The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
Many of the Northeast’s heaviest snowfalls in over a century of reliable record keeping have occurred since 1990. How can we reconcile the rash of large snowstorms with our warming climate?
I’m an atmospheric scientist. Let’s take a look at a fundamental law of physics, and some theories that could help explain these changes.
Warmer air, more moisture
First, warmer air can retain more moisture than colder air.
Think about the atmosphere as a sponge. Air retains approximately 4% more water vapour for every degree Fahrenheit of temperature increase (that’s about 7 percent per degree Celsius). The physical law that explains this relationship is known as the Clausius-Clapyron relation.
This increased atmospheric moisture is helping intensify the water cycle. The Northeast and Mid-Atlantic have become more wetter, not just in winter but also in spring, summer, and fall. In addition to more total precipitation over a season and year, the additional moisture also fuels extreme events, like more intense hurricanes and flooding rains. The Northeast has seen an increase of more than 50% in the heaviest precipitation events in recent decades, the largest increase of any region of the U.S.
In the early 1900s, winters across the Northeast typically averaged around 22 degrees Fahrenheit. Now, 26 degrees is the official new “normal” temperature, defined as the average over 1991-2020. A few recent winters have been over 30.
In the Northeast, however, the climate has warmed but is still below freezing. Or, to put it another way, the cold regions of the world that are suitable for snow have been sufficiently warmed to allow storms capable of holding more moisture. The region experiences heavy snowfall, rather than intense downpours.
The warming ocean plays a role
The historic blizzard that buried Boston under nearly 2 feet of snow in January 2022 was fueled by ocean waters in the western Atlantic that are warmer than normal. This is also part of a pattern.
The oceans have been absorbing more than 90% of the additional heat attributable to rising atmospheric greenhouse gases from human activities, particularly burning fossil fuels. The oceans now contain more heat energy than any time since measurements began six decades ago.
Scientists are investigating whether global warming is causing a slowing down of the ocean conveyor belt that transports water around the world. Satellite imagery and ocean measurements show that warmer waters have “piled up” along the East Coast, a possible indication of a slowing of the Atlantic Meridional Overturning Circulation.
The majority of the energy needed for tropical and mid-latitude extratropical cyclones is provided by ocean water. This is commonly known as nor’easters.
The Arctic influences the snow pattern, too
While tropical storm systems are fueled primarily by warm water, nor’easters gain energy from sharp temperature gradients where cold and warm air masses meet. Climate change may also be contributing to the recent increase in extreme snowfall events.
Recent research has suggested that a warming Arctic, including declines in Arctic sea ice and snow cover, is influencing behavior of the polar vortex, a band of strong westerly winds that forms in the stratosphere between about 10 and 30 miles above the Arctic every winter. The winds surround a large pool containing extremely cold air.
When the Arctic is relatively warm, the polar vortex tends to be weaker and more easily elongates or “stretches,” allowing extremely cold air to dip south. Episodes of polar-vortex stretching have markedly increased in the past few decades, leading, at times, to more severe winter weather in some places.
Arctic amplifying, the enhanced warming of our north, may paradoxically be helping to shuttle cool air to the Eastern Seaboard in polar vortex disruptions. Here, the cold air can interact and mix with the warmer, moister air from the warmer than normal western Atlantic Ocean. The historic blizzard was made possible by the most recent stretched Polar Vortex event.
Global climate models project an increase in the most extreme snowfall events across large areas of the Northern Hemisphere with future warming. In some other parts of the world, like Western Europe, intensification of the hydrological cycle will mean more winter rain than snow as temperatures rise.
Winter temperatures will remain cold enough to allow storms to bring heavy, if not unprecedented, snow to the east coast of North America and Northern Asia. Extreme snowfalls, according to climate models, will become less common but not necessarily less intense in the second half century as more storms produce rainfall.
A warming climate is expected to lead to a sharp rise in the number of high-impact Northeast winter storms. As extreme events become more frequent due to climate change, this is another risk that the U.S. must be prepared for.