigate whether global warming will cause changes in regional precipitation.

Specifically, USGCRP should answer two questions:

  • How will regional precipitation change in the U.S. during the next 30 years?
  • How much rain will fall next summer in the United States?

These are not easy questions. Predicting rainfall change is at least 10 times more difficult than measuring and predicting temperature changes, Shuttleworth says.

Temperature and rainfall are linked. "But global average temperature is too easy a target and a little irrelevant" Shuttleworth says. "We know that the hydrologic cycle will increase because air has the ability to store more water vapor at warmer temperatures. But the thing that matters is whether the rising temperature will change the variation in precipitation. Will the storms be bigger? Are the droughts more intense and prolonged?"

Shuttleworth proposes that USGCRP take the following policy actions:

  • Decide whether it's more critical to predict rainfall and other global change influences in the U.S. or outside the country. Changes in water distribution can have effects on political and economic stability in other parts of the world.

  • Determine time periods that engineers can safely assume will be essentially stable in terms of rainfall extremes in the U.S. The period should be short enough that the change in rainfall is small compared to the total natural variability.

  • Formulate policies that will work to keep the rate of climate change as low as possible. "We can't stop climate change, but we can mitigate it," Shuttleworth says. Banning CFCs to protect the Earth's ozone layer is one example.

  • Refocus USGCRP research to carefully examine rainfall variability, with the goal of predicting what that variability will be over the next 30 years.

By taking these steps, USGCRP can become a positive force for helping to foster reasonable, sustained development, Shuttleworth argues.

In addition to his work in UA HWR, Shuttleworth serves on the U.S. Global Change Research Committee and the National Research Council Panel on the Global Ocean-Atmosphere-Land System. He is active in scientific steering committees of the World Climate Research Programme, the International Biosphere-Geosphere Program, and the International Council of Scientific Unions.

During the past decade, Shuttleworth has led two major international studies to investigate the relationship between Amazonian deforestation and climate, and participated in international studies of African desertification.

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