By Janet Raloff
Earth’s warming in recent years has had an exaggerated impact in the Arctic. There, temperatures have soared relative to temperate areas, resulting in an increased summer melting of sea ice. But new research indicates that the local warming would be even more dramatic if it weren’t for salt sprays kicked up by whitecaps from the Arctic’s increasingly open waters.
Snow and sea ice reflect much of the sun’s warming rays back into space. As an increasing share of the Arctic Ocean’s year-round cover of sea ice has disappeared, the sea surface has darkened — or reduced its albedo — and become an increasingly better absorber of solar energy. The open water starts to develop in spring and doesn’t ice over again until fall. Year-round ice is ice that survives the summer.
Clouds can deflect incoming solar rays. And climate scientists have realized that some of the salt from sea spray could enter the lower atmosphere, forming diffuse filtering clouds of tiny particles. The question had been how effectively these clouds might offset warming due to the Arctic Ocean’s diminishing summer albedo.
The majority of sea-salt particles emitted into the air are well under a micrometer in diameter, notes climate scientist Hamish Struthers of Stockholm University. These tiny particles, known as aerosols, can persist in the air for days or even weeks, he says, and rise to altitudes of a kilometer or more. For the new study, he and his colleagues input into a computer program satellite measurements of seasonal changes in the Arctic albedo and surface measurements of a host of features including temperatures, ice cover and sea salt kicked up by waves on the ocean surface.
As expected, the salt clouds can exert a subtle cooling of the Arctic, the team reports online April 13 in Atmospheric Chemistry and Physics. Struthers says his group’s new calculations indicate that “the size of this [salt] aerosol effect is most likely 10 percent or less of the albedo [warming] effect” due to sea-ice melting. The effect is small, he acknowledges, but not negligible.
“Natural feedbacks in the climate system, such as the one described here, are potentially very important,” says Natalie Mahowald, an atmospheric scientist at Cornell University. “This is the first time I have seen this feedback mechanism discussed or quantified,” she says, making it “an interesting paper that describes a new mechanism for modulating climate in the Arctic region.”
