How dust in the wind may be quickening Greenland's ice melt

Dust blown onto Greenland's vast ice sheet from from elsewhere in the Arctic may be accelerating the pace of mass loss from the globe's second largest reservoir of fresh water.

Dust is known to absorb sunlight, then re-radiate it as heat. When it falls on snow, it triggers an earlier snowmelt. This exposes ice beneath the snow to the sun earlier in the season on the top of the ice sheet and along its margins.

Dust-related melting at Greenland could add at least 2 centimeters (0.79 inches) more to globally averaged sea-level rise by 2100 than current estimates suggest, writes Marie Dumont, a researcher at the National Center for Meteorological Research's Snow Study Center in Grenoble, France, in an e-mail.

Between 1992 and 2010, the ice sheet lost an average of 12.9 billion tons of mass a year, Dr. Dumont and colleagues note, in their study published June 8 in the journal Nature Geoscience. Currently the ice sheet is losing between 200 billion and 450 billion tons a year. The team estimates that the additional loss from dust's effect adds another 27 billion tons a year to these losses.

The contribution could be higher if the dust is triggering blooms of bacteria and ice algae on the ice sheet and its margins. Dust is a natural fertilizer for these organisms, which remain dormant in winter and blossom during the melt season. Colonies of these technicolor microbes darken ice and snow. The team suggests that this may be happening with Greenland's ice sheet, particularly around its margins.

"There is an urgent need of in situ measurements on that question," Dumont writes.

Satellite measurements have shown that during the past 10 years, the surface of Greenland's ice sheet has darkened during the melt season. The effect has been most pronounced since 2009.

The coming of the melt season alone can lead to a darker surface as temperatures warm. Snow on the surface during the melt season develops larger grains as the temperatures rise. Larger grains trigger additional warming, which begets even larger grains.

Meanwhile, global warming has increased the inventory of potential darkening agents: soot from an increase in large wildfires in northern Russia and in Alaska, plus an increase in dust as a longer snow-free season has exposed soils for longer periods of time.

In addition, climate change has altered atmospheric circulation patterns in ways that have enhanced warming at high latitudes.

When the team modeled the effects of these circulation patterns on snow melt at altitudes above about 6,500 feet, where the least amount of melting tends to occur, they found that changes to the snow grains was insufficient to darken the surface as much as satellite measurements had revealed. Moreover, the darkening trend held up even in 2011, when the spring season atop the ice sheet was the coldest in a decade.

This led them to explore other darkening agents.

Soot is more effective at warming than dust, but annual measurements of dust and soot in samples taken from the North Greenland Eemian Ice Drilling site, along with measurements taken since 2006 by others at different locations around Greenland, showed that concentrations of dust have run as much as 150 times higher than concentrations of soot, swamping soot's effect. And spectral analysis of the ice-sheet surface indicated that the predominant darkening agent was reddish in color, typical of dust. 

That left dust as a major co-conspirator, along with general warming itself, enhancing the ice sheet's mass loss. When the team plugged the dust's effect into their snow model, they found that it contributed the additional 27 billion tons of mass loss a year.

And while two Icelandic volcanoes experienced significant eruptions in 2010 and 2011, the team notes that the melt-season darkening trend began prior to 2010.

The results are important because they represent a mechanism for reinforcing ice-mass loss that current models don't take into account, writes Jason Box, a glaciologist at the Geological Survey of Denmark and Greenland, in an e-mail from Copenhagen, where the geological survey is based. Dr. Box heads the Dark Snow project, a crowd-funded research program to directly measure the effect darkening agents have on Greenland's ice sheet.

The results imply that "sea-level rise projections are probably underestimates," he writes.

That's almost a certainty, he suggests, noting that the number of factors reinforcing mass loss from Greenland's ice sheet outnumber the factors retarding it by about 7 to 1.

Moreover, based on studies of past glacial and interglacial periods, ice builds gradually at the start of an ice age, while it declines rapidly as the climate warms again.

In the past, however, that warming started from a much colder climate than Earth is experiencing today – an interglacial period, he adds.

Greenhouse gases from burning fossil fuel and from land-use changes are adding to that natural interglacial warmth, pushing the climate into what he dubs "a super interglacial state" – something for which "there is no precedent."