The Greenland Ice Sheet
A large ice sheet and several smaller ice caps cover just over 80% of Greenland’s surface area (2.17 million square kilometers). Each year the ice sheet accumulates roughly 680 cubic kilometers of ice from snow fall, which it must lose at the same rate to maintain a steady size. In the center of the ice sheet, where little or no melt occurs and the ice thickness can exceed 3000 meters, the ice sheet sheds mass thorough glacial flow, which acts as a large conveyer belt to transport the ice to the lower elevations. While people often speak of the ice sheet as melting, only about half of the annual ice loss is from direct melting at the ice sheet’s lower elevations. The other way the ice sheet loses ice is through its many (200+) fast moving (1-12 km/yr) outlet glaciers that calve large icebergs (Figure 1), which ocean currents carry away to melt at somewhere in the north Atlantic. In contrast to Greenland, there is little surface melt in Antarctica and nearly all the annual ice loss is accomplished by ice discharge to the ocean.
The mass balance of the ice sheet is the difference between the annual snowfall and the combined loss from melting and iceberg calving. The recent IPCC report indicates that the Greenland Ice Sheet had a negative mass balance and lost ice at a rate of 20 cubic kilometers per year from 1961 to 2003 (0.05 mm/yr contribution to sea level). This rate increased substantially to 92 cubic kilometers (0.21 mm/yr) for the more recent interval from 1993-2003. Several published results for the period after 2003 indicate the annual mass loss is now somewhere between 120 and 220 cubic kilometers (0.3 to 0.55 mm/yr). While these contributions represent a relatively small fraction of the current rate of present sea level rise, the increase by roughly 0.5 mm/yr over a period when mean summer temperatures in Greenland have risen by about 1oC is cause for concern in the face of much greater potential future warming. Were the Greenland Ice Sheet to melt completely, it would raise sea level by 7.3 m (24 feet). Relatively conservative model predictions suggest that this could happen over the next 1000 years if not sooner.
Processes Governing Change
In addition to direct melt from warmer temperatures, two processes related to ice dynamics are likely to influence the rate at which Greenland loses ice in the coming centuries. The first effect has received widespread press attention. In this situation faster flow results as warmer summers improve lubrication by producing more surface melt that reaches the ice sheet’s base through large fissures in the ice called moulins. This effect is readily observed on alpine glaciers throughout the world and seasonal speedup has been observed at locations in Greenland. While the concept of increased lubrication is conceptually clear, our understanding of the details of the process is far from clear and very few observations exist to test the sensitivity of this effect to warmer temperatures.
The second effect, which seems to be driving much of the current change, has received less public attention. The ice sheet discharges much of its ice through fast moving glaciers that flow through narrow fjords, much like toothpaste being squeezed from a tube. Over the warmer summers since 2000, the trunks of many of these glaciers have retreated several kilometers inland along their narrow constricting fjords. Once this happens, the friction of the ice in contact with the fjord bed and walls is lost, removing a major impediment to flow and causing ice upstream to accelerate dramatically. To continue the toothpaste analogy, imagine the situation when pressure is applied to the tube to remove a dried gob of paste after the cap has been left off. Once the offending gob squirts free, the effect can be catastrophic if the pressure is maintained. Jakobshavn Isbrae, Greenland’s largest outlet glacier just east of Illulisat, flowed at roughly 6.7 km/yr in the 1990s. Following the collapse of the ice tongue in its fjord in the early 2000s (Figure 2), the glacier’s speed almost doubled to 12.6 km/yr to increase the contribution to sea level by roughly 0.06 mm/yr. This speedup along with similar accelerations of many other glaciers accounts for most of the change in mass balance over the last several years.
Current State of Knowledge
With the aid of spaceborne observations supplemented by field-based research, the last decade has seen tremendous progress in ice-sheet research. Whereas a decade ago few if any point measurements were available for individual glaciers, we can now visualize flow over the entire ice sheet in both time and space as Figures 1 and 2 indicate. Gone is the notion that ice sheet flow responds sluggishly taking centuries to millennia to respond to change. We now know that individual glaciers can respond rapidly, dramatically altering their flow over a period as short as a single warm summer.
In rough proportion to our gains in knowledge, are the realizations of the large gaps in our understanding of many of the processes driving change. While we know that glaciers can respond suddenly and dramatically with a strong sensitivity to changes in temperature, we do not know magnitudes of these sensitivities nor do we know the exact links to climate. For example, while we know that large speedups occur as a glacier’s calving front retreats up its fjord, we do not understand how warmer summers cause this retreat to begin. In addition, our window of reliable observations is just over a decade old, at best, making it difficult to separate short-term variability from long-term change. The IPCC highlighted these uncertainties when they stated "The ice sheets of Antarctica and Greenland could raise sea level greatly. Central parts of these ice sheets have been observed to change only slowly, but near the coast rapid changes over quite large areas have been observed. In these areas, uncertainties about glacier basal conditions, ice deformation and interactions with the surrounding ocean seriously limit the ability to make accurate projections."