SAN FRANCISCO, CA, DECEMBER 15, 2014 – If Arctic sea ice is the canary in the coal mines that are causing global warming, Greenland is the 800-pound gorilla that sits in the corner, quiet and ponderous, but packing a heavy wallop and ready to use it.
Scientists at the American Geophysical Union’s Fall Meeting here have been watching the gorilla carefully, and are learning things — surprising and troubling new things — about the stirring of this big beast of ice. Among them are the discovery of aquifers and buried lakes of water that are liquid year-round, unexpected dense layers of ice high on the sheet that may contribute to flooding, and that Greenland had rapid melting from 1900 to 1930 as the Earth came out of the Little Ice Age — more rapid even than today’s melting.
“New tools are allowing us to see these subsurface processes for the first time,” said Mike MacFerrin, a researcher at the University of Colorado at Boulder’s CIRES research institute. Ground-penetrating radar pulled behind snowmobiles and systematic flyovers from NASA’s Operation Icebridge have led to vast quantities of new data that have brought quick discoveries and insights over just the last few years.
The island of Greenland holds almost 3 million billion metric tons of ice, enough to raise sea level over twenty feet if it all warms away. It’s now losing almost 400 billion tons of ice a year, and the loss rate is accelerating, up almost 150 percent since the first half of last decade. MacFerrin said, “In the next century, Greenland melt may raise global sea level by one to three feet.”
Surprisingly, the nature of the melt has changed in recent years. When once it was mostly streaming glaciers discharging calves into the sea, surface melting and runoff surpassed discharge around 2007. This runoff, rather than ice dynamics, is expected to dominate Greenland’s ice sheet’s contribution to sea level rise this century.
Also surprising is that the last few decades aren’t the first period of rapid melt Greenland has recently seen. Anders Bjork, a researcher at the Natural History Museum of Denmark, for the first time reconstructed the retreat of glaciers early in the 20th century, analyzing photos of Greenland taken then by Danish and Norwegians explorers, and comparing them to today.
He found that, while Greenland has warmed by 2.3 degrees F per decade in the last 20 years, it warmed by 3.6 degrees F per decade from about 1920 to 1940, long before satellites existed. Though it’s warmer now, glaciers were retreating about three times faster early last century in the wake of the Little Ice Age.
Early explorers of Greenland’s ice sheet (credit: Anders Bjork, Natural History Museum of Denmark).
New Tools, New Findings
Ground-based radar, beamed into the ice sheet from a snowmobile or plane, reveals boundaries where densities change. This has led to some of the most surprising recent discoveries in Greenland by Rick Forster of the University of Utah, Lora Koenig of the National Snow and Ice Data Center in Boulder, Colorado, and their colleagues: the presence of under-the-ice aquifers and buried lakes of liquid water.
The aquifers come from surface water percolating through firn, the partially packed porous snow layer that is in a state between snow and glacial ice. The buried lakes are along almost the entire perimeter of the ice sheet; because liquid water is warmer than ice, the lakes spread relative warmth in their locales, contributing to yet more melting. This water interacts with the ice sheet in unexpected and, so far, largely unknown ways, science that is crucial to gauging Greenland’s contribution to sea level rise over this century.
Credit: Lora Koening, NSIDC CIRES.
MacFerrin and his colleagues made a separate discovery when, instead of firn, their ice drill unexpectedly encountered dense layers of ice high on the ice sheet, about 15 feet below the surface. These “ice lenses” are thick enough to block surface meltwater; in the record-breaking melt of the summer of 2012, an unprecedented series of lakes and rivers formed. When they drained, coastal Greenland saw some of its worst flooding on record.
The ice lenses extend dozens of miles inland, far more than expected, and those known cover over 27,000 square miles.
Greenland’s melt is primarily from warmer atmospheric temperatures, a consequence of the buildup of human-emitted greenhouse gases. The melting is only expected to increase as we pass into the future.
“Every few years, the ice sheet surprises us, doing something we never knew it could,” said Mike MacFerrin. “It’s anyone’s guess what we might discover within the next several years. Using the tools we currently have, we’re doing our best to keep up right now.”
from the AGU Fall Meeting in San Francisco, December 15th, 2014.