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Ice storms, downpours, heavy snow, no snow: Diagnosing ‘warming winter syndrome’

Richard B. (Ricky) Rood, University of Michigan, The Conversation on

Published in Science & Technology News

One of the most robust measures of Earth’s changing climate is that winter is warming more quickly than other seasons. The cascade of changes it brings, including ice storms and rain in regions that were once reliably below freezing, are symptoms of what I call “warming winter syndrome.”

Wintertime warming represents the global accumulation of heat. During winter, direct heat from the Sun is weak, but storms and shifts in the jet stream bring warm air up from more southern latitudes into the northern U.S. and Canada. As global temperatures and the oceans warm, that stored heat has an influence on both temperature and precipitation.

The warming is evident in changes to growing seasons, reflected in recent updates to plant hardiness zones printed on the back of seed packages. These maps show the northward and, sometimes, westward movement of freezing temperatures in eastern North America.

The shift of this freezing line between snow and rain can mean ice storms in places and at times when communities aren’t prepared to handle them, as several parts of the U.S. saw in early 2024.

I study the impact of global warming and have documented changes to the climate and weather over the decades.

On average, freezing temperatures are moving northward and, along the Atlantic coast, toward the interior of the continent. For individual storms, the transition to freezing temperatures even in the dead of winter can now be as far north as Lake Superior and southern Canada in places where, 50 years ago, it was reliably below freezing from early December through February.

 

When temperatures are close to the freezing point, water can be rain, snow or ice. Regions on the colder side, which historically would have been below freezing and snowy, are seeing an increase in ice storms.

The character of snow also changes near the freezing line. When the temperature is well below freezing, the snow is dry and fluffy. Near freezing, snow has big, wet, heavy flakes that turn roads into slush and stick on tree branches and bring down power lines.

Because the climate in which snowstorms are forming is warmer due to global accumulation of heat, and wetter because of more evaporation and warmer air that can hold more moisture, individual snowstorms can also result in more intense snowfalls. However, as temperatures get warmer in the future, the scales will tilt toward rain, and the total amount of snow will decrease.

Indeed, on the warmer side of the freezing line, winter rain is already becoming the dominate type of precipitation, a trend that is expected to continue. With the warmer oceans as a major source of moisture, the already wet Eastern U.S. can expect more winter precipitation over the next 30 years. Looking to the future, soggy wet winters are more likely.

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