Lightning strikes the Willis Tower (formerly Sears Tower) during a summer thunderstorm in downtown Chicago.

(Scott Olson/Getty Images)

At a Glance

Climate change is making summer weather systems weaker.Weaker systems means pollution could linger in cities longer, instead of being cleared out quickly.Heat waves could also last longer.At the same time, thunderstorms are more destructive.

Climate change is altering North America's summer weather systems in a way that could mean more lingering pollution, stronger thunderstorms and longer heat waves, a new MIT study says.

Using temperature and humidity data going back to 1979, the study found global warming is shifting energy in the atmosphere away from extratropical cyclones, the huge weather systems that circulate across thousands of miles, bringing summertime wind and rains, according to about the study.

"Extratropical cyclones ventilate air and air pollution, so with weaker extratropical cyclones in the summer, you're looking at the potential for more poor air-quality days in urban areas," Charles Gertler, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences and author of the study, said in a statement. "Moving beyond air quality in cities, you have the potential for more destructive thunderstorms and more stagnant days with perhaps longer-lasting heat waves."

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A tropical cyclone is a broad term describing a low-pressure system that is tropical in nature, meaning it has a warm core. In the Atlantic and Eastern Pacific basins, a tropical depression, tropical storm and hurricane are all examples of a tropical cyclone, said weather.com meteorologist Brian Donegan. An extratropical cyclone is simply a non-tropical cyclone, such as a low-pressure system that produces snow, rain or thunderstorms in the United States.

and meteorologist Bob Henson said extratropical cyclones typically form at higher latitudes, where they feed off the contrasts between warm and cold air.

The greater the difference, the stronger extratropical cyclones are. However, global warming has increased the temperature in the Arctic faster than in other areas and reduced the difference between those northern and southern latitudes. The result: weaker extratropical cyclones in June, July and August.

The study found the energy available for large-scale extratropical cyclones has decreased by 6 percent.

The energy now goes to sudden, stronger thunderstorms at the extremities of the cyclones that Bloomberg reported. The energy for these storms has gone up by 13 percent.

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Meanwhile, the extratropical cyclones don't circulate as much. That means they aren't clearing out pollution, and they're not pushing summer storms along.

"Researchers are finding these trends in winds and rainfall that are probably related to climate change," Gertler said. "But this is the first time anyone has robustly connected the average change in the atmosphere to these sub-daily timescale events. So we're presenting a unified framework that connects climate change to this changing weather that we're seeing."

The study will be published this week in the Proceedings of the National Academy of Sciences.