(CN) – Surface ice melting from Greenland – one of the key drivers of global sea level rise – has shifted into overdrive and shows no signs of abating, according to a study published Tuesday in the journal Nature.
The study provides new evidence of the effects of climate change on Arctic melting and global sea level rise. It suggests that if the trend of warmer summers continues causing unprecedented rates of melting it could accelerate the already fast pace of sea level rise.
“Melting of the Greenland Ice Sheet has gone into overdrive. As a result, Greenland melt is adding to sea level more than any time during the last three and a half centuries, if not thousands of years,” said lead author Luke Trusel, a glaciologist at Rowan University’s School of Earth & Environment and former post-doctoral scholar at Woods Hole Oceanographic Institution. “And increasing melt began around the same time as we started altering the atmosphere in the mid-1800s.”
Human alteration of the atmosphere began with the Industrial Revolution of the mid-19th century. Surface melting across Greenland’s mile-thick ice sheet ramped up dramatically during the 20th and early 21st centuries, according to scientists.
Trusel said the new research provides evidence rapid melting observed in recent decades is highly unusual when put in historical context. Another noteworthy aspect of the findings is how little additional warming it now takes to cause huge spikes in ice sheet melting.
“Rather than increasing steadily as climate warms, Greenland will melt increasingly more and more for every degree of warming, Trusel said. “The melting and sea level rise we’ve observed already will be dwarfed by what may be expected in the future as climate continues to warm.”
Icebergs calving into the ocean from the edge of glaciers represent one component of raising sea levels. But more than half of the water entering the ocean comes from runoff from melted snow and glacial ice atop the ice sheet.
To determine how intensely Greenland ice has melted in past centuries, the research team used a drill the size of a traffic light pole to extract ice cores from the ice sheet itself and an adjacent coastal ice cap, at sites more than 6,000 feet above sea level. The scientists drilled at these elevations to ensure the cores would contain records of past melt intensity as far back as the 17th century.
During warm summer days in Greenland, melting occurs across much of the ice sheet surface. At lower elevations, where melting is the most intense, meltwater runs off the ice sheet and contributes to sea level rise but no record of the melt remains. At higher elevations, however, the summer meltwater quickly refreezes from contact with the below-freezing snowpack sitting underneath. This prevents it from escaping the ice sheet in the form of runoff. Instead, it forms distinct icy bands that stack up in layers of densely packed ice over time.
Scientists at four ice core labs in the United States measured physical and chemical properties along the cores to determine the thickness and age of the melt layers. Dark bands running horizontally across the cores, like ticks on a ruler, enabled the scientists to visually chronicle the strength of melting at the surface from year to year. Thicker melt layers represented years of higher melting, while thinner sections indicated years with less melting.
Combining results from multiple ice cores with observations of melting from satellites and sophisticated climate models, the scientists found the thickness of the annual melt layers they observed clearly tracked not only how much melting was occurring at the coring sites, but also much more broadly across Greenland. This breakthrough allowed the team to reconstruct meltwater runoff at the lower-elevation edges of the ice sheet – the areas that contribute to sea level rise.
“From a historical perspective, today’s melt rates are off the charts, and this study provides the evidence to prove this,” co-author and Woods Hole glaciologist Sarah Das said. “We found a 50 percent increase in total ice sheet meltwater runoff versus the start of the industrial era, and a 30 percent increase since the 20th century alone.”
Ice core records provide critical historical context because satellite measurements used to understand melt rates caused by climate change have only been around since the late 1970s according to co-author Matt Osman, a graduate student in the MIT-WHOI Joint Program.
“We have had a sense that there’s been a great deal of melting in recent decades, but we previously had no basis for comparison with melt rates going further back in time,” he said. “By sampling ice, we were able to extend the satellite data by a factor of 10 and get a clearer picture of just how extremely unusual melting has been in recent decades compared to the past.”
Osman said understanding how Greenland has already responded to climate change will allow scientists to answer what might happen there next.
“What our ice cores show is that Greenland is now at a state where it’s much more sensitive to further increases in temperature than it was even 50 years ago,” he said.
Das agreed. “Even a very small change in temperature caused an exponential increase in melting in recent years, so the ice sheet’s response to human-caused warming has been non-linear,” she said.
Trusel concluded: “Warming means more today than it did in the past.”
Besides Woods Hole Oceanographic Institution, ice core samples were examined at the U.S. National Science Foundation Ice Core Facility in Denver, Wheaton College in Norton, Massachusetts, and the Desert Research Institute in Reno, Nevada.
The U.S. Department of Defense, the Netherlands Organization for Scientific Research, the Netherlands Earth System Science Center, and the Belgian National Fund for Scientific Research also provided institutional support to the effort.