News•June 24, 2015

Extreme Temperatures Linked to Changing Air Patterns

Scorching summertime heat waves in Europe, Asia and North America, as well as extreme cold snaps in central Asia, have become more likely because of changes in the way air is flowing over those regions, a new study detailed in the journal Nature suggests.

A corporal of the Moscow Kremlin Guard helps another soldier on duty during the extreme heat wave that hit during July 2010.
Credit: Boris SV/flickr

The overall warming of the atmosphere that has resulted from the buildup of greenhouse gases has generally tipped the odds in favor of more extreme warm temperatures and fewer cold ones. But the way areas of high and low pressure meander around the globe can reinforce those odds, or counteract them. That leads to different patterns of temperature extremes in different places at different times.

“It’s important to determine where we believe that some of the recent trends in circulation could potentially be linked with climate change, rather than just natural variability,” Ted Shepherd, an atmospheric scientist at the University of Reading in the U.K., said in an email. Shepherd, who was not involved in the study, wrote an accompanying commentary on it in Nature.

Using atmospheric data from the last 35 years, study author Daniel Horton, a Stanford University postdoc, and his colleagues found that persistent areas of high pressure in certain places were linked with extreme heat waves in Europe, western Asia and eastern North America. The position of the systems affected how air was directed over those areas. An example is the deadly Russian heatwave of 2010, which was the result of such a “stuck” high-pressure system that kept a large mass of hot, dry air parked over the region for weeks.

Conversely, an increase in cold extremes over central Asia was associated with a pattern that led more Arctic air to flow in over the region. The trend in more cold extremes was strongest during the period since pronounced Arctic warming emerged, or about the last 25 years, which lends at least some support to the possibility that that warming is helping fuel the trend, Shepherd said. The potential influence of rapid Arctic warming on such extremes has been a hot research topic in recent years, though it is much debated in the climate community.

This map shows how much temperatures over Russia varied from normal between July 20-27, 2010. The strong concentration of deep red over eastern Russia reflects the weeks-long heat wave that gripped the region that summer.
Credit: NASA

Whether or not these changes in atmospheric circulation are themselves linked to global warming wasn’t something the study tried to answer. Judah Cohen, who has conducted several studies on the Arctic warming-cold extremes connection, said that while the new study was “a nice analysis” and consistent with other findings, he thought it would “do little to settle or alleviate the differences” between the different camps on that question. Cohen, an atmospheric scientist with Atmospheric and Environmental Research, was also not involved in the study.

While the new research didn’t answer what led to the particular atmospheric patterns associated with extreme temperatures, Horton hopes that they can use the same approach from the study to try to figure that out. He called the effort “a work in progress,” adding, “We don’t have answers yet.”

Figuring out that answer is important to understanding what changes different regions might face in a warming world, because having a particular system parked over one area for a long time can also to lead to issues like drought and flooding.

The ongoing California drought, for example, has been linked to a persistent high-pressure system that has kept much needed rains away.