Scientists have found dramatic evidence that the ice sheet covering Greenland disappeared almost entirely under natural climate conditions for a lengthy time period during the last 1.4 million years, suggesting that current global warming — if it persists — could drive melting there to a perilous threshold, prompting a rapid rise in sea levels.
“The fact that we lost most of the Greenland ice sheet for extended periods under natural forcing is certainly a somber note, given the recent temperature trends,” says Joerg Schaefer, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, and lead author of the study published this week in the journal Nature. “The probability that we approach a tipping point is real, and we need to understand the dynamics of the Greenland ice sheet under rather extreme temperature forcing — and we need to understand this fast.”
Their findings contradict assumptions in some quarters that the Greenland ice sheet has been relatively stable during the recent geologic past. “Unfortunately, this makes the Greenland ice sheet look highly unstable,” Schaefer says.
The ice sheet covering Greenland, a country roughly four times the size of California, contains about 684,000 cubic miles of ice, and could increase sea levels as much as 24 feet if it melts, a scenario that raises important policy and environmental questions about the fate of coastal communities in future years if current temperature trends continue. If the ice warms slowly, it could take thousands of years for Greenland to completely melt. If it warms quickly, it could take just hundreds.
Sea levels already are rising, and Greenland’s melting is a significant contributor. Loss of Greenland ice has roughly doubled since the 1990s, according to Schaefer. “During the last four years, by some estimates, it shed more than a trillion tons,” he says. It accounts for about a quarter of currently rising sea levels of about 3 millimeters a year, but experts believe this could speed up.
To better understand what will happen to the ice sheet in the coming years, researchers have sought to learn about its behavior in the past, especially when the Earth was three or more degrees Fahrenheit warmer than it is today. The scientists studied a chunk of bedrock drilled from nearly two miles below the summit of the ice sheet, and found that chemical isotopes indicate its surface was exposed to open sky for at least 280,000 years during the last 1.4 million years.
“I think the paper says that at least for a little time, the center of Greenland was getting a bit of a cosmic ray burn, so the ice was off of it,” says Richard Alley, a glaciologist at the University of Pennsylvania, and a co-author of the study. “The study is also telling us that Greenland is quite sensitive to warming, and if you warm it a lot, it will melt all that much faster. An ice cube melts faster in hot tea than in cold.
“It’s a sea level issue, and even melting a little bit of it matters a lot if you live on the coast,” he adds. “The more we warm the world, the more rapidly Greenland’s ice will melt and contribute to sea-level rise, with clear implications for wise decisions about our energy system.”
A second paper, also published this week in Nature, found something a little different, although both sets of scientists insist that the results do not necessarily conflict with each other.
University of Vermont geologist Paul Bierman and his colleagues studied deep cores of mud at the bottom of the ocean containing bits of bedrock eroded off the east side of Greenland. The samples indicate it has been actively scoured by glacial ice for much of the past 7.5 million years. This likely means that the ice sheet on the Eastern flank of the island has not completely melted for long, if at all, during the past several million years.
“These two studies are actually relatively easy to reconcile,” Schaefer says. “Our data from the bedrock underneath the summit of the Greenland ice sheet show that nine-tenths of the [sheet] was gone for extended periods, but we also clearly say that it is likely that a small ice-cap remained in the Eastern Highlands of Greenland.”
Bierman’s paper “shows consistently that glacial erosion of these small glaciers/small ice-cap in the Eastern Highlands was at work at most times,” Schaefer says. “So both papers are consistent with small ice preserved during the inter-glacials, but the main part of the GIS melted.”
Bierman agrees. “These results appear to be contradictory — but they may not be,” he says. “Their study is a bit like one needle in a haystack, and ours is like having the whole haystack, but not being sure how big it is.”
Schaefer acknowledged, however, that proof of sea-level rise during the period studied is scant. “Evidence for past sea level is rare. Even for the last interglacial, Marine Isotope Stage 5e, some 125,000 years ago, there are big controversies,” he says. “Before that, sea-level records get even more difficult and only point observations are available. So no, we do not have robust evidence that would help us from the sea-level records here. But vice versa, our study indicates that sea level might have been higher than assumed during several inter-glacial periods.”
Alley, however, is more confident “we have had high sea levels in the past,” he says. “If you go to the tropical islands and sit on the beach, you can see corals 135,000 years old — and they are above you. Clearly sea level has been higher in the past, and probably multiple times.”
In studying the rock, Schaefer and his colleagues found traces of radioactive beryllium-10 and aluminum-26, isotopes produced by tiny particles from outer space that constantly bombard the surface of the planet.
These isotopes decay at known rates, and cannot be created if the rock is ice-covered. Modelers agree that the site of the rock core would be among the last to melt, should the ice sheet disappear; thus, the authors conclude that the sheet must have been down to less than ten percent of its current size when the site was ice-free.
Past studies of seafloor sediments off various parts of Greenland have found remains of pollen and other materials dated to a periodic warming cycle about 400,000 years ago, suggesting that Greenland could have been largely ice-free then. Other studies suggest that the ice surface shrank during the last major warming cycle some 120,000 years ago, raising sea levels by 12 to 18 feet.
But these studies provide no clear information on how long these periods lasted, and their evidence is less direct than the current study. “Here we have no question — we interrogated the surface directly,” says study co-author Robert Finkel, of UC Berkeley, who participated in the 1993 drilling expedition that recovered the rock. While the drilling took place more than two decades ago, it is only recently that have scientists developed the sophisticated lab techniques needed to analyze and extract information from the rocks.
Paleoclimatologist Jeff Severinghaus, of the Scripps Institution of Oceanography, who was not involved in the study, describes the research as “very direct and incontrovertible,” adding that it “challenges some prevailing thought on the stability of the ice sheet in the face of anthropogenic warming. We can now reject some of the lowest sea-level projections, because the models underpinning them assume continuous ice cover during the last million years.”
Thomas Stocker, a climate scientist at Switzerland’s University of Bern, who also was not involved, says the work “shows that the Greenland ice sheet has been much more dynamic than thought.”
No one knows exactly what it would take to make the ice collapse, or how long it might take. But Schaefer says he hopes the nation — and the world — “will unite behind careful environmental science” and do whatever is necessary to cope with the consequences.
“What we see right now, and in the near future, is beyond the natural forcing of the recent geologic past,” he says. “We, as a human civilization, have committed to a lot of sea-level rise already, and need to face and prepare for it.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.