When University of Montana geologist Joel Harper was a kid growing up in Durango, Colo., he spent his winter days collecting snow and building “glaciers” to see if they’d last through summer.
Harper, who teaches snow and ice at UM, understands why his childhood glaciers didn’t survive the season’s change. Now, he’s looking to see if the Greenland ice sheet will last the century, and what its melt means for rising sea levels.
“The amount of area that melts each summer on average has been going up,” Harper said this week, while discussing his new study in his campus office. “The big question is what happens to that melt? Does it make it to the ocean or not?
Harper led a team of researchers from several universities to Greenland in 2007 through 2009 to answer that question. The results of their $1.3 million study, funded by the National Science Foundation, were published last month in the prestigious journal Nature.
“The melt generated from low elevations easily escapes to the ocean, but there’s a larger area at higher elevation where the fate of the melt is highly uncertain,” Harper said. “Quantifying how that melt can infiltrate and refreeze in cold snow and firn is critical to understanding sea level changes.”
In the wake of storms like Sandy, the consequences of rising sea levels have jumped to the forefront of the nation’s discussion on climate change. While that may extend beyond Harper’s purview, the effects of a warming climate are evident across Greenland, where the ice sheet continues to retreat.
Yet where the melt water goes has been something of a mystery. Melting ice along Greenland’s low-lying coastal areas flows into the sea, that much is clear. But melt from the island’s interior, where the ice is several thousand feet thick, may be retained in the firn, or layers of compacted snow up to 300 feet thick.
Harper described the firn as a giant sponge. The sponge is absorbing water from melting ice, keeping it from reaching the ocean. But like all sponges, the firn is getting soggy and soon, Harper believes, it will lose its ability to retain the melt.
“Under a range of plausible climate scenarios, filling this firn will take on the order of decades,” Harper said. “It’s not a year or two of heavy melt that’s going to do it, but it’s also not centuries.”
Harper believes the firn can hold an additional 322 to 1,289 gigatons of water before it becomes saturated and begins to release runoff into the sea. One gigaton of water equals roughly one cubic kilometer of water.
It’s a massive amount of melt, but Harper notes there remains a massive amount of ice. If the Greenland ice sheet were to melt completely, Harper suggests sea levels would rise more than 20 feet, a devastating figure for coastlines around the world.
“There’s 7 meters of sea level rise sitting there in the ice,” Harper said. “Nobody believes that sea level rise can come in a very short period of time, but it can happen in the next decades to century.”
Harper and his team of researchers from the universities of Wyoming, Colorado and de Liege in Belgium, were dropped off in Greenland by planes fitted with skis.
They spent the next month traversing the Greenland ice, skiing behind two snowmobiles and sleeping in tents dug into the snow. They drilled ice cores and installed temperature sensors to measure the rate at which the water was refreezing in the firn.
“Greenland is a giant region of flatness,” Harper said, noting the location of their traverse on a wall map. “Every day is just different sameness. You’re out there in the middle of nowhere.”
The initial study took two years to complete, and came at a crucial time for Greenland. Satellites have monitored the ice sheet’s melt for more than 30 years. The area of melting ice and the duration of that melt both are increasing.
Harper said this past year marked the first time since monitoring began that the entire ice sheet has experienced melt. Yet core samples suggest it likely has happened at points in the past.
“The sea level is definitely coming up, and Greenland is contributing to that,” Harper said. “But there’s a lot of uncertainly over how much it’s coming up or will come up in the future because of Greenland.
“We have this observation that melt is increasing, but we don’t know how much of that goes into the ocean and how much stays behind.”
Researchers are now drilling through the depths of the Greenland ice and placing sensors at the bottom to measure pressure and flow. The water that makes it to these depths, Harper said, reaches the ocean.
The question remains, when will the flows increase, and by how much?
“We’re looking at that water underneath and how it influences the motion of the ice sheet,” he said. “The more water you put down there, the faster the ice may move. It’s speeding up. That’s what we’re trying to do is figure out how fast it’s speeding up.”