Summer heat waves and downpours have become more frequent in the northern hemisphere this century, apparently because extreme weather can get trapped for weeks in the same place in a warming world, a study showed Aug. 11.

Disruptions to the jet stream, which forms huge meandering waves as it blows at high altitude around the planet, and cold air from a thaw of Arctic ice may explain why weather systems are stalling more often, the scientists wrote.

"Weather extremes in the summer — such as a record heat wave in the United States that hit corn farmers and worsened wildfires in 2012 — have reached an exceptional number in the last 10 years," the Potsdam Institute for Climate Impact Research said of its findings.