Research Cites Role of Warming in Extremes

http://www.nytimes.com/2013/09/06/science/earth/research-cites-role-of-warming-in-extremes.html

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Scientists have long predicted that global warming will worsen heat waves and torrential rainfalls. In some parts of the world, that is exactly what happened last year, climate scientists reported Thursday.

Rising temperatures add energy to the atmosphere, and computer models warn that this will produce wider and wilder swings in temperature and rainfall and alter prevailing wind patterns. In examining a dozen extreme weather events last year, scientists found that evidence that human activity — in particular, emissions of heat-trapping carbon dioxide from the burning of fossil fuels — was a partial culprit in about half of them.

Yet other extreme weather events, including the drought that withered the Midwest, appear to be just part of a natural pattern, the scientists concluded. The research, a series of 19 studies by 18 teams, was published in a special issue of the Bulletin of the American Meteorological Society.

While global warming will likely increase the number and severity of extreme weather events, climate scientists have been reluctant to attribute a particular heat wave, storm or drought directly to global warming, because of the natural variations of weather. But with advances in computer modeling and analysis of climate data, they are now able to tease out the contributions of human civilization.

The extreme weather events “would have likely occurred regardless of climate change,” said Thomas R. Karl, director of the National Climatic Data Center in Asheville, N.C.

“The importance of attribution research comes with understanding, however, the impact that climate change adds, or doesn’t add, to any extreme event,” he added.

The articles’ editors likened climate change to someone habitually driving a bit over the speed limit. Even if the speeding itself is unlikely to directly cause an accident, it increases the likelihood that something else — a wet road or a distracting text message — will do so and that the accident, when it occurs, will be more calamitous.

Even when global warming contributes to extreme weather, “natural variability can still be the primary factor in any individual extreme event,” the editors wrote.

To examine causes of the Midwest drought last year, the most severe since the 1950s, researchers ran computer models comparing two situations: one with present-day concentrations of carbon dioxide and other greenhouse gases, the other with the much lower greenhouse gas concentrations before the Industrial Revolution. They found little difference in the frequency of Midwest droughts.

But scientists performing a similar comparison for the heat wave that blanketed much of the United States in July last year estimated that such heat waves now occur four times as frequently because of the influence of greenhouse gas emissions.

“It was really clear and it was really stark,” said Dr. Thomas Peterson, principal scientist at the climate center. “Things had changed dramatically.”

Another team, reviewing data on a heat wave in the eastern United States in the spring of last year, estimated that the activities of humans contributed about one-third of the 6.6-degree spike in temperatures.

An analysis of Hurricane Sandy did not look at the dynamics of the storm, but rather how often floodwaters have reached the heights seen last October. Because sea levels have been rising, the chances of Sandy-like flooding inundating the Battery at the southern tip of Manhattan have risen from once every 2,330 years in 1950 to once every 1,570 years today, the researchers said.

If sea level rise over the next 40 years is low, about half a foot, then the chances of flooding increase slightly. If sea level rise is at the top end up of predictions — two yards — then much smaller storms would cause as much flooding as Sandy did and Lower Manhattan could be inundated every couple of years by 2100, the researchers said.

“Coastal communities are facing a looming crisis due to climate-related sea level rise,” said William Sweet, an oceanographer with the National Oceanic and Atmospheric Administration and one of the authors of the Sandy study.

Record-low ice cover in the Arctic in 2012 was partially caused by global warming, researchers also said. But heavy rains in northern Europe, China and Japan were all explainable by natural variability.