New research suggests weakening summer storm activity could lead to more severe heat extremes.
Storm activity across the U.S., Europe and Russia has calmed down in recent decades, but a recent study suggests this phenomenon could lead to devastating heat waves, droughts and wildfires, Potsdam Institute for Climate Impact Research reported.
"When the great air streams in the sky above us get disturbed by climate change, this can have severe effects on the ground," said lead author Dim Coumou. "While you might expect reduced storm activity to be something good, it turns out that this reduction leads to a greater persistence of weather systems in the Northern hemisphere mid-latitudes. In summer, storms transport moist and cool air from the oceans to the continents bringing relief after periods of oppressive heat. Slack periods, in contrast, make warm weather conditions endure, resulting in the buildup of heat and drought."
Past studies have focused primarily on winter storms, but this new research looked at observational data from weather stations and satellites to reveal a significant decrease in summer storm activity. Researchers looked at a specific type of turbulence called synoptic eddies to calculate the total energy of wind speeds. They found storm intensity has dropped by about one tenth since the year 1979.
"Unabated climate change will probably further weaken summer circulation patterns which could thus aggravate the risk of heat waves," said co-author Jascha Lehmann. "Remarkably, climate simulations for the next decades, the CMIP5, show the same link that we found in observations. So the warm temperature extremes we've experienced in recent years might be just a beginning."
The researchers suggested rapid warming in the Arctic could be partially responsible for the observed changes in circulation. Arctic sea ice is believed to be shrinking as a result of global warming, causing the polar region to take up more heat. The dark water is less reflective than ice, so less ice cover means less sunlight is reflected back into space. Temperature differences are known to drive air motion, and a reduction in these differences is believed to be causing the jet-stream to weaken.
"From whichever angle we look at the heat extremes, the evidence we find points in the same direction," Coumou said. "The heat extremes do not just increase because we're warming the planet, but because climate change disturbs airstreams that are important for shaping our weather. The reduced day-to-day variability that we observed makes weather more persistent, resulting in heat extremes on monthly timescales. So the risk of high-impact heat waves is likely to increase."
