This article was originally featured on The Conversation.
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 U.S. has been feeling this warming in the winter of 2023-24. Snowfall has been below average in much of the country. On the Great Lakes, the ice cover has been at historic lows. Late February saw a wave of summerlike temperatures spread up into the central and eastern U.S., accompanied by the potential for dangerous thunderstorms and wildfire risk. And forecasters expected another above-average warm spell in early March.
The longer warming trend 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.
Ice storms and wet snow
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.
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