The 10,521-foot summit of Greenland's desolate ice sheet was the site of a rare phenomenon in 2012 -- the top layer of dry snow at that high elevation melted. Ninety-seven percent of the surface of Greenland's ice sheet melted that summer, including the rarely-changed, remote, cold summit.
Such an event is noticeable on the white expanse, said one witness.
"The surface gets kind of slushy," said Kaitlin Keegan, a doctoral student at Dartmouth who was there at time. "You can definitely see that the snow is wet."
Widespread melt like that had not occurred on the high-latitude snow surface since 1889, when about 80 percent of the Greenland ice sheet's surface thawed.
Now Keegan and her research partners have pinpointed the cause of the extraordinary melt in those years -- soot from extensive wildfires in other parts of the northern hemisphere that dusted the snow with dark, solar-heat-absorbing particles, combined with unusually warm conditions that summer.
The findings were detailed in a study published online Monday in the Proceedings of the National Academy of Sciences.
To create the surface-melt conditions seen in 2012 and 1889 takes a special combination of heat and black carbon, said Keegan, the lead author. Ice cores showed that there were some warmer years, and some years with more wildfires, but that the precise confluence of melt factors was rare, she said.
There were extensive wildfires in 2012 in Siberia and North America, although there is no data yet to determine the exact origins of the wildfire soot that drifted to Greenland, Keegan said.
Historical records also say that 1889 was a big wildfire season in the western United States, including the notorious Santiago Canyon fire that burned in California.
Keegan, who worked at the research observatory on Greenland's summit that is operated year-round by the National Science Foundation and is home to an atmospheric observatory operated the National Oceanic and Atmospheric Administration, examined the cores drilled into the ice sheet. Those cores clearly showed, in their layers, the point where wildfire soot fell on the surface and where the dry, white snow thawed and then refroze, she said.
The good news about the extensive surface melt? It did not send extra meltwater to Greenland's coastal edges to empty into the ocean and contribute to sea-level rise, Keegan said.
"We're not talking about ice-mass loss," she said. The thawed snow later refroze.
The bad news? The wide areas of thawed and refrozen snow damaged Greenland's ability that summer to reflect sunlight, thus drawing in more solar heat and potentially sending ripple effects through the Arctic. The refrozen layer did a poorer job of reflecting the sun's light -- a function known as albedo -- than the normal surface of dense, compacted dry snow known as firn.
There could be even worse news, according to the paper. Under a worst-case climate scenario, a similarly widespread surface melt over Greenland's ice sheet would be not a once-in-a-century event, but the summertime norm. If the Arctic is warmer in summer by 16.2 degrees Fahrenheit at the end of the century and wildfire frequency doubles, annual chances for such widespread surface melt would be 94 percent, according to the study.
Under the most conservative scenario, with an Arctic-wide summer temperature increase of 3.6 degrees, and no increase in wildfire frequency, the annual probability of such a widespread surface melt would be 17 percent by 2100, the study says.
Other scientists have studied the usual surface melt of 2012 and 1889.
NOAA, in its 2013 Arctic Report Card, showed that Greenland's albedo powers rebounded significantly in 2013 after the steep drop in 2012, but that they remain on an overall downward trajectory since 2000.
A study led by University of Colorado scientists and set to be published in the Journal of Geophysical Research evaluated the numerous atmospheric and oceanic factors that sent unusual heat to Greenland in 2012 and 1889.
Those factors, plus the spread of wildfire soot, combined to bring surface temperatures at Greenland's summit above freezing, the study says. The scientists created animated recreations of the 2012 and 1889 surface melts.
Another study, led by scientists at the University of Edinburgh, compared ice melt and movement in 2012 and 2009 and concluded that widespread surface melt in the middle of Greenland will not accelerate mass loss. That study, published Dec. 3 in the Proceedings of the National Academy of Sciences, found that the 2012 record summer melt was offset by a later slowdown in winter ice movement, meaning not much impact on Greenland's contribution to sea-level rise.
However, that study said more research is needed to determine if there is an effect of interior surface melt on Greenland's tidewater glaciers. Those glaciers are steadily shrinking, according to NOAA's Arctic Report Card, with area loss in 2010 and 2012 that was more dramatic than other recent years.