The enormous sheets of ice that lie atop Greenland may not be as prone to catastrophic melting as many scientists thought, even if the planet continues to warm and temperatures remain high for hundreds of years. But while that may sound like good news, new evidence also suggests that parts of the even vaster ice sheets that lie atop Antarctica could be more unstable than once believed.
That’s the conclusion of scientists who have been drilling deep into the Greenland ice sheet since 2007, in a Danish-led project known as the North Greenland Eemian Ice Drilling (NEEM). Their results, published Wednesday in a landmark paper in Nature, show that temperatures rose some 8 degrees Celsius (14.5 degrees Fahrenheit) higher than they are today during the so-called Eemian period, a stretch of natural global warming that occurred between about 115,000 and 130,000 years ago.
“This is higher than most paleo-climate models have suggested,” said Dorthe Dahl-Jensen of the University of Copenhagen, the project leader and lead author of the new study, in an interview.
In one sense, the surprisingly high temperatures could seem reassuring: even the most pessimistic climate models project only eight or nine degrees Fahrenheit of warming by the end of this century, implying that Greenland’s ice could remain largely intact. But combined with other research, the new paper also suggests that the Antarctic contributed correspondingly more to sea level that was up to 25 feet higher than we see today.
The ancient warming, say the scientists, also caused extensive melting on what was the surface of the ice sheet more than a thousand centuries ago.
Extensive laboratory analysis of ancient ice samples show that the result of these soaring temperatures, significantly higher than the 5 degrees Celsius of warming climate scientists project at worst by the end of this century, caused extensive melting on what was the surface of the ice sheet more than a thousand centuries ago.
The same sort of thing happened last summer, when 97 per cent of the Greenland’s surface ice had undergone some melting by mid-July – something Dahl-Jensen experienced first-hand. “We had an event where it rained at the NEEM site,” she said. “That’s very unusual; we’d never seen it before.” When that happens, she said, “the rain doesn’t run off; it penetrates into the snowpack and refreezes into an ice layer.”
The scientists studying ice from the NEEM drilling cores saw exactly the same sort of refrozen layers in ice from the Eemian period. They also sampled tiny air bubbles trapped by snowflakes that fell during the Eemian, and which were eventually compressed into glacial ice.
The amount and the relative abundance of oxygen, nitrogen and other gases in the bubbles told scientists how high the ice surface was above sea level when the Eemian began and when it ended – since then, as now, air has a different composition and density at different altitudes. The conclusion: 128,000 years ago, shortly after the Eemian started, the ice sheet at the NEEM site in northern Greenland was 650 feet higher than it is today. Six thousand later it had melted back by more than a thousand feet, ending up at about 425 feet lower than the present level.
That sounds like a lot, but as Jensen said, “it’s not like the whole ice sheet disappeared.” In fact, she said 75 per cent of the ice remained intact, even after 6,000 years of very high global temperatures. This would have driven global sea level up by no more than 2 metres. For those who recently experienced the storm surge from Hurricane Sandy, that may seem pretty awful, given that Sandy’s destructive power was amplified by a mere foot or so of sea-level rise since 1900.
But studies of ancient shorelines all over the world have shown that the actual sea-level rise during the Eemian period was as much as 8 meters, an almost incomprehensively destructive change. Back then, said Hubertus Fischer, at Switzerland’s University of Bern, one of the study’s co-authors, “there were very few humans, and no coastal cities. If we saw such a huge increase now, the consequences would be drastic even if it took a couple of thousand years to unfold.”
It might not take that long, however. If Greenland only accounted for a quarter of that long-ago inundation, Dahl-Jensen said, “this means Antarctica must have contributed the rest.”
As the planet warms under the thickening blanket of greenhouse gases humans are pumping into the atmosphere, Antarctic ice may play a major factor again. “West Antarctica, and parts of East Antarctica are unstable, meaning that the bed of the ice sheet lies as much as a mile below sea level,” Dahl-Jensen said. “If you reduce the thickness of this ice, it could pop up like an iceberg and start floating in the ocean. This would give you rapid sea-level rise, which would be much more dangerous than slow changes.”
While this scenario doesn’t come directly from the NEEM analysis, but rather from the chain of reasoning that leads from it, evidence from other observations suggests there could be something to the idea. A paper that appeared in Nature in April of 2012 noted a thinning in some of Antarctica’s ice shelves tied to warm ocean currents eating at the ice from underneath. A paper in Nature Geoscience a few weeks later projected even more and more rapid, thinning in the future, again thanks to deterioration from below.
None of this necessarily changes the best current projections of sea level rise by the end of this century, which still stand at about 1 to 2 meteers by 2100 – especially since temperatures were significantly higher during the Eemian. That’s going to be plenty destructive, agree scientists and urban planners.
But even if the planet never reaches Eemian-level temperatures, Fischer said, “we expect about 5 degrees Celsius of warming if we keep blowing out carbon dioxide for the next hundred years.”
If the mercury stays that high for any significant length of time, it will inevitably send sea level even higher, and the message from the NEEM project is clear.
“The West Antarctic ice sheet is clearly more sensitive to warming than we thought,” Fischer said. “This is really a huge message.”