Air pollution reduces Arctic cloud lifetime, study suggests

WASHINGTON — Fossil fuel emissions from Asia and Europe may be cutting down the life expectancy of Arctic clouds, reducing the clouds’ ability to regulate temperatures in the polar region, according to new research.

A new study published in Geophysical Research Letters, a journal of the American Geophysical Union, suggests pollution plumes coming predominately from Northeast Asia and Northern Europe travel to the Arctic region and allow cloud droplets to freeze at higher temperatures.

This phenomenon triggers earlier than normal snowfall and can reduce the clouds’ lifetime, according to the new research.  The shorter the clouds live, the less they are able to regulate temperatures at the surface, the study’s authors said.

Pollution has been known to disrupt Arctic temperatures by introducing greenhouse gases into the atmosphere, but the new finding suggests another process by which pollution from mid-latitudes can disrupt polar temperatures. This previously unquantified effect represents a valuable piece of the Arctic climate change puzzle, according to the study’s authors.

“The Arctic has a climate that is changing very rapidly, and the warming is more intense than the warming that we have in the middle latitudes,” said Quentin Coopman, an atmospheric scientist at the Institute of Meteorology and Climate Research at Karlsruhe Institute of Technology in Karlsruhe, Germany and lead author of the new study, which was completed while Coopman was a graduate student at the University of Utah and the University of Lille.

Along with increasing temperatures, the Arctic is experiencing record lows of sea ice extent in recent years according to the National Oceanic and Atmospheric Administration.

“We focused on the pollution and cloud interaction, but it is part of a bigger system including the sea ice and the atmosphere of the Arctic and (greenhouse) gas and aerosols for example,” Coopman added. “Our results will help modeling studies better predict the evolution of the climate in the Arctic.”

Not many sources of pollution exist in the Arctic but pollutants from combusted fossil fuels coming from other areas of the world can invade the region through atmospheric circulation patterns, Coopman said. Once the pollution arrives, it becomes trapped for weeks or months under a temperature inversion, where a layer of warm air rests above cooler air near the surface and prevents the pollution from escaping into the upper atmosphere or depositing on the surface.

Clouds can either cool or warm surface temperatures in the Arctic, depending on where they form and how much sea ice is present. Clouds above sea ice trap some of the sunlight reflected and heat emitted by the ice, which can warm the surface. But clouds above ocean water, which is much less reflective than ice, block sunlight and have a cooling effect. Collectively, these processes are key in regulating Arctic surface temperatures.

Previous research conducted by Coopman and his colleagues showed Arctic cloud properties are extremely sensitive to pollution. They found clouds in the Arctic were two to eight times more sensitive to air pollution than clouds at other latitudes.

In the new study, the researchers wanted to further investigate how air pollution affects Arctic clouds. They combined data from satellite images of Arctic clouds with atmospheric models used to simulate carbon monoxide, a by-product of incomplete combustion used as a tracer for pollution coming from mid-latitudes.

The new study’s results suggest pollution plumes lower the amount of cooling needed for cloud droplets to freeze by about 4 degrees Celsius (7.2 degrees Fahrenheit), a much stronger impact than expected, Coopman said. This means cloud droplets can freeze at higher temperatures. When cloud droplets freeze more readily, snowfall occurs sooner, which can decrease the clouds’ lifetimes and inhibits their ability to regulate temperatures at the surface, according to the study’s authors.

The new study did not examine how much this change in cloud formation is affecting surface temperatures but the study’s authors said previous work suggests a reduction of cloud lifetime would have an overall cooling effect on the surface and a warming effect in the upper atmosphere.

“Small changes can have very strong consequences in the Arctic region because the atmosphere is very dry, the temperature is very cold, and the clouds are at the edge of existence, so any addition of pollution will have a strong impact on the clouds,” Coopman said.

Marc Salzmann, a research scientist at the Institute for Meteorology at the University of Leipzig in Germany who was not involved with the new study, noted that although the study suggests combustion aerosols to be the cause of the change in freezing temperature, more research needs to be done to show what exactly about the plumes leads to this shift.

“Carbon monoxide is used as a marker for air pollution in this study, but it is obviously not the carbon monoxide itself that causes this,” Salzmann said. “It would therefore certainly be very interesting to find out which physical processes may cause this correlation.”

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