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The Northern Hemisphere's mid-latitudes have experienced many heat waves in recent years, such as one that fueled Rocky Mountain wildfires in summer 2012. Warmer-than-normal temperatures appear red in this NASA image of North America on June 28, 2012. (Image by Jesse Allen, using MODIS data provided by Land Processes Distributed Active Archive Center (LPDAAC).)

Summer Heat Waves May Be Linked To Sea Ice Loss

As ice melts, the jet stream gets stuck in the north, causing warm weather to linger in the south--but the reason why this occurs remains unknown

smithsonian.com

As much of the United States shivers through a cold spell, readers may be hard pressed to remember the summer heat waves that have been coming in increasing frequency. The southwestern U.S. baked during this past summer. High heat in the Midwest and East Coast in summer 2012 killed 82 people, which followed a record summer in 2011. And that came after a 2010 summer that saw high heat across the Northern Hemisphere, from Asia to Europe to North America.

These events are not random and can be blamed on the disappearance of sea ice from the Arctic Ocean and, to a lesser extent, the melting of snow cover in the Arctic, say climate scientists from the Chinese Academy of Sciences in Beijing and Rutgers University. Their study was published December 7 in Nature Climate Change.

The ice that blankets the Arctic Ocean increases in winter and shrinks in extent in the summer. Likewise, Arctic lands become covered in snow in winter, and that snow melts in warmer months. This cycle is natural, but it’s been changing in recent years. The summer ice has been shrinking more, and the winter snow has been melting more. The region is warming more quickly than the rest of the world, and it’s having a variety of consequences, from alterations to the food web to a melting of permafrost to the opening up of shipping channels.

But climate scientists are also trying to figure out if the loss of snow and ice might be having larger effects on Earth’s weather patterns. Snow and ice act like mirrors, reflecting some of the Sun’s energy back out into space. When that mirror shrinks, the darker land and ocean can suck up more heat, which not only leads to more melting and a warmer Arctic but may also alter weather far away.

Arctic sea ice reaches its smallest extent in September, and that area has declined by about 8 percent every 10 years since the 1980s. Arctic snow cover, which reaches its minimum in June, has been shrinking even faster, declining about 18 percent every decade since 1979. In the new study, the researchers linked this data, as gathered from satellite observations, with atmospheric data and found that shrinking sea ice was associated with the jet stream moving northward. Snow cover also played a role but a smaller one, even though it is disappearing faster than the sea ice.

The jet stream is a ribbon of air that flows around the Northern Hemisphere from west to east and separates cold Arctic air from warmer air masses to the south. A jet stream stuck farther in the north helps to keep unbroken the warm weather patterns to the south, “increasing the probability of extreme weather events such as heat waves and droughts,” the researchers write, particularly in the eastern half of North America, eastern Europe and eastern Asia.

This study “provides further evidence linking snow and ice loss in the Arctic with summer extreme weather in mid-latitudes,” the researchers write. “As greenhouse gases continue to accumulate in the atmosphere and all forms of Arctic ice continue to disappear, we expect to see further increases in summer heat extremes in the major population centres across much of North America and Eurasia where billions of people will be affected.”

Though a heat wave may sound like a good thing right now, as many of us look out through frost-covered windows onto snowy streets, these are expensive, deadly events that kill more people than cold, cause droughts and contribute to devastating wildfires.

But the link between changes in the Arctic and heat waves in the populous mid-latitudes isn’t certain. The study showed an association, but climate scientists have yet to figure out the mechanism that might provide the link and most remain skeptical that such a link exists. “I would have more confidence in the linkage being ‘real’ if there was a well-understood and proven mechanism to support the correlations,” James Screen, a climate researcher at the University of Exeter in England, told Climate Central. And there is evidence that Arctic melting can also be associated with extremes in winter cold.

Though climate scientists have yet to understand exactly how the changes in the Arctic may be influencing weather elsewhere in the world, there is enough evidence to convince them that they should keep investigating, climate scientist James Overland of the NOAA/Pacific Marine Environmental Laboratory in Seattle, writes in an accompanying News & Views article. “The potential for an Arctic influence remains high given the outlook for further declines in summer sea-ice and snow cover over the next few decades and Arctic amplification of global temperatures.”

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About Sarah Zielinski
Sarah Zielinski

Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.

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