Sweltering summer heat waves are on the rise across the Northern Hemisphere. The rise is being driven by changes in the atmosphere spurred by a warming Arctic, new research shows.
Scientists came to that conclusion after examining 35 years of weather data. They spotted a decline in the strength of summer storms. These storms bring welcome relief to northern continents in the form of cool, moist air. Wind pattern changes brought on by the rapidly warming Arctic weakened summer storms. The researchers described their findings online March 13 in Science.
Heat waves are devastating because of their length, notes Dim Coumou. A climate scientist at the Potsdam Institute for Climate Impact Research in Germany, he led the new study. “If you have several weeks of extremely high temperatures, then you tend to see massive damage to crops and heat-related deaths.” A summer heat wave in 2003 killed an estimated 70,000 people in Europe.
Summer storms can snap a heat wave. These storms drag cool, wet air from the oceans and on across the hot land. A river of wind in the sky, known as the jet stream, drives the track that these storms take. The jet stream’s winds blow west to east around the North Pole. The big difference in temperature between the Arctic and the rest of the Northern Hemisphere powers these high-altitude winds.
Increases in carbon dioxide and other “greenhouse” gases have been slowly warming Earth’s atmosphere. These gases trap heat close to the ground, creating what is known as global warming. But the increase in temperature hasn’t been the same everywhere. The Arctic has warmed twice as fast as the rest of the Northern Hemisphere. That is due to the disappearance of reflective sea ice and snow cover. Instead of reflecting the sun’s energy, the far North now absorbs more of it. And that has reduced the difference in temperature between the Arctic and the rest of the Northern Hemisphere. The result: weaker polar winds and a weakened jet stream.
Studies in the past have investigated the impacts of the dwindling jet stream on lower-latitude weather. But these studies mostly have focused on the autumn and winter. Coumou and his colleagues instead focused on weather changes during the overlooked summer months. They pulled together weather data from 1979 through 2013. In those data, the team found changes in the atmosphere’s summer behavior.
The summertime jet stream slowed by 5 percent between 1979 and 2013, the researchers now report. This drop caused a 10 percent decline in the energy available to power those cooling summer storms. Fewer storms boost the risk that summers in the Northern Hemisphere will become dangerously warm.
“We can’t say for sure that Arctic warming caused a particular heat wave,” Coumou notes. “But we have seen extreme heat waves happening more and more often.” Predictions by computer models of climate also call for similar decreases in jet-stream speed by the end of the century, the researchers point out.
The impact of Arctic warming on summertime storms is surprisingly dramatic and unlikely to change course in the near future, says Jennifer Francis. She is an atmospheric scientist at Rutgers University in New Brunswick, N.J.
“We don’t expect the Earth to start cooling anytime soon, and certainly not the Arctic,” Francis says. “So this increasing frequency and intensity of heat waves that we’ve been seeing over both North America and Eurasia is probably only going to intensify.”
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Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice.
Arctic sea ice Ice that forms from seawater and that covers all or parts of the Arctic Ocean.
atmosphere The envelope of gases surrounding Earth or another planet.
carbon dioxide A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
climate The weather conditions prevailing in an area in general or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
continent (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctic.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
jet stream A fast-flowing, high-altitude air current. On Earth, the major jet streams flow from west to east in the mid-latitude regions of the Northern and Southern Hemispheres.
latitude The distance from the equator measured in degrees (up to 90).
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Original Journal Source: D. Coumou, J. Lehmann and J. Beckmann. The weakening summer circulation in the Northern Hemisphere mid-latitudes. Science. Published online March 13, 2015. doi: 10.1126/science.1261768.
Original Journal Source: P.A. Stott, D.A. Stone and M.R. Allen. Human contribution to the European heatwave of 2003. Nature. Vol. 432, December 2, 2004, p. 610. doi: 10.1038/nature03089.