Glaciers/Snow Melt

Warming temperatures and the resultant changes in surface reflectivity have implications for glacial and snow melt. The stories below suggest that melting trends are significant and in some regions accelerating. One of the most telling tales of ice sheet melt is the recent calving off of the 66 square kilometers (equivalent to 11,000 football fields) Ayles ice shelf, one of six remaining ice shelves in Arctic Canada. While the event transpired in 2005, scientists only recognized the loss in 2006 by looking at satellite images of the area. The ice shelf was at least 3,000 years old.

While most surveys and projections of the Greenland ice sheet report a net loss of glacial coverage, these studies argue that many previous assessments underestimate the loss, as they both ignore some of the dynamic nature of ice loss as well as ice loss in a number of major glaciers that were not included in surveys. Employing satellite radar and remote sensing, Rignot and Kanagaratnam find that Greenland's ice mass loss doubled in the last ten years -- from approximately 90 cubic kilometers/yr to 220 cubic kilometers/yr, which indicates that Greenland's contribution to global eustatic sea level rise increased from 0.23 ±  0.08 mm/year in 1996 to 0.57 ±  0.1 mm/year in 2005. Most significantly, the large majority of this loss was caused by ice discharge from glaciers, resulting from velocity increases and changes in outlet glaciers which draw from interior basins, rather than runoff. This loss is confirmed by Chen, Wilson, and Tapley using other satellite gravity measurements. According to their analysis, previous estimates were off by a factor of three (earlier numbers suggest a net loss of 82 ± 28 cubic kilometers/ year, while the new assessment shows a loss of 239 ± 23 cubic kilometers /year). Chen et al. attribute the discrepancy to substantial increase in melting within the last year and a half, as well as better resolution in their models.

Implications: The methods employed in these studies produce results that far exceed previous estimates of glacial loss. They suggest that earlier global models both significantly underestimated glacial loss, and represent an artificially low estimate of total ice sheet melting. This, in turn, has implications for the timing of large scale Greenland ice melt, for general ocean circulation, and for consequent sea level rise. It might be noted that if it were entirely melted, the Greenland ice sheet would contribute roughly 7 meters of global sea level rise.

The UN's Intergovernmental Panel on Climate Change projects in its 2001 report that the next century will see some net ice gain -- rather than net loss -- from Antarctica as precipitation (and hence snow thickness) increases with global warming. However, Velicogna and Wahr argue that this projection fails to take into account coastal regions, where the ice has low thresholds for resistance to temperature rise. Using data from the Gravity Recovery and Climate Experiments (GRACE) survey satellite, the scientists were able to measure the ice loss. They found that Antarctica contributed 0.4 ±  0.2 mm of sea level rise per year during the period 2002 to 2005, the majority of which was coming from the Western Antarctic ice sheet. In a separate study, Monaghan et al. demonstrate that snowfall over the Antarctic continent has not increased since the 1950s, and, therefore, sea level rise will likely continue unmitigated, as Antarctica is characterized by a net mass loss.

Implications: Antarctic melt is usually not included in medium- and short-term models of global warming impacts and sea level rise. If the findings in these studies turn out to be true, such melting must be included in climate models -- and the results will raise overall anticipated sea levels.

Overpeck et al. assert that sea level rise may very well be substantially faster and more significant than initially thought. Examining the last interglacial period 127,000 to 130,000 years ago (the Eemian interglacial), during which warming was approximately equivalent to that projected for the year 2100, they find sea levels 4 to 6 meters above those of the present day. Moreover, they found that sea level rise (specifically related to melting of the Greenland ice sheet and the West Antarctic ice sheet) was extremely rapid. They conclude that the potential for massive ice sheet loss is real, and that it may be triggered even with modest warming.

Implications: If this result is correct, it suggests the linear projections of ice sheet melt -- assumed to be quite modest in most global warming scenarios -- are vastly understated. The expectation that global warming could exceed levels of the last interglacial further increases the significance of these results, implying a clear potential for sea level rise of several meters by 2100 rather than only the tens of centimeters suggested.

While scientists have predicted a future

decline in winter sea ice coverage in the Arctic, so far, those losses had been thought to still be minimal. However, Comiso analyzed trends in winter ice coverage loss in the Arctic and found that winter ice loss was at record levels in both 2005 and 2006. Using satellite data, surface temperature data, and data on natural variability and wind, Comiso shows that ice has retreated in the region roughly 66 percent more than in previous years. He suggests that warm temperatures prevent the refreezing of the ice, which leads to a downward spiral in ice loss. Comiso asserts that many forces are at play: ice-albedo feedbacks, which occur when ice retreats and, in turn, solar radiation reflectivity is altered, increasing absorption rates; higher greenhouse gas levels, which lead to more warming; and the power of long-wave radiation during darker months in winter.

In addition, research led by Walt Meier has detected a large polynya (an area of open water that persists despite being surrounded by sea ice) north of Alaska. While the scientists have yet to determine whether the polynya appearance can be attributed to warming, Meier did note that climate change would likely result in polynya features. At its greatest extent, the polynya was as big as the state of Indiana.

Implications: These results show that the projections of winter ice retreat have already arrived in the Arctic -- well ahead of expectations. However, it is too early to determine whether this is a true trend, unlike that of summer sea ice, which is known to have thinned and retreated significantly over the last several decades. The disappearance of winter ice is likely to lead to a more rapid retreat of ice in the summer and, in turn, an increase in the rate of annual loss of Arctic ice cover, bringing closer the date at which the Arctic is ice-free.

According to Zemp et al., alpine glaciers in the European Alps had already lost almost 50 percent of their 1850 mass by 2000, with the majority of ice disappearing since 1985. Employing measurements, remote sensing, and modeling, Zemp et al. quantified past glacial loss as well as projected area change. They find that the large majority (approximately 90 percent ) of alpine glaciers are less than 1 square kilometers in size, which will further increase their vulnerability to future warming. Modeling future ice loss, the authors suggest that under a scenario of 3 degrees C warming, only 10 percent of the area covered by glaciers in 1850 would be left. A 5 degrees C warming scenario would leave the European Alps essentially completely ice free.

Implications: The loss of the European Alps' glaciers will transform ecosystems as glacial runoff disappears: sediment loads will be altered, affecting river composition; stability of montane slopes will weaken; and water resources will be at risk. The same consequences would exist for other regions of the world with mountain glaciers.