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How global warming causes both extreme heat and cold weather – ZME Science

Texas Guardsmen assist a motorist stuck on snow and ice on February 17, 2021 in Abilene, Texas.  Photo: Texas Army National Guard/Staff Sgt. Yvonne Ontiveros

It’s obvious how global warming can lead to record-breaking surface temperatures and heatwaves. After all, there’s “warming” in the term that describes the rise of global average temperatures as a result of more heat-trapping greenhouse gases emitted into the atmosphere. But the same process is responsible for driving record-low temperatures as well, which seems counterintuitive. A new study explains how global warming can drive both sweltering heat waves and frostbiting blizzards by analyzing three extreme events from the past winter.

Harsh winters and blazing summers: what’s the connection

On February 26, 2015, Senator James Inhofe (R-Okla.), who quite ironically used to be the chair of the Senate Environment and Public Works Committee, appeared with a snowball on the Senate floor to demonstrate, once and for all, that climate change is a hoax.

“You know what this is?’” asked Inhofe, the author of The Greatest Hoax: How the Global Warming Conspiracy Threatens Your Future. “It’s a snowball, from outside here. So it’s very, very cold out. Very unseasonable.” Then he tossed it to a congressional page.

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This tasteless stunt occurred during a period when it happened to be very cold in the eastern United States. Since then, June 2021 has officially become the hottest June on record in North America while July 2021 was the worst for wildfires since records began.

But while global warming and extreme cold might sound antithetic, they’re actually quite well connected. And if Inhofe would have been of good faith and listened to climate scientists, he would have known this, too.

Research has connected the collapse of the polar vortex — a huge ring of low-pressure, cold winds in Earth’s stratosphere above the North Pole — with extreme sub-zero temperature events. This is owed to the polar vortex’s connection with the jet stream, a band of strong air currents flowing from west to east about 10 kilometers above the surface. When the temperature abruptly increases due to global warming, the interaction between the polar vortex and jet stream can be dramatically altered. The resulting extreme weather due to the interplay can be further amplified by changes in ocean temperature or Arctic Ocean sea ice.

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Researchers led by Xiangdong Zhang, a professor of climate and atmospheric sciences at the University of Alaska Fairbanks, showed this effect in action when they examined two record-setting cold air outbreaks in China from late December 2020 to mid-January 2021. During this time, the cities of Beijing and Tiajin saw their lowest temperatures in 54 years at -19.7°C (-3.46°F) and -19.9°C (-3.82°F), respectively.

Record-breaking cold weather in North America’s midwest and deep south in February 2021 was also examined, which saw the coldest recorded temperatures in almost a century in Texas and Austin, at -13.3°C (8°F) and -8.3°C (17°F), respectively.

Using observational data from the past 42 winters, the researchers plugged temperature readings into climate models that simulated how sea and atmospheric events may impact extreme weather.

“Even though global warming and loss of Arctic Sea ice occurs every year, such extreme weather events that we investigate are intermittent — they do not occur every year,” said co-author James Overland, research oceanographer at the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory in the U.S. “This is because they are caused by a combination of new global warming and extreme, but naturally occurring, precursor weather conditions in the jet stream and polar vortex.”

“The overarching problem to solve is why extreme weather events have more frequently occurred in a warming climate during recent decades and if the Arctic warming amplification plays a leading driving role,” Zhang added. “The extreme events of the 2020-21 winter provide a unique opportunity to examine what physical processes or mechanisms drive these events.”

The results suggest that all three events were linked to sudden stratospheric warming, although the downstream effects were different for each scenario. During the first East Asia cold event, polar air modulated the midlatitude jet stream, steering cold air southward. In the second event, the polar vortex split, deepening the region of low pressure driving more cooler Arctic air into the region. Finally, in North America, the polar vortex also split but this time the low-pressure atmosphere settled more deeply over the south.

The study shows that large-scale atmospheric circulation and temperature anomalies can lead to extreme weather across the world, but due to slightly different factors. Satellite remote sensing may help validate these model simulations; and if that’s the case, scientists may use these models to better predict how precursor events influence the monsoon season in East Asia and other weather events. But whatever may be the case, the common denominator is global heating.

“By studying these record-breaking cold spells, we can see the ’big picture‘ of extreme weather events.” said co-author, Zhe Han, scientist in the Institute of Atmospheric Physics, Chinese Academy of Sciences. “Though the events can be different, they might share similar underlying mechanisms that are related to global warming. Along with the warming, the Arctic amplification and intensified ocean thermal anomalies may interact with the atmospheric circulation, such as the polar vortex and sudden stratospheric warming, to cause the occurrence of extreme cold or hot events.”

The findings appeared in the journal Advances in Atmospheric Sciences.

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