Linking Temperature Rise To Effects

A report from the National Research Council estimates how much change in precipitation and crop yields is likely to occur for each degree of human-caused global warming.

The report differs from most other analyses because it links these changes to global average temperature rise rather than to atmospheric concentrations of greenhouse gases.

Most of the effects of climate change will stem from rising temperatures, says Katharine Hayhoe, a member of the NRC committee that prepared the report. A notable exception is ocean acidification, which will be due to increased carbon dioxide levels, added Hayhoe, who is an associate professor of geosciences at Texas Tech University.

The report estimates a reduction in rainfall of 5 to 10% in southwest North America, the Mediterranean area, and southern Africa per degree Celsius of warming. In addition, the report projects crop yields will fall 5 to 15% per degree for corn grown in the U.S. and Africa and for wheat in India. These estimates apply for global warming in the range of 1 to 4 degrees C.

The report stops short of making recommendations for stabilizing greenhouse gas levels in the atmosphere.

"Scientists can't make the decisions of how much risk is acceptable. That's a political decision," explains Hayhoe. "Rather, we're trying to give policymakers the information they need to make these decisions."

Emissions of carbon dioxide during the 21st century will essentially determine the magnitude of global warming, the report says. It concludes that "The world is entering a new geologic epoch ... in which human activities will largely control the evolution of Earth's environment."