"Long before it's in the papers"
March 28, 2015


Global warming doesn’t cause extreme winters, study says

March 28, 2015
Courtesy ETH Zurich
and World Science staff

Global warming doesn’t cause extreme winters, a study has found—instead it tends to re­duce tem­per­a­ture vari­abil­ity.

That a warming trend doesn’t cause cold snaps might seem obvious, but it had been a question. Tem­per­a­tures plunged far be­low freez­ing across the east­ern Un­ited States in the past two win­ters. Parts of the Ni­ag­a­ra Falls froze, and ice floes formed on Lake Mich­i­gan. Some theories pointed to global warming as a culprit. 

In one hy­poth­e­sis, stronger warm­ing of the po­lar ar­eas of the globe rel­a­tive to the rest has weak­ened the “po­lar jet stream,” a wind cur­rent sev­er­al miles or kilo­me­ters over the ground. This could cause the po­lar jet stream to be­come “wavy,” lead­ing to more tem­per­a­ture fluctua­t­ions in mid-latitudes.

The re­search­ers be­hind the new study used cli­mate sim­ula­t­ions and the­o­ret­i­cal ar­gu­ments to ar­gue that the op­po­site will hap­pen—not only will cold snaps be­come rar­er simply be­cause the world is warm­er, but al­so be­cause fluctua­t­ions are smaller. On the oth­er hand heat waves may be­come more com­mon, be­cause of the first ef­fect, said Tapio Schnei­der, pro­fes­sor of cli­mate dy­nam­ics at ETH Zu­rich, a uni­vers­ity in Zu­rich, Switz­er­land, who led the stu­dy. The find­ings ap­pear in the lat­est is­sue of the Jour­nal of Cli­mate.

The stu­dy’s point of de­par­ture was that the poles are in­deed warm­ing faster than the equa­tor; this means the tem­per­a­ture dif­fer­ence be­tween them is de­creas­ing. Sup­pose they had per­fectly equal tem­per­a­tures: this would mean all air mass­es have the same tem­per­a­ture, re­gard­less of their di­rec­tion of flow, so there would be no tem­per­a­ture vari­abil­ity. This ex­treme case won’t hap­pen, the re­search­ers said, but it il­lus­trates the log­ic.

Us­ing a highly sim­pli­fied cli­mate mod­el, they said, they ex­am­ined var­i­ous cli­mate sce­nar­i­os to ver­i­fy their the­o­ry. Cli­mate mod­el sim­ula­t­ions by the Intergo­vernmental Pan­el on Cli­mate Change showed si­m­i­lar re­sults, they said: as the cli­mate warms, tem­per­a­ture dif­fer­ences in mid-latitudes de­crease. So does tem­per­a­ture vari­abil­ity, es­pe­cially in win­ter.

“The wav­i­ness of the je­t stream that makes our day-to-day weath­er does not change much,” Schnei­der said. He added the changes in the north-south dif­fer­ence in tem­per­a­tures affect tem­per­a­ture vari­abil­ity more.

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Cold snaps like the ones that hit the eastern United States in the past winters aren’t due to climate change, a study has found. The scientists behind the work concluded that global warming actually tends to reduce temperature variability. Repeated cold snaps led to temperatures far below freezing across the eastern United States in the past two winters. Parts of the Niagara Falls froze, and ice floes formed on Lake Michigan. Some questioned whether climate change could be responsible. One hypothesis to support this idea proposes that stronger warming of the polar areas of the globe relative to the rest has weakened the “polar jet stream,” a wind current several miles or kilometers over the ground. This could cause the polar jet stream to become “wavy,” leading to more temperature fluctuations in mid-latitudes. The researchers behind the new study used climate simulations and theoretical arguments to argue that the opposite will happen—not only will cold snaps become rarer simply because the world is warmer, but also because fluctuations are smaller. On the other hand heat waves may become more common, because of the first effect, said Tapio Schneider, professor of climate dynamics at ETH Zurich, a university in Zurich, Switzerland, who led the study. The findings appear in the latest issue of the Journal of Climate. The study’s notes point of departure is that the poles are indeed warming faster than the equator; this means the temperature difference between them is decreasing. Suppose they had perfectly equal temperatures: this would mean all air masses have the same temperature, regardless of their direction of flow, so there would be no temperature variability. This extreme case won’t happen, the researchers said, but it illustrates the logic. Using a highly simplified climate model, they said, they examined various climate scenarios to verify their theory. Climate model simulations by the Intergovernmental Panel on Climate Change showed similar results, they said: as the climate warms, temperature differences in mid-latitudes decrease, and so does temperature variability, especially in winter. “The waviness of the jet stream that makes our day-to-day weather does not change much,” Schneider said. He added the changes in the north-south difference in temperatures play a greater role in temperature variability.