by Curt Stager
Even though the weather was a bit warmer than usual, sailing open vessels across the North Atlantic in medieval times was still a grueling and risky endeavor. Only fourteen of the original boatloads survived the stormy crossing to plant two settlements on the southern tip of what is now known to be the world’s largest island. They tilled the reluctant dirt as best they could and sat out the long dark winters in low stone-and-turf houses that were warmed by the radiant heat of their own bodies and those of their livestock as much as by smoky fires. They had the place pretty much to themselves because in those days most native Greenlanders lived much farther up the frigid coast, where the floating ice was more reliable and the seals and walrus more abundant. Rarely interacting with the southern farmers, the native hunters had only colonized the place from more northerly regions of the Canadian Arctic about two hundred years before Eirik showed up.
After 1408 AD, all documented Norse contact with Greenland ended, and extinction of the colonies was complete by the end of the fifteenth century. Nobody knows for sure what did them in. Some scholars blame hostilities with the native hunters. Archaeological studies also suggest that the white settlers overused the land, ruining its tenuous productivity through soil erosion and deforestation. But another compelling possibility is that they succumbed because the climate changed around them.
Floating pack ice had already begun to spread farther south again as early as the fourteenth century. As the global cooling period known as the Little Ice Age gathered strength, warm summer weather became less and less dependable in agricultural Europe, glaciers in the Alps slouched downhill to crush Swiss farms and villages, and the first winter “frost fairs” were held on the consistently frozen Thames River. Because Greenlandic colonists depended on Iceland and Norway to keep them supplied with iron, tools, extra food, and long wooden planks for the construction of vital watercraft, the effects of encroaching sea ice on trade could have been deadly, as could a shortening of growing seasons on already marginal farms. On top of the real and possibly fatal threat that it posed for them, the relentless cooling must have resonated cruelly with the Norse belief that Ragnarøk, the fated end of the world, would begin with a fierce and prolonged “winter of winters.”
Whether or not the Medieval Warm Period and subsequent Little Ice Age really orchestrated the birth and death of the colonies, those climatic disturbances revolutionized the environment of Greenland over the course of centuries and made life first easier and then dramatically harsher for those who lived there. And as the twenty-first century once again leads us into warmer conditions, Greenland has retaken center stage in a new saga of climate change, one whose telling draws a global audience.
Today, Greenland is still shared by Arctic natives and Scandinavians: the modern-day Inuit and the Danes. Tiny Denmark, a diminutive mitten of land and associated islands that constrict the mouth of the Baltic, gained title to that territory in 1814 when the former owner, Norway, lost it in the Treaty of Kiel. Disputes with Norway and the United States cropped up occasionally after that but Danish claims were finally settled in 1933 by an international court. Greenland is now a self-governing administrative division of Denmark, not exactly a colony any more but not fully independent, either.
For the rest of the world, Greenland’s coastal rind of habitable land has had only minimal impacts on media, economies, and politics. But Anthropocene warming is changing that quite dramatically.
The melting of Greenland stands front and center in public discussions of global climate change nowadays, and with good reason. The unstable West Antarctic ice sheet is also on the radar screen, but Greenland is different. For one thing, it is home to more than fifty thousand people; there are no “Antarcticans” in the Southern Hemisphere. The stability of the Antarctic ice is more significantly compromised by the breakup of floating ice shelves that are now becoming less and less capable of holding the seaward ends of land-based glaciers in place. And although the northern ice mass is less likely to slide bodily into the ocean than the West Antarctic sheet seems to be, much of it lies far enough from the pole to thaw extensively in summer. The southernmost tip rests well below the Arctic Circle, at about the same latitude as Anchorage, Oslo, and Helsinki. Greenland is a glacial dinosaur, a holdover from the last ice age that makes its own self-refrigerating weather. The vast expanse of reflective whiteness repels enough solar energy to keep it cooler than it would otherwise be, and the great height of the central ice domes—as much as 2 miles (3.4 km) in places—adds to the effect. Mean annual temperatures in the highest northern sites are fiercely cold, close to 23 degrees below zero (-30°C).
But at lower elevations and in places exposed to milder conditions near the coasts, things are changing rapidly. Tourists, journalists, scientists, and local residents now tell of meltwater lakes filling and draining from furrowed ice surfaces, of glacial blue-and-white avalanches thundering away from the snouts of retreating ice streams, of seal hunters who no longer trust the thinning sea ice on local fjords to support them.
These are the basics of a well-known tale, but they also raise questions that are often left unanswered. How quickly could so much ice disappear? And what might the place be like after the most dramatic changes have run their course and less icy conditions become the norm?
In order to determine how long Greenland’s ice cover may survive long-term warming we must know how much frozen water there is to liquefy and how much is lost from year to year. High-tech surveys have recently updated the volume of Greenlandic ice from 624,000 cubic miles (2.6 million km3) to 696,000 cubic miles (2.9 million km3), enough to build a symmetrical cube 90 miles (142 km) tall. Space-based measurements, particularly those made by a pair of U.S.-German satellites (GRACE, or Gravity Recovery and Climate Experiment) also reveal more ice loss along the margins than we knew about just a few years ago, and each new report from the field seems to indicate faster rates. Glaciologist Konrad Steffen, of the University of Colorado in Boulder, recently summed up some of the latest findings for me: “GRACE results reveal an ice loss of about 200 gigatons per year now, of which 40 to 50 percent is due to surface melt and the rest to increased iceberg calving. Greenland’s mass balance has been negative since 1995, and the rate is increasing.” One research group headed by Scott Luthcke, of NASA Goddard Space Flight Center, has equated the annual melting with the flow of six Colorado rivers, and another group estimates that recently accelerated outflows have released enough water to fill Lake Erie. Implicit in such descriptions is the suggestion that the ice sheet is rapidly disintegrating before our eyes. But is it?
To begin to clarify what’s going on in Greenland during this early warming phase of the Anthropocene, it helps to maintain a proper sense of scale. An update on how much ice was lost this year is of little use if we don’t also know how it compares to the overall ice budget. Reports of Colorado Rivers gushing from Greenland’s white flanks make it sound like a dying whale in some Melville novel. But if we understand how big that ice sheet is, such metaphorical rivers shrink in relation to their monstrous source. Consider a recently reported loss of 48 cubic miles (200 km3) of ice per year between 2003 and 2008. If that rate continues far into the future, then the entire ice sheet could vanish in as little as … 14,500 years.
A warmer future might speed things up, of course. Computer modelers have already made numerous estimates of what additional warming could do to Greenland, but their conclusions vary because the models don’t yet consistently express the full complexity of ice sheet dynamics. If Arctic climates warm by 11 to 12°F (6 to 6.5°C) and stay that way indefinitely, various models suggest that Greenland might take as many as 20,000 years to melt down or as few as 3,000 years. Some models also calculate that a higher sustained rise of 14°F (8°C) could destroy most of it in 5,000—or in as little as 1,000—years. Such conditions are well within the range of possibility for a runaway greenhouse scenario, but even in the most extreme situations the ice will take far longer to disappear than many of us imagine.
When we focus on the net loss of ice by comparing outflows to buildups, we also find that Greenland’s current contribution to sea-level rise is smaller than one might expect. For now, most of the melting and surging is happening along the margins, at relatively low elevations where milder temperatures and undercutting by seawater speed those processes along. Estimates of the net annual outflow of meltwater vary, but they have recently ranged between 100 and 300 gigatons and accounted for a tenth to a third of the rate of global sea-level rise.
The volumes of runoff and ice wasting away from Greenland are large, not only because the world is warming up but also because Greenland itself is gigantic. Even though it is classified as an island, it spans about 1,550 miles (2,500 km) from north to south and up to 680 miles (1,100 km) crosswise. It’s as large as Saudi Arabia, twice the size of Ontario, three times bigger than Texas, and about four times larger than France. If we treat that ice mass simply as an enormous chunk sitting passively under a heat lamp, then we’ve got nearly 700,000 cubic miles of it to consume by thawing layer after slushy layer away from the surface. Little wonder, then, that the time frames on which surface melting alone can demolish Greenland’s ice cover are counted in thousands of years.
But in reality, it’s not simply a matter of dripping ice cubes. Much of the loss is dynamic, meaning that sizable pieces are tumbling into the oceans en masse rather than trickling away as surface runoff. If you want to know why we’re still so short on specifics about the future of polar ice sheets, consider the following list of complicating processes that we now know are at work under those snowy white surfaces.
Coastal “outlet glaciers” are rivers of creeping ice that drain the main pile and pour their contents out to sea in the form of icebergs. Sometimes they surge and sometimes they slow, and nobody’s yet sure exactly why they do what they do at any given time. For instance, the Helheim glacier on the eastern coast recently doubled its discharge but then slowed down quite a bit. In contrast, the huge Jakobshavn ice stream, which drains the west central sheet, doubled its pace after 1992 and showed no sign of slowing as of 2009. Some say that a floating ice shelf that once resisted it has now vanished and therefore no longer stems the flow. Others say that meltwater and wet muds under the ice lubricate it like a banana peel underfoot.
Another complicating factor is that thick ice develops an odd molecular structure under the tremendous pressure of overlying layers, so it behaves more like pliable putty than the brittle cubes floating in a cocktail glass. This transformation endows ice sheets with the gift of motion; when they grow heavy enough, they ooze sideways under their own weight. That’s why the last ice age left so many scratches and grooves in the bedrock of formerly glaciated lands; boulders and pebbles became gouges and chisels in the grip of sliding basal ice. When retreat time finally arrived, the northern ice sheets lost their lateral mobility as they thinned from the top downward. Rather than dragging themselves back toward the pole, the stagnant southern margins simply dropped their embedded stone-working tools and decayed in place like snowdrifts in spring.
If compressed ice is freed of its overburden, as would occur after prolonged surface melting, it reverts to its preferred, more voluminous crystal structure. Researchers who drill long ice cores from thick polar ice sheets may wait for weeks while their prizes swell and stabilize in cold storage; otherwise, they can shatter when sampled. In the field, the relaxation and cracking of decompressed layers trigger icequakes and might help to fracture some of the remaining ice. Even more powerful are the massive tremors that shudder through advancing outlet glaciers as heavy bergs snap off and fall away from their crumbling seaward faces.
Such fractures may cause even more complications in addition to direct structural weakening. Meltwater lakes and streams that form on top of the ice in summer can plunge into fissures rather than staying put until winter freeze-up or just running off the edges. The weight of water pounding down into a deep, V-shaped seam can split glacial ice like a hammer-driven wedge splits firewood. And where the fissures meet bedrock, the tunneling meltwater can thaw and lubricate the base of the ice sheet, helping it to slide seaward all the more rapidly.
Radar imagery shows that extensive pools of meltwater lie beneath the Greenland ice sheet, probably as a result of surface seepage and basal melting against the warmer basement rock. If the lake-sized pools become large enough, or if seawater leaks in around the edges and seeps into deep valleys in the underlying bedrock, the overlying sections of the ice sheet might destabilize. I asked glaciologist Gordon Hamilton about this during a conference at the University of Maine’s Climate Change Institute. Could such a thick, heavy pile of ice actually float?
“Sure it could,” he replied. “Water doesn’t compress, so if it seeps down under there it can lift the ice away from the bedrock.” I recalled a presentation earlier that morning that showed a snakelike coastal glacier rising and falling slightly with the tides, almost as if it were breathing. “And if the ice pulls back from the rim of a rocky basin that it sits in, seawater might fill that space, too. Tidal cycles could then warp the overlying ice up and down until it cracks off.”
Still another way to help demolish an ice sheet is to lower its high, cold surface. At present, Greenland counteracts some of its marginal ice loss by piling more snow on top of itself, especially where the central plateau is so high and cold that it doesn’t thaw much. But that’s not enough to cancel today’s net deficit, which is only likely to grow larger as northern air and oceans continue to warm.
Hamilton continued. “If the ice margins lose lots of mass to melting and dynamic thinning, then the high central portion of the ice sheet will begin to sink lower into warmer elevations where it could start to melt in summer. That melting would drop it even lower, and so on.” This process would presumably continue until the pile becomes so thin that it doesn’t flow laterally anymore, at which point surface melting would be left to finish the job.
And then there’s crustal rebound. A heavy ice sheet bends Earth’s crust beneath it like a spongy seat cushion. Remove or lighten the load, and the crust or cushion bounces back. Rebound earthquakes still shake the northeastern United States even though the last ice age ended more than 10,000 years ago, and newly generated rebound is measured routinely in Greenland today. Between 2001 and 2006, the Helheim glacier alone drained enough mass from the southeastern sector to let the underlying bedrock rise by more than 3 inches (8 cm). As Greenland loses more weight, rebound movements might further fracture the remaining ice.
This list of ice-eating mechanisms suggests that recent estimates of Greenland’s disintegration rate may have been too conservative. Some glaciologists now worry that a wholesale collapse could follow a perfect storm of surface melting, basal lubrication, coastal surging, and the like. Others, however, strike a more measured tone, suggesting that the drawdown could slow significantly once the main body of ice retreats far enough from the ocean’s edge to reduce the thawing influence of marine currents and to stem the outflow of icebergs. But the lack of consensus on such details shouldn’t be surprising. We’re still new at this, and we don’t yet fully understand all the mechanisms at work. As a result, we still don’t have many firm answers about the future of Greenland other than the obvious conclusion that continued warming will cause continued shrinkage at any one of a wide range of possible rates.
Geological records of past warmings don’t tell us as much about ice stability as we might like, either. What long-term history we do have, however, shows us how stubbornly stable Greenland’s ice heap is and tends to favor the less apocalyptic arguments. Technically speaking, it can be rather surprising that the ice sheet exists today at all. It lies at lower, warmer latitudes than larger ice age behemoths that once lay closer to the pole but still disintegrated under the thermal assault of postglacial warming thousands of years ago.
Such resistance to warming apparently also existed in the more distant past, as well. Roughly 13,000 years of warmth during the Eemian intergl
acial still left between a third and a half of Greenland’s ice cover in place. That’s amazing, considering that a sediment record from Baffin Island, based upon ancient insect remains, showed that summer temperatures there were an impressive 9 to 18°F (5 to 10°C) higher than today. And an earlier, 30,000-year interglacial didn’t de-ice Greenland completely, either.
On the other hand, history also shows that megamelting can and does happen, especially when Earth is more generously adorned with mobile ice than it is now. About 14,500 years ago, a melt volume equivalent to two or three liquefied Greenland ice sheets poured into the oceans over the course of 500 years or so. Because ice cores show that Greenland was still heavily glaciated then, much of the loss must have occurred elsewhere, probably from other ice masses that covered more of the polar regions at the time.
The edges of the gigantic Laurentide ice sheet that once buried most of eastern Canada and the northeastern United States gradually decayed as climates warmed during the early stages of deglaciation, but gigantic sections of the central dome also destabilized suddenly from time to time. One notable collapse occurred over what is now Hudson Bay, perhaps because seawater seeped into a depression under the creeping belly of the ice. The resultant rush of bergs into the North Atlantic disemboweled the Laurentide monster, stranding isolated remnants on Baffin Island, Labrador, and northern Quebec.
Three important points arise from these findings. First, a composite ice volume larger than Greenland’s was once destroyed within a matter of centuries. Second, most of Greenland’s ice nonetheless survived millennial-scale warmings of the past. And third, dynamic undercutting by seawater probably heaved more polar ice into the ocean than atmospheric warmth alone did. In other words, a fully glaciated Greenland today is unusual not because its thick white mantle should necessarily have vanished long ago; its interior may be too isolated from marine attack to suffer the fate of the Laurentide ice sheet. Rather, it’s unusual because it’s so alone. If it weren’t for postglacial sea-level rise and the low-lying nature of the lands around Hudson Bay, there might still be a second major ice mass perched atop northeastern Canada, too.