Much of the world, including the drought-plagued American West, will face more deadly heat waves, intense rainstorms and prolonged dry spells before the end of the century, according to a new climate change study released Thursday.
Focusing not on averages but on extremes, the new research draws on nine climate models to predict what will happen if worldwide greenhouse gases keep increasing.
Longer periods of high heat and heavy rainfall are predicted for nearly all areas by 2080 to 2099. In addition, dry periods will last longer in the Southwestern United States, southern Europe and several other areas, the scientists reported.
“It’s the extremes, not the averages, that cause the most damage to society and to many ecosystems,” said Claudia Tebaldi, lead author of the report by the National Center for Atmospheric Research in Boulder, Colo., Texas Tech University and Australia’s Bureau of Meteorology Research Center.
In California, if the predictions come true, it could mean a triple whammy: more temperatures soaring over 100 degrees, longer rainless periods and more powerful winter storms that could trigger flooding.
“In the future, rising frequency, intensity and duration of temperature extremes ... are likely to have adverse effects on human mortality and morbidity,” says the scientists’ report, “Going to Extremes,” which will be published in the December issue of the journal Climatic Change. “Changes in precipitation-related extremes such as heavy rainfall and associated flooding also have the potential to [cause] significant economic losses and fatalities.”
The federally funded analysis is among the first to use supercomputer simulations developed in the U.S., Japan, France and Russia for an international committee of scientists, the Intergovernmental Panel on Climate Change.
Since California frequently suffers droughts and floods and was gripped by a prolonged summer heat wave that caused more than 100 deaths and triggered power outages, many state officials are alarmed by the dire predictions, which echo other scientists’ warnings.
“The signs of climate change are all around us, but the impacts can be dramatically lessened if we take action now to reduce greenhouse gas emissions,” said Linda Adams, California’s secretary for Environmental Protection.
The scenarios were based on three projections in volumes of greenhouse gas emissions, which come largely from the burning of gasoline, coal and other fossil fuels.
Even under the lowest-emissions scenario, more extreme events were predicted, although the trend was significantly weaker. That means reducing greenhouse gas emissions will lower the risk of severe heat waves and heavy storms.
Also, the report contained some good news for California and the rest of the Southwest: The increase in dry spells occurred only under the highest-emission scenario, so if global emissions decline, there may be no higher frequency of droughts. The West is in the eighth year of its most severe drought since record-keeping began in 1895.
Wetter weather was one of the most significant and consistent patterns that showed up in the modeling, the study shows.
“Depictions of a wetter world and greater precipitation intensity emerge unequivocally,” the report says.
Extra precipitation is tied to global warming because warm oceans evaporate more and warm air holds more moisture.
The higher latitudes, above 40 degrees north -- in the United States, north of Reno, Denver and Philadelphia -- are expected to feel the most effects of more extreme precipitation.
“We see increases in precipitation intensity almost everywhere, but particularly at higher latitudes,” said co-author Gerald Meehl, a scientist in the National Center for Atmospheric Research’s climate and global dynamics division.
Along with more precipitation, more days will pass between rain events in the Southwest, he said.
“The reason these can both happen simultaneously is that you can have longer dry spells between rainfall events, but when it does rain, it rains harder,” Meehl said.
The effects on water supplies would be complex. In the Sierra Nevada, which provide much of California’s water, models predict some decreased snowpack, but more water in the rivers.
In the Southwest, the more frequent heat waves would be caused by changes in atmospheric circulation created by greenhouse gases.
Other changes that were called pronounced in the models include a longer growing season and fewer frost days in the Northwestern U.S. and Eastern Europe, and more heat waves in Northern Australia.
The scientists looked at 10 indicators of extremes: heat wave duration, the difference between a year’s high and low temperatures, growing season length, frost days, warm nights and five factors involving precipitation.
Other recent studies have predicted more widespread wildfires, droughts, and die-offs of plants and wildlife.
Although scientists say global temperatures have already increased, the report says the rate will be magnified.
Generally, the more greenhouse gases that are emitted, “the more extreme things get,” Meehl said. He warned, however, that “it’s not a simple linear relationship”; if gases decline 20% that doesn’t mean there is an equivalent decline in the effect, because localized phenomena guide climate extremes. Still, even small declines, he said, mean fewer extremes.
Gov. Arnold Schwarzenegger and the Legislature have agreed on a plan to cut by 25% the greenhouse gases emitted from California industrial sources by 2020, which would make California the first state to fight global warming by capping emissions.