President Trump mixed political sloganeering with a warning to take shelter for Northeasterners caught in a deadly winter storm that dumped 2 feet of snow in some places earlier this month.

He tweeted on Jan. 20, “Wouldn’t be bad to have a little of that good old fashioned Global Warming right now!”

Extreme winter weather often prompts a fresh wave of public skepticism from those who doubt the existence or severity of human-caused climate change. Trump, for his part, has made many similar comments.

As experts point out, short-term variations in the weather—even when they result in extreme snowfall or record-breaking cold—don’t negate the long-term trend of global warming. Decades of data unambiguously demonstrate that average temperatures all over the world are on the rise.

But when it comes to winter weather, and specifically snow, teasing out the effects of climate change is a special challenge. It seems as though snow—arguably the most iconic feature of a cold climate—should be one of the most obvious indicators of global warming. But scientists are finding that it’s not that simple.

For one thing, there are a variety of ways to measure snow—the amount of snow that falls from the air, its depth on the ground, the amount of total water the snowpack contains, and the total surface area it covers on the earth. That’s just a few.

Now add climate change. It has conflicting influences on snow. In many places, rising temperatures may increase the chances that snow will turn to rain before it hits the earth or that it will melt faster once it’s on the ground. On the other hand, warmer air is capable of holding more moisture and producing more precipitation. That means in some very cold climates, where there’s still plenty of room for temperatures to stay below freezing, warming could actually cause an increase in snow.

And what happens to individual snowstorms, like the one that blasted the East Coast two weekends ago, is an even more complex question. Some research suggests that severe snowstorms are actually occurring more frequently in some areas, even places where scientists otherwise expect average snowfall or snow cover to decline under global warming.

In short, there are “no easy answers” to the question of climate change and snow, according to David Robinson, New Jersey’s state climatologist and head of the Rutgers University Global Snow Lab.

Scientists broadly agree that snow will change in most places as the climate continues to warm, he told E&E News. But exactly how and why, from one location to the next, may be among the most challenging questions about weather and climate change.

“I mean, it’s been the bane of my existence for my entire career,” said Robinson, laughing.

Clear and complicated

“Our winters are getting sick, and we know the reason why,” said climate scientist Amato Evan of the Scripps Institution of Oceanography in a presentation at the fall meeting of the American Geophysical Union. “It’s global warming, it’s rising temperatures, and that’s the only logical explanation for what’s happening.”

Evan was referring to the snow season in the western United States—one of the regions where global warming has had some of its clearest effects on snow. Multiple studies have found that snowpack in the Western states, including the Sierra Nevada and the Rocky Mountains, has been declining for decades (Climatewire, Dec. 13, 2018).

paper published last March found that about a third of all monitoring sites in the western United States are showing significant snowpack declines and that the total amount of water stored in the average April snowpack has declined by 21 percent since 1915. A 2011 paper said the recent declines are likely the most dramatic the region has seen for a thousand years.

It’s a concerning trend. Mountain snowpack provides a crucial supply of fresh water when it melts each spring, helping feed the mighty Rio Grande and the Colorado River, both of which millions of people depend on. Those rivers have been experiencing low flow in some places in recent years, a big problem for water managers in the United States and Mexico with fears of impending water shortages.

Fresh water from melting snow also helps moisten the soil each spring and promote plant growth. Some experts are concerned that dwindling snowpack could contribute to drier landscapes and bigger, hotter wildfires in the Western states, which have already seen record-breaking blazes in recent years.

Scientists say one problem is that the snow season itself—the time of year when snow falls and accumulates on the ground—is shrinking. At the time of his presentation, Evan had just published a paper saying that fall is coming later and spring is coming earlier. More and more, the snow season at high altitudes in the mountains is starting to resemble the snow season at lower, warmer elevations.

The western United States is one of the clearest examples of a changing snowscape, but the pattern of earlier springs seems to be happening across much of the Northern Hemisphere. Data from the Rutgers Snow Lab, which tracks variations in snow cover across the Northern Hemisphere from one year to the next, suggest that there’s a trend toward below-average spring snow cover over at least the past decade or two, compared to the typical spring snow cover in previous decades.

2016 analysis of satellite records also pointed to a broad pattern of decreasing snow cover across the Northern Hemisphere, particularly in the spring, with the strongest effects seen in western North America and Eurasia.

That’s no surprise. Numerous studies have found that winters are growing shorter and spring is arriving earlier all over the world, even in places that don’t get much snow. But not every trend makes so much sense.

The same analysis also found that the depth of snow on the ground is generally decreasing across North America, with significant declines mainly seen in Canada. This makes sense in most places, but comes as a surprise in a few regions, the researchers point out.

In many places, a warmer, wetter atmosphere might simply lead to an increase in rain. But the researchers note that the coldest parts of North America—namely, Canada and parts of the northern United States—might be expected to actually see an increase in the amount of snow that falls to the ground. The analysis seems to suggest this isn’t happening in some of those places.

Other complex factors are also at play. In Canada, some research has found that the point in the season when snow is at its deepest is happening earlier and earlier in the winter. And snow seems to be melting faster once it’s on the ground. This could help explain why the overall trend in snow depth, when averaged across the entire winter, appears to be declining.

“The core of your winter might have as much or more snow on the ground at times, but it may melt off faster come spring,” Robinson said. “Some things are counterintuitive in some respects—and snow can be a perfect example of that.”

Gerard van der Schrier, a scientist with the Royal Netherlands Meteorological Institute, says that two seemingly conflicting snow trends can be true at the same time. In certain regions, heavy snowstorms in the dead of winter may indeed produce greater volumes of snow.

On the other hand, the same warming that causes heavier snow during the coldest months may also cause earlier spring melts or intermittent warm periods of no snow at all throughout the winter. So overall average snow cover can decline.

In other words, when conditions are right for snow, it really snows—but the melting in between or after these snow storms may be more severe, as well.

Some of van der Schrier’s research published last year found widespread declines in snow depth across much of Europe. But in the very coldest places, he pointed out, the opposite appears to be true.

Zooming into specific sites, the complex effects of climate change are even starker.

On Alaska’s Mount Hunter, a 14,000-foot peak in Denali National Park, snowfall is dramatically increasing. A 2017 study found that summer snowfall on the mountaintop has risen by about 49 percent since the mid-19th century. Winter snowfall has increased by a colossal 117 percent.

But even there, the reasons for the change are not entirely straightforward.

Local warming is part of it, with more moisture in the air leading to more precipitation. That region of Alaska has seen 2 to 3 degrees Fahrenheit of warming in the last 50 years alone.

But researchers say warming in the tropical Pacific and Indian oceans is also playing a role. Even from so far away, these ocean temperatures have a strong influence on atmospheric patterns in the Gulf of Alaska—and the warming seems to be strengthening a low-pressure system there that drives warm, moist, snow-friendly air across the state.

Snow and a climate debate

Alaska isn’t the only place where rippling, long-distance climate effects may be affecting the local snow. Some scientists suggest that rapid warming in the Arctic may actually have the ability to influence snowstorms in the United States.

Some data suggest that major snowstorms may be happening more frequently in the northeastern United States than in previous decades. That’s despite the fact that scientists generally expect average snowfall in the region to decline as it continues to warm.

Some researchers have proposed that these large snowstorms are being influenced by changes in major atmospheric air currents, driven by climate change at the top of the world.

Because the Arctic is warming faster than the rest of the planet, the difference in temperature between the North Pole and the equator is rapidly shifting. Atmospheric temperatures play a big role in regulating the flow of air around the world. As the temperature gradient shifts, some scientists say it may be disrupting major air currents, like the Northern Hemisphere’s jet stream, causing the flow of air to become wobbly or wavy as it moves around the globe.

Some scientists have suggested that these kinds of processes may be driving changes in the polar vortex, a swirling mass of air that lies over the Arctic. Shifts in the polar vortex may be launching more giant snowstorms at the eastern United States.

In a presentation last month at the American Geophysical Union’s annual meeting, meteorologist Judah Cohen of analytics firm Atmospheric and Environmental Research Inc. suggested that anytime there’s a period of “pulsating warmth” in the Arctic, it tends to coincide with a bout of severe winter weather on the East Coast.

In each case, he said, the reason seems to be linked to a temporary weakening of the polar vortex, which sends cold Arctic air streaming down into the mid-latitudes. The same processes are thought to be responsible for a recent pattern of colder, harsher winters in other parts of the world, such as Siberia.

The strong influence of the polar vortex on mid-latitude weather is very clear—scientists can see what kind of consequences it has in real time. Earlier this month, meteorologists reported that the polar vortex had split into several pieces and was likely to bring a spate of severe winter weather to the eastern United States.

That said, models have often had a hard time capturing a clear link between Arctic warming and these kinds of shifts, and not all scientists are convinced. Robinson, for his part, said he believes “the jury is absolutely still out” on the issue—but added that “it’s an area ripe for further research.”

In the meantime, research suggests that the largest snowstorms may continue to have a strong presence in the eastern United States under future climate change—even without accounting for the possible influence of Arctic warming. One recent modeling study suggested that snowstorms in the Northeast will generally decline by the end of the century under a severe climate change scenario. But it found that the declines will be greatest in small or moderate snowstorms and that severe snowstorms will be much less affected.

The reason, once again, comes back to the conflict between warmer temperatures and more moisture in the atmosphere.

The models suggest that the total number of days each winter suitable for snow will decline in a warmer world, said study author Colin Zarzycki, an atmospheric scientist at Pennsylvania State University. But on days when conditions happen to be right, the amount of snow produced will be higher than before. In other words, when snowstorms do occur, they’ll tend to be large ones.

“Essentially, what we’re finding is these really ‘snowmageddon’-type storms, these big nor’easter events, are not as significantly mitigated in our climate runs,” he told E&E News.

‘Melting cannibalism’

To better understand how climate change is affecting snow, some scientists are zooming in to look at the issue up close: the snowflake.

A warming climate doesn’t just affect the amount of snow that falls from the air, said Marco Tedesco, a snow and ice expert at Columbia University. It can alter the size and shape of the snowflakes themselves.

Snowflakes that form in warmer conditions may partially melt and refreeze again—sometimes several times in a row—as they fall through the air and accumulate on the ground. This process alters the structure of the snowflakes, often making them rounder at the edges or causing them to stick together in little globs.

These structural changes can cause the snowflakes to melt more easily. When that happens, large patches of snow cover may disappear faster than they would if the snow had formed in colder conditions.

“I call it melting cannibalism,” Tedesco said. “It’s the snow eating its own self.”

Tedesco recently launched a project he dubbed X-Snow, which aims to monitor changes in snowflakes and other factors like snow depth, focusing particularly on the eastern United States. He’s recruiting members of the public, or “citizen scientists,” to aid in the effort by attending local training sessions and then collecting their own measurements.

The campaign has already kicked off in New York’s Catskill Mountains.

“There’s a very little scientific effort on the East Coast, especially the New York area,” Tedesco said, when it comes to snow research. In the Western states, where concerns about issues like water management and wildfires tend to be higher, there’s a bigger focus on monitoring snow trends.

East Coast snow has a lot of importance, too, Tedesco said. It helps fuel local water resources and nurture natural ecosystems, as well, and it also has high economic value in some places, supporting ski resorts, winter sports and tourism.

“Snowflakes, they contain information on both the atmosphere where they come from and what happens to the snowpack on the ground where they fell,” he told E&E News.

Understanding how climate change is working at the snowflake level may help scientists better understand how snow has changed in recent decades.

“Simulating clouds and precipitation is still one of the hardest things to do in the climate models,” Tedesco said.

Even though debates exist around the future of snow, scientists do have some broad ideas about where it’s heading if global greenhouse gas emissions don’t start falling immediately.

“I think there is certainly a consensus that you’re going to have a shorter snow season, both in terms of snowfall events and snow on the ground,” said Robinson. “You’re going to have areas that commonly have snow have it less common. You’re going to have areas that commonly have snow have more rain events associated with that snowpack.”

And in some places, he added, snowfall or severe storms might increase. What’s clear is that climate change is causing the old patterns to shift, in one direction or another, even if scientists are still struggling to understand what’s happening and why.

“I think the consensus is you’re going to see in most every area a change in its historic snow regime,” Robinson said.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at