Making sense of a warmer planet with more extreme winters

Making sense of a warmer planet with more extreme winters
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This has been a year of extremes most closely associated with the 2021 summer weather. There have been unprecedented events, including heatwaves most notably in the U.S. Pacific Northwest and Southwestern Canada, devastating floods in the New York City metro area, Germany, China and Italy, droughts accompanied by wildfires and rapidly intensifying and destructive tropical storms. All of these weather-related hazards — affecting millions of people and causing billions of dollars in losses — are consistent with our understanding of the increasing risks from weather extremes related to climate change, and they are projected by our global climate models to increase. And some of the principal scientists instrumental in the development of these climate models were just awarded the Noble prize in physics.

But winter 2021 also featured extreme winter weather, including record snowfalls and record cold temperatures in North America, as well as Europe and Asia. The winter weather event that likely had the most devastating consequences was the Texas freeze of February 2021 that included generational cold and an unprecedented widespread snowfall. Hundreds of people died and the economic cost from the extreme winter weather is estimated over $150 billion dollars in Texas alone.

But his was not a stand-alone winter event — it follows a decade with a surprising number of winter weather extremes including “snowpocalypse” and “snowmageddon.” It is intuitive and supported by our climate models that a warming planet would contribute to less extreme cold and paralyzing snowfalls, especially where the warming is most rapid in the high latitudes especially over the Arctic Ocean. Also, those skeptical of the link of climate change to anthropogenic activities have highlighted extreme winter events as proof that concerns over climate change are hyperbolic and fear-mongering.

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How to reconcile a surprising number of extreme winter events and climate change has become a topic of increasing scientific inquiry and of vigorous debate. Some climate scientists argue that there is no physical relationship between climate change and severe winter weather. However, there are a minority of climate scientists arguing a more iconoclastic idea: Rapid Arctic change related to accelerated warming can contribute to more severe winter weather.

The two most popular theories argue that either a weaker equator to North Pole temperature difference favors a slower and wavier Jet Stream or that heterogeneities in Arctic change are favorable for disrupting the stratospheric polar vortex (known as a sudden stratospheric warming because of dramatic and rapid warming of the polar stratosphere). A wavier Jet Stream and a disrupted polar vortex lead to more vigorous exchange of air masses necessary for extreme winter weather. The historical and costly Texas freeze brought the debate of the linkage between climate change and the extreme winter weather to a climax, with even the White House arguing for the physical connection between the two. The problem is that the Texas freeze was not the result of a wavier Jet Stream or a large polar vortex disruption.

Instead, a recently published study that I co-authored presented analysis demonstrating that the Texas freeze was a result of a more obscure and minor stratospheric polar vortex disruption where the polar vortex stretches like a rubber band or taffy. We showed using machine learning that for the months of October through February these stretched polar vortex events have roughly doubled from 25 days on average to 50 days on average over the satellite era (roughly since 1980).

These events are related to extreme cold in the Eastern U.S. more so than any other configuration of the polar vortex. Although extreme winter weather is not observed on all stretched polar vortex days, the probability of extreme winter weather is elevated when the polar vortex is stretched. 

Empirical evidence suggests that Arctic change including less Arctic sea ice loss and increased snowfall across Siberia during the fall months favor a stretched polar vortex more so than any other polar vortex state or configuration. Therefore, we conclude that Arctic change made the Texas freeze and its devastating consequences more likely. 

As the Northern Hemisphere approaches a new winter season there are important and relevant societal lessons from our findings. It is incontrovertible that the climate system is more energetic now than any recent period. More energy in the system can be harnessed by the atmosphere to changes that are intuitive such as more intense heat waves, greater flooding and drought and more rapidly developing tropical cyclones. But we are also learning that energy can result in unforeseen and counter-intuitive changes to our climate.

For example, in the Southern Hemisphere, it can result in a stronger polar vortex and Jet Stream that insulates the Antarctic continent from milder air masses to the north, leading to a record cold Antarctic winter and extensive sea ice ringing the continent in 2021. In the Northern Hemisphere increased energy can result in more vigorous mixing of air masses bringing air masses typically confined to Alaska and Northern Canada to Texas while mild air from Hawaii is transported north of the Arctic circle. This is a sobering new understanding of the impacts of climate change and our quest for adaptation to extreme weather.

Energy prices are high due to short supplies and our study argues not to bank on climate change to suppress prices this winter. The anticipated demise of winter weather due to climate change is not guaranteed by a rapidly warming Arctic or at least at the rate predicted by our climate models and ignoring the hazards of extreme winter weather can have devastating impacts on our society, as exemplified by the Texas freeze of February 2021. The weather models are predicting for the polar vortex to be unusually weak later this month. This does not guarantee that it will be weak during the winter months when its impact is greatest, but our study suggests that this is not random but related to a rapidly changing Arctic and consistent with recent trends, which will likely continue for the foreseeable future.

Judah Cohen, Ph.D., is director of seasonal forecasting and principal scientist at Atmospheric and Environmental Research (AER). In addition to his research interests on Arctic mid-latitude linkages, Cohen is leading AER's development of seasonal forecast products for commercial clients. Cohen also has a research affiliate appointment in the Civil and Environmental Engineering Department of MIT.