In the high-stakes race against sea level rise, understanding what's causing the Greenland Ice Sheet to melt is critical. The problem isn't just rising temperatures: soot from ships, wildfires and distant power plants, as well as dust and a living carpet of microbes on the surface of the ice, are all speeding up the melting.

Right now, predictions for sea level rise range from about 1 to 10 feet by 2100—a wide difference for coastal communities trying to plan seawalls and other protective measures.

The more we understand about how pollutants affect the ice, the more accurate those projections will be. So, let's take a look at what's happening on the ice sheet now—and the risks ahead.

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First, temperatures are rising in the Arctic at about twice the global average. That causes melting around the edges of the ice sheet each year and reaches across more of the surface during summer heat waves.

In areas near the edge of the ice sheet, things get even more interesting: a carpet of microbes and algae mixed with dust and soot, a short-lived climate pollutant, is darkening the ice sheet, absorbing the sun's rays and accelerating the melting of the ice.

New research shows this dark zone is growing.

Yellow and red research tents dot the ice sheet along the dark zone. Credit: Dark Snow Project, via NASA

Roughly 70 percent of Greenland's contribution to sea level rise today comes from the melting of the ice sheet, rather than from glaciers calving, so what happens in these acceleration zones matters for Miami and other cities along the coasts.

Greenland's Living Carpet of Algae

In western Greenland, the dark zone is about the size of West Virginia. It grew by 12 percent between 2000 and 2012, and new research suggests it's likely to continue to expand, according to climate researcher Jason Box, who travels wide swaths of the ice sheet each summer to collect samples for the Geological Survey of Denmark and the Dark Snow project.

The new research, published in the journal Nature Communications, describes a geological feedback loop on the ice that's expanding the dark zone: Warming melts the western edge of the ice sheet, releasing mineral dust from rock crushed by the ice sheet thousands of years ago. That dust blows to the surface of the ice, nurturing the microbes and algae living there. Those organisms produce colored pigments as sunscreen, which contribute to the darkening of the surface, reducing reflectivity and increasing melting.

A scientist takes samples of dark biological material from the Greenland Ice Sheet for testing. Credit: Dark Snow Project

Box was part of a research group that used images collected by drones to assess a 25-kilometer slice of the dark zone. They found that a mix of soot, dust and algae account for 73 percent of the darkening, with the terrain and other unknown variables accounting for the rest.

The exact breakdown of the different substances is still being studied, but the last few years of research show the algae and microbes are the dominant cause of the darkening.

Black Carbon: A Climate Double-Whammy

Soot, also known as black carbon, is another big concern because it's a climate double whammy: It's much more potent than CO2 at trapping heat in the atmosphere in the short term, and when it falls to the surface, it gradually darkens Greenland's snow and ice.

Those two impacts are key reasons the UN's International Maritime Organization is currently moving toward new restrictions in the Arctic on heavy fuel oil, shipping's fuel of a choice and a major source of black carbon. They're also reasons several countries are working to expand the Gothenburg Protocol to cover black carbon. (Never heard of it? Read this.)

So far, black carbon falling on the ice sheet has not reached a critical level that would sharply speed up or expand Greenland Ice Sheet melting on its own, scientists say. Samples of melted snow, airborne sensors and satellite readings suggest that black carbon concentrations in Greenland have not increased significantly since the early 2000s, when James Hansen calculated that black carbon reduced Arctic albedo by about 1.5 percent. That's in part due to policies that have helped cut back on industrial emissions.

However, black carbon in snow is still the most efficient climate forcing agent because the fallout season (when the particles fall to the surface in snow or rain) coincides with—and exacerbates—the spring snowmelt cycle, University of Washington climate scientist Stephen Warren said during the European Geosciences Union (EGU) conference last week.

A study in the European Alps published in 2013 hints at the climate-changing power of soot, showing that black carbon pollution from early in the Industrial Age triggered a meltdown of alpine glaciers.

What Can Be Controlled?

Increased Arctic shipping, as warming opens up more of the once-ice-covered Arctic Ocean, is likely to bring more black carbon to the region in the coming years. And increases in Northern Hemisphere forest fires and peat fires on Greenland as the planet warms could add to it.

But black carbon is also one accelerator of Greenland's melting that can be curtailed by policies because of its human sources.

(The NASA animation below uses satellite data from the past 15 years to show how Greenland has been losing more ice each summer.)

Last summer, unusual peat fires in Greenland's drying permafrost released about 23 tons of carbon near the ice sheet. By the scale of North American or Siberian wildfires, the Greenland fires weren't all that big, but their proximity increased the amount of black carbon particles falling on the ice sheet, Norwegian researcher Andreas Stohl said during the EGU meeting. He noted that Greenland's permafrost is thawing faster than expected now.

Intense wildfires in Siberia and northern Canada, where last summer's blazes generated an "unprecedented" smoke plume that curled toward the Arctic, are also adding to the black carbon risks, said NASA researcher Douglas Morton.

"These are the dirtiest clouds on Earth. They are chimneys for pollution," Morton said. The tiny particles from the fires lingered in the Northern Hemisphere's atmosphere for a month, each molecule of black carbon trapping 3,200 times more heat than CO2 over a 20-year span. Caught up in seasonal northwest winds, some of that soot ended forming nuclei for snowflakes that fell on Greenland.

Even relatively small amounts of black carbon can make a difference by triggering a chain reaction in delicate snow crystals, Box said.

"Just the little bit of extra heat from a tiny soot particle can start transforming feathery and highly reflective snow crystals into darker, rounded grains that absorb more heat," he said.