A massive new study by 16 authors has calculated just how much ice the Greenland ice sheet has lost since the year 1900. And the number, says the paper just out in the journal Nature, is astounding: 9,103 gigatons (a gigaton is a billion metric tons).
That's more than 9 trillion tons in total. And moreover, the rate of loss has been increasing, the research finds, with a doubling of annual loss in the period 2003 to 2010 compared with what it was throughout the 20th century.
The study was led by Kristian K. Kjeldsen of the Natural History Museum of Denmark at the University of Copenhagen. The complex work involved inferring the loss of mass of the total ice sheet over a very large stretch of time using a merger of multiple information sources - the distinct marks left by retreating glaciers on the landscape, extensive aerial photography from 1978 through 1987, and - beginning in 1983 - satellite and aircraft observations.
All were merged to provide the new mass loss estimates. "It's the first observational based study that shows where Greenland has lost its mass over the last 110 years," said Kurt H. Kjær, the paper's senior author and also of the Natural History Museum of Denmark at the University of Copenhagen.
The result is not only a better understanding of Greenland's massive, centennial contribution to sea level rise, but also disturbing confirmation that its loss of mass is now accelerating.
"We find that 2003-2010 mass loss not only more than doubled relative to the 1983-2003 period, but also relative to the net mass loss rate throughout the twentieth century," the study notes. It states that mass loss in this most recent period, ending in 2010, was 186 gigatons per year on average, though other estimates have put that number even higher for the most recent years. NASA currently states that Greenland is losing 287 billion tons of ice per year.
The fact that Greenland was losing mass throughout the entirety of the 20th century may seem surprising. But the paper suggests that we are coming out of the Little Ice Age, a cooler period, and so glacier retreat was more or less kicked off around 1900 - and then accelerated as major human-caused global warming kicked in.
Since 1900, the research finds, Greenland's major mass loss has been coming quite consistently from a few key regions - the northwest and southeast of the ice sheet, as well as at Jakobshavn glacier in the southwest, Greenland's single fastest moving glacier. It is currently losing 25 to 35 gigatons of ice annually.
However, there has been an alarming turn of late - in recent years a new sector has awakened, the northeast of the ice sheet, which features two major glaciers named Zachariae Isstrøm and Nioghalvfjerdsfjorden. They hold back fully 12 percent of all of Greenland's ice, as a feature called the "northeast Greenland ice stream" drains through this region.
"The northeast ice stream has been really active in the latest period, which we do not see any of the previous periods," said Kjær. "So that has a bit of a surprise when you put that into context."
Ice loss from Greenland today occurs through two key mechanisms - melting on the surface of the ice sheet followed by runoff into the ocean, and large calving events at marine based glaciers, which are followed by more flow of ice outward from behind them.
The latter process can be quite dramatic - capable of triggering huge earthquakes as gigaton-sized icebergs detach, roll in the water, and crash into glaciers behind them.
The new research also suggests that Greenland was a major player in the global sea level budget throughout the last century, and accounts for much sea level rise that the U.N.'s Intergovernmental Panel on Climate Change had previously not attributed to the Greenland. If the estimate of 9,103 gigatons of ice loss in the last 110 years is correct, then Greenland would have contributed about 2.5 centimeters of sea level rise over the period - roughly an inch.
That may not sound like much, but it's enough water to submerge the entire U.S. interstate highway system 98 feet deep - and to do so 63 times over, says Jason Box, a professor with the Geological Survey of Denmark and Greenland. Taken in total, a melting of the entire Greenland ice sheet would lead to roughly 20 feet of sea level rise.
Since the end of the study period in 2010, mass loss has only continued, with a particularly stark loss in the year 2012 - but fortunately, it does not appear that there has been another doubling of the rate of mass loss since 2010, says Box. Not yet, anyway.
One of the most intriguing implications of the research is that a truly gigantic amount of fresh water has traveled into the North Atlantic ocean from Greenland and from melting Canadian Arctic glaciers over the course of the last century - and has likely stayed there, says Box.
"The Gulf Stream presents a wall in the ocean south, where this fresh water just can't go. So the fresh water must be accumulating in the North Atlantic," he says.
This could be interfering with the Atlantic meridional overturning circulation, or AMOC, which is driven by differences in temperature and salinity of ocean waters. Cold salty water sinks in the North Atlantic and travels southward at depth, pulling more warm water north. But if waters freshen too much in the North Atlantic, that could slow or eventually shut down the circulation.
This is believed to have happened in the Earth's past many times, including prior to the Younger Dryas, a cold period that occurred some 13,000 years ago. Scientists think this cool period may have been caused when Lake Agassiz, an enormous lake sitting atop the Laurentide Ice Sheet that then covered much of North America, suddenly partly drained and injected a huge volume of freshwater into the ocean.
One 2002 study suggested that the volume of freshwater at the time was about 9,500 cubic kilometers. Because a gigaton is "approximately the same as one cubic kilometer of ice," this is similar to the amount that Greenland has lost since 1900. However, the pulse from Lake Agassiz would have been much more sudden and rapid in comparison. So it is not clear that a direct comparison here is possible.
Nevertheless, the fact that Greenland has already contributed so much fresh water to the oceans, and that now its ice loss is speeding up, is sobering. The key question for the future of Greenland's ice is how high temperatures will go and how long they will stay there. The world's recent Paris goal of keeping warming well below 2 degrees Celsius (or even better, 1.5 degrees Celsius), if achieved, may just be enough to prevent a scenario in which a total melt occurs over time.
"The ice sheets are doomed in plus 3 Celsius world," says Box.