Arctic clouds are more vulnerable to pollution than once thought

Air pollution could drive warming across the Arctic to a far greater extent than it does further south.

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This April 9, 2015 natural-color satellite image shows sea ice and cloud streets between Labrador (Canada) and Greenland. (NASA Earth Observatory)

When Tim Garrett was a graduate student, his research brought him to what was then Barrow (and today Utqiagvik), the northernmost settlement in Alaska. Here, he boarded a plane to analyze how clouds affected the region below. Soaring through the air, he saw what he called “mystery plumes” — hazy air that looked like the smog found circling Los Angeles.

“It was really quite surprising,” Garrett, now professor of atmospheric sciences at the University of Utah, said during a call with ArcticToday. “Occasionally we would see pollution that was in the same place of the clouds and we never could figure out where that pollution was coming from.”

Curious, Garrett dove into historical records once back in the Lower 48. He found that this phenomenon wasn’t new: Arctic explorers dating back to the late 19th century had recorded instances of hazy clouds. Perhaps the first person to notice the haze was the Swedish explorer Adolf Nordenskiold, although he attributed it to more cosmic forces — he thought the pollution was coming from outer space.

Fridtjof Nansen, the famous Norwegian polar explorer, observed some of this pollution during his first trips to the region more than 100 years ago. But it took until the 1970s for scientists to name the phenomenon, “Arctic haze.” After being transported to the Arctic from lower latitudes, pollution then settles under a temperature inversion where cold air is trapped under a cap of warm air.

Now, a study and published in the journal Geophysical Research Letters by researchers including Garrett, looks at how this pollution impacts low level Arctic clouds. Focusing on this event has implications for the whole region, as clouds can influence how much the already-fragile Arctic warms.

For the study, Garrett and the article’s lead author Quinten Coopman used a model to simulate air pollution plumes from different sources between March and September, 2005 to 2010. They also projected what would happen when the pollution interacted with Arctic clouds.

This is one of the first studies to incorporate both pollution data and large scale meteorological data into one model data set. The last point is particularly important, as the authors could correctly say if a cloud was the result of pollution, or just naturally occurring.

“It’s so easy to get perhaps the right answer for the wrong reason or perhaps the wrong answer for the wrong reasons,” said Garrett. “To just say, ‘oh look, this cloud has gotten thicker in the presence of a pollution plume.’ But, that may have nothing to do with the pollution plume, it’s just that the plume arrived with a moist air mass.”

Jessie Creamean, an aerosol chemist at the Cooperative Institute for Research in Environmental Science in Boulder, Colorado, has done similar research looking at pollution over the North Slope of Alaska. She appreciates the scope of the new study—how it looks at the Arctic as a whole, rather than just one location.

“It’s nice to see it taken to the next level,” said Creamean.

The authors then calculated how sensitive Arctic clouds were to two types of air pollution: human-made aerosols from places including Europe and China, and smoke from forest fires. They found that Arctic clouds were two to eight times more sensitive to air pollution compared to clouds from lower latitudes.

Clouds in the Arctic can already contribute to rapid warming observed in the region by radiating back some of the energy emitted from sea ice. This effect can be intensified when the clouds are sensitive to pollution—more aerosols can lead to more water droplets in the clouds, which in turn help the clouds warm the surface more.

Garrett speculates that the clouds are more sensitive to air pollution because the temperature inversion found throughout the Arctic makes it harder for air to move. In mid-latitudes, low level clouds are warmer and tend to be more turbulent, which mixes cloudy air and clear air. But in the Arctic, the inversion makes sure this turbulence doesn’t happen as often.

“Because the air is trapped vertically, it just can’t move up and down with the same ease as it would if there wasn’t an inversion. And if that’s the case and there is very low mixing between cloudy air and clear air…the effects of pollution on clouds are much more apparent,” Garrett said.

In an email, Coopman also said that clouds may be more sensitive in areas with less pollution like the Arctic.

“The pollution concentration is initially low, and any change in pollution will have a larger effect,” he wrote.

The authors also found that Arctic clouds were much more sensitive to anthropogenic pollution than smoke from forest fires. This was a limitation of the study, as they couldn’t provide a precise explanation for this phenomenon. Garrett speculates that it might be a result of where the plumes end up in the atmosphere — due to its high buoyancy, air from forest fires might rise higher than the low level clouds the study was primarily focused on.

“It does not mean that [forest fire pollution] does not have the potential to interact with clouds, it is just that it does not end up at the same place of clouds, and so it cannot interact with them,” Coopman wrote.

As the Arctic opens to more industrialization, these findings might become more relevant. In early January, the Trump administration released a proposal to open up almost all of Alaska’s federal waters to oil and gas leasing (a move now challenged by the state’s lawmakers). And China recently released its first official Arctic policy white paper encouraging moves for more Arctic shipping routes.

Garrett says that if these plans materialize they will have an impact on the Arctic environment.  But he wasn’t sure how much. If the Arctic warms, that might bring more precipitation, which might wash out some of the pollution collecting in clouds.

Still, knowing the impacts of aerosols may help inform future decisions.

“It’s important to know what they are doing to assess the problem now, but to also assess how it may have an impact in the future,” said Creamean.