WASHINGTON — Climate change may increase the risk of extreme rainfall in the tropics and drought in the world's temperate zones, according to a new study led by NASA.
"These results in many ways are the worst of all possible worlds," said Peter Gleick, a climatologist and water expert who is president of the Pacific Institute, an Oakland research organization. "Wet areas will get wetter and dry areas will get drier."
The regions that could get the heaviest rainfall are along the equator, mainly over the Pacific Ocean and the Asian tropics. Increased aridity and drought could have a greater effect on human life, however, because those conditions are more likely to occur where most of the world's population lives.
In the Northern Hemisphere, drought-prone areas include the Southwestern United States, Mexico, North Africa, the Middle East, Pakistan and northwestern China. In the Southern Hemisphere, drought could become more likely in South Africa, northwestern Australia, coastal Central America and northeastern Brazil.
"Large changes in moderate rainfall, as well as prolonged no-rain events, can have the most impact on society because they occur in regions where most people live," said William Lau, the study's lead author and a scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
"The regions of heavier rainfall, except for the Asian monsoon, may have the smallest societal impact because they usually occur over the ocean," he added.
The study is based on the results of 14 models that show agreement on the possible rainfall trends, Gleick said. The study will be published in the journal Geophysical Research Letters.
The study arrives as a large wildfire has burned thousands of acres in Ventura County. Although many factors have shaped the spread and severity of the fire, the land may have been primed by low rainfall in California.
Climate change does not cause forest fires but does contribute to their likelihood, Gleick said, adding: "It's not about causality but influence."
For every 1 degree Fahrenheit increase in global average temperature because of greenhouse gas emissions, heavy rainfall will increase globally by 3.9%, the study predicted. The paper defines heavy rain as months that get an average of 1/3 of an inch or more of rain daily.
Climate change could lead to heavier rainfall because warmer air holds more moisture.
On the flip side, for every degree Fahrenheit of warming, the length of time a region goes without rain could increase globally by 2.6%.
Scientists have said that the world needs to keep global average temperatures from exceeding 2 degrees Celsius, or about 3.6 degrees Fahrenheit, above pre-industrial global temperatures to avert catastrophic changes to nearly all aspects of life. In the last 150 years or so, the Earth's average temperature has already risen about 1 degree Celsius, or 1.8 degrees Fahrenheit.
To prevent the 2 degree Celsius rise and its effects, including extremes in rainfall, the world has to keep emissions of heat-trapping carbon dioxide below a ratio of 400 parts per million. According to the Scripps Institution of Oceanography at UC San Diego, there have been isolated measurements of 400 parts per million in the Arctic, and scientists expect readings in Hawaii to exceed 400 parts per million this month.
[For the record, 7:20 p.m. May 5: An earlier version of this story said a rise of 1 degree Celsius equals a rise of 1.3 degrees Fahrenheit. It would be 1.8 degrees.]