Global warming

Record Arctic Snow Loss May Be Prolonging North American Drought

Wired,

28 September, 2012

Melting Arctic snow isn’t as dramatic as melting sea ice, but the snow may be vanishing just as rapidly, with potentially profound consequences for weather in the United States.

Across the Arctic, snow melted earlier and more completely this year than any in recorded history. In the same way ice loss exposes dark water to the sun’s radiant heat, melting snow causes exposed ground to heat up, adding to the Arctic’s already super-sized warming.

This extra heat retention appears to alter the polar jet stream, slowing it down and causing mid-latitude weather patterns to linger. It’s even possible that the ongoing North American drought, the worst since the Dust Bowl of the 1930s, was fueled in part by climate change in the Arctic, making it a preview of this new weather pattern’s ripple effects.

“In the past, whatever happened in the Arctic stayed in the Arctic. But now it seems to be reaching down from time to time in the mid-latitudes,” said climatologist James Overland of the National Oceanographic and Atmospheric Administration. “When you combine the new influence of the Arctic with other effects, such as El Niño, we’re seeing the more extreme weather events.”

Over the last several weeks, public attention has been seized by the disappearance of ice in the Arctic Ocean, which in September covered a smaller area than at any other time in the climate record, a fitting exclamation point to its 50 percent decline since the late 1970s.

Variation from the post-1967 historical norm in June snow cover, June 2012. Dark orange corresponds to between 75 and 100 percent below average. Image: Rutgers University Climate Lab

In June, Arctic snow cover also reached historic minimums. At the time, the news received little attention. Though the snow has retreated for several decades, and has even declined as precipitously as the sea ice, freshly exposed ground simply lacks the visual impact of open water.

It’s also harder to put the decline into context: Scientifically useful Arctic snow records only date back to the beginning of satellite photography in the 1960s, a relatively short period of time. The role of snowmelt has received less research attention than sea ice, and scientists are just starting to understand the interactions between climate patterns in the Arctic and lower North America.

“This is cutting-edge science,” said climatologist David Robinson, who runs the Global Snow Lab at Rutgers University. The research is maturing, however, and the implications are troubling.

To understand what snow loss could do, it’s instructive to study what happens when sea ice melts, a process described in a Geophysical Research Letters paper published in March by climatologists Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin-Madison.

As the sea, now deprived of its reflective cover, absorbs heat, surface temperatures rise. That heat returns to the atmosphere during fall and winter, reducing the difference in temperature between the Arctic and latitudes below. This difference is what propels the northern hemisphere’s polar jet stream, the globe-spanning atmospheric current that pushes vast amounts of cold air south and warm air north.

“Think of it like a hill. Normally the Arctic is much colder than areas to the south. Because warm air takes up more space than cold air, the atmosphere to the south is thicker. If you’re sitting on top of one of these layers, you’ll slide down the hill to the Arctic. Earth’s spin turns you towards the right, and that’s what generates the jet stream,” explained Francis. “If you’re warming the Arctic more, the hill is less steep, and you won’t roll as fast.”

The jet stream loses speed. As this happens, say Francis and Vavrus, its path also changes, dipping far to the south and reaching to the north. This is what happens in fall and winter after Arctic sea ice melts in the summer. In the summer, after snow melts in spring, “we think a similar mechanism is going on with the snow,” Francis said. “If you lose all the snow earlier on high latitude land in the spring, when the sun is strongest, you’ve got dark soil exposed earlier, warming up earlier. It’s another way to make the Arctic warm faster than the rest of the hemisphere.”

'People talk about the new normal. There's nothing normal about this. It's going to continue to change.'For now, Francis says, this is still a hypothesis, albeit supported by North American climate patterns in recent years and similar observations from Siberia. “There’s just basic physics behind it. We’re dealing with a very different energy budget up in polar regions than previously, because we’re exposing the land earlier in the season to the warming rays of the sun,” Robinson said. “The physics are indisputable.”

Indeed, it’s reasonable to speculate about the effects of the jet stream’s new patterns — and that’s where things get really interesting. In another Geophysical Research Letters paper now in press, Francis and Overland describe how atmospheric pressure patterns generated by extreme spring snowmelts in the last several years seem to have channeled warm air across the central Arctic Ocean.

The winds accelerate the sea ice’s melt and push it into the Atlantic Ocean. They also seem to have hastened Greenland’s ice sheet melt, which reached unprecedented rates this July. “The winds used to be light,” said Overland. “Now we have more steady winds that blow from the Bering Strait across the north pole and out into Atlantic.”

The connection between snowmelt and the new winds hasn’t been directly proven, Overland said, but the pieces fit. “In the last three years, we’ve had a real major loss in snow cover. That’s why we think there may be a tie between the loss of snow, higher atmospheric pressure and the changes in the winds,” he said.

As the polar jet stream slows and meanders, the regional weather patterns it influences could end up persisting longer than usual, rather than being carried away by the stream. Whether this would extend to temperate latitudes during the summer isn’t certain, said Francis, since the polar jet stream tends to be weaker in summer than in winter, but it’s plausible.

“It’s harder to show in summer, because the waves are more amorphous, but the same mechanisms should happen,” Francis said. If so, that could at least partly explain why the North American drought, which started in the spring, is so severe. In a year without such an extreme Arctic snowmelt, it might have been a dry spell dispelled by the jet stream. Instead it stuck around.

Northern hemisphere snow coverage, in millions of square kilometers, as compared to the post-1967 average. Image: Rutgers University Climate Lab

More research is needed to be certain this hypothesized cascade of snowmelt, jet stream changes and drought lockdown in fact happened — Overland cautioned that “it’s very, very difficult to say” — but it raises the possibility that the Arctic climate is even more intertwined with lower-latitude weather than most researchers thought.

If so, extreme lower-latitude weather events will become more likely. “As the waves work more slowly, the weather wherever you happen to be will tend to change more slowly,” Francis said. “If that goes on long enough, you have extreme weather. If you have a cold snap for a day or two, it’s not a big deal. If it goes on for weeks, it’s an event. Same with drought.”

The next question is whether the extreme Arctic snowmelt is a result of human-caused warming. According to Francis, that’s likely the case. “There’s nothing else that can explain it. It’s so dramatic. It’s almost certainly mostly anthropogenic,” she said.

Robinson said climate scientists generally agree that some Arctic warming is human-forced, but would disagree as to precisely how much. As for himself, “I believe we see the fingerprint of man in it,” he said, saying there is a “preponderance of evidence” that greenhouse gases are to blame. “We see multiple changes going on there. These things are happening just as the models suggest they should happen.”

Even a small amount of unnatural Arctic warming is a problem. “That little bit of warming starts all these physical processes, like loss of snow and ice, so you start absorbing more solar energy rather than reflecting it to space. That amplifies the signal,” said Overland, who says people are responsible for an Arctic uptick of about 2 degrees Fahrenheit. “It’s not just the initial warming. It’s the cascade of events.”

Some researchers have also linked the drought to an intersection of human-caused warming in the Indian Ocean, where warmer temperatures are historically associated with mid-latitude droughts, and natural La Niña cooling in the central Pacific, which generates dry spells in southern North America. Add this “perfect ocean for drought” to the Arctic snowmelt, and the combination may have been catastrophic.

Northern hemisphere snow coverage, in millions of square kilometers, as compared to the post-1967 average. Image: Rutgers University Climate Lab

That, of course, remains a hypothesis. “I wish we had years more data. I wish we had models that could give us order-of-magnitude improvements in temporal and spatial resolution. But that’s science. You put the pieces together, and you conduct your investigation,” Robinson said.

In a few years, scientists may have a better idea. In the meantime, the Arctic will continue to melt. “We are seeing changes that most of us never imagined we would see in our careers,” Robinson continued. “People talk about the new normal. There’s nothing normal about this. It’s going to continue to change.”