Arctic sea ice
In recent years, the area covered by Arctic sea ice, particularly the minimum extent in summer, has rapidly shrunk. This is thought to result from a combination of factors: over the past several decades surface air temperatures in the Arctic have been warming at about twice the global rate, and the ocean has been warming. Other possible factors may be changes to ocean and atmospheric circulation patterns, which play a role in flushing ice out of the Arctic Basin. Once melting is initiated, the lower ALBEDO of water compared to ice also provides a reinforcing feedback mechanism that accelerates further melting, because the open water is able to absorb more heat from the sun.
Arctic sea ice extent changes since the mid-1970s
Source: UK Department of Environment, Food and Rural Affairs, Met Office Hadley Centre, Climate change and the greenhouse effect—a briefing from the Hadley Centre, December 2005, p. 35. © British Crown Copyright 2005, the Met Office.
Modelling suggests that under several of the IPCC's emissions scenarios, ice in the month of September (when it is at its minimum extent in the annual cycle) will have almost completely disappeared on average by 2100. However, the recent rapid decrease in Arctic sea ice is occurring faster than predicted by IPCC's Fourth Assessment Report. A new summer minimum was set in 2007 which is around 30 years ahead of a range of simulation model forecasts (see the Climate Institute’s Evidence of Accelerated Climate Change). An ice-free Arctic Ocean might be achieved well ahead of the timeframe indicated by IPCC modelling; if so, this suggests that the Arctic is even more sensitive to greenhouse warming than suspected to date. An animation of possible Arctic ice loss to 2100 can be viewed at http://www.metoffice.gov.uk/climatechange/science/projections/
Present day and projected Arctic sea ice fractional concentration
At the opposite end of the globe, the Antarctic Peninsula also appears to be warming. The collapse of the Larsen B ice shelf in 2002 was the largest single event in a series of retreats by ice shelves in the Peninsula during the last 30 years. The rate of warming in this region is approximately 0.5°C per decade, compared to the global rate of about 0.2°C per decade. The warming is indicated by increased summer snowmelt, loss of ice shelves and the retreat of marine and tidewater glacial fronts. Flow rate measurements for Antarctic Peninsula glaciers indicate accelerating trends. The Southern Ocean is also warming more rapidly than the global ocean. These changes are impacting the flora and fauna of the Peninsula: sea-ice adapted Adelie penguins are being replaced by the more open-water oriented Chinstrap penguins, and there has been increasing plant cover.
However, surface temperatures over the rest of Antarctica have remained approximately stable, and the amount of snow falling in Antarctica appears to be increasing. Although regional snowfall is not easy to measure there (partly because the snow can blow considerable distances and accumulate) the number of days of precipitation has increased. Antarctica is the world's driest continent, but global warming—by increasing global evaporation—is likely on theoretical grounds to increase precipitation in many areas.
The total average area of Antarctic sea ice has more or less stayed the same over the last three decades. In contrast, the mass of ice in the two large ice sheets over the continent, and associated ice shelves that represent the extension of the ice sheets over the ocean, may be changing. Antarctica contains most of the current global ice mass—enough to cause a sea level rise of about 60 metres if the ice sheets disappeared completely. However, large scale melting and dynamical loss of the Antarctic ice sheets is thought to be unlikely in the next century. Most at risk is the smaller West Antarctic ice sheet, which has the potential to contribute about 6 metres to sea level rise if it were lost completely. Current evidence suggests that the West Antarctic ice sheet is losing mass, which is partially offset by smaller gains in the East Antarctic ice sheet.
There is considerable uncertainty over the influence of atmospheric and ocean circulation patterns on Antarctic temperatures, snowfall distribution and amount and how these patterns may change with global warming, which complicates efforts to predict changes in the continent's ice mass balance. Models are in general agreement in predicting that for the next century enhanced snowfall on the continent of Antarctica should exceed warming-induced ice losses, and this increased accumulation of ice should offset some of the sea level rise that would otherwise occur.
Greenland ice sheet
The Greenland ice sheet contains the equivalent of about 7 metres of sea level rise. Recent studies suggest that the ice sheet has been experiencing a net loss (losses due to melting and ice flow discharge are exceeding gains due to snow accumulation), and that the rate of loss is increasing. Ice mass loss from the Greenland ice sheet is thought to have contributed to a sea level rise of 0.05 millimetres per year from 1961 to 2003, with the rate increasing to 0.21 millimetres per year from 1993 to 2003. There is a high degree of year-to-year variability in the ice mass balance, driven largely by variability in the amount of summer melting, as well as variability in the rate of discharge via glaciers.
Models predict that the Greenland ice sheet may shrink substantially over the next few hundred years in response to global warming. Results also suggest the ice sheet could disappear completely if temperatures rise above a critical threshold, and that this threshold could be crossed this century. The melting process would occur slowly, raising global sea level by about 7 metres over more than 1000 years, as shown in model simulation below. It is uncertain whether melting of the ice sheet could be reversed once the process was substantially underway, as the absence of ice would change the albedo to allow more of the sun's heat to be absorbed, and surface temperatures would also be enhanced by an overall lowering of surface elevation.
Simulated melting of the Greenland ice sheet under atmospheric CO2 concentrations stabilised at 4x pre-industrial levels
Evolution of the Greenland surface elevation and ice sheet volume versus time in the experiment of Ridley et al. (J. Climate, vol. 17, p. 3409, 2005) with the UKMO–HadCM3 AOGCM coupled to the Greenland Ice Sheet model of Huybrechts and De Wolde (1999) under a climate of constant quadrupled pre-industrial atmospheric CO2.
Source: Intergovernmental Panel on Climate Change, Fourth Assessment Report, Working Group I report—the physical science basis, Chapter 10 Global climate projections, Figure 10.38, p. 830.
Outlet glaciers of the Greenland and West Antarctic ice sheets provide one of the mechanisms of ice loss from these large ice sheets. This dynamic drainage of ice can account for most of the observed Antarctic net ice mass loss, and about half of the Greenland mass loss (the remainder being due to melting of the ice sheet in excess of replenishing snowfall). Recent evidence suggests that the flow rate of these outlet glaciers is increasing, thereby enhancing the rate of mass loss from the ice sheets. This may foreshadow a more rapid rise in sea level that could have a potentially dramatic effect on coastal regions worldwide.
Crucial to the survival of a glacier is its mass balance, the difference between accumulation and ablation (melting and sublimation). Climate change may cause variations in both temperature and snowfall, causing changes in mass balance. A glacier with a sustained negative balance is out of equilibrium and will retreat. A glacier with sustained positive balance is also out of equilibrium, and will advance to re-establish equilibrium. Currently, there are a few advancing glaciers, although their modest growth rates suggest that they are not far from equilibrium.
As a general rule, the world's glaciers have been retreating since the 1850s. Mid-latitude mountain ranges such as the Himalayas, European Alps, Rocky Mountains, Cascade Range, and the southern Andes, as well as isolated tropical summits such as Mount Kilimanjaro in Africa, are showing some of the largest proportionate glacial loss. The rate of retreat of most glaciers has increased since 1990. The observed decline in mass balance of glaciers and ice caps (excluding those surrounding the Greenland and West Antarctic ice sheets) can be translated to an equivalent sea level rise of about 0.3 millimetres per year from 1960 to 1990; with the rate doubling to about 0.6 millimetres per year of equivalent sea level rise from 1990 to 2004.
Retreat of the world’s glaciers
Large-scale regional mean length variations of glacier tongues. The raw data are all constrained to pass through zero in 1950. The curves shown are smoothed with the Stineman method and approximate this. Glaciers are grouped into the following regional classes: SH (tropics, New Zealand, Patagonia), northwest North America (mainly Canadian Rockies), Atlantic (South Greenland, Iceland, Jan Mayen, Svalbard, Scandinavia), European Alps and Asia (Caucasus and central Asia).
Source: Intergovernmental Panel on Climate Change, Fourth Assessment Report, Working Group I Report—the physical science basis, Chapter 4 Observations—changes in snow, ice and frozen ground, Figure 4.13, p. 357.
There is much information supporting the retreat of glaciers. Perhaps the most striking evidence relates to the retreat of European glaciers. The World Glacier Monitoring Service monitors changes in the mass, length, volume and area of glaciers worldwide. Between 1995 and 2000, 103 of 110 glaciers examined in Switzerland, 95 of 99 glaciers in Austria, all 69 glaciers in Italy, and all 6 glaciers in France were in retreat. As an example, since 1870 the Argentière and Mont Blanc Glacier have receded by 1150 metres and 1400 metres respectively. The rate of retreat appears to be increasing: the Trift Glacier in Switzerland retreated over 500 metres or 10 per cent of its total length in the three years 2003–2005. Closer to home, in both Papua New Guinea and New Zealand glaciers have retreated rapidly over the last 60 years, coinciding with warming over this period.
Glaciers stockpile rock and soil that has been carved from mountainsides at their terminal end. These debris piles often form dams that impound water behind them and form glacial lakes as the glaciers melt and retreat from their maximum extents. These are commonly unstable and have been known to burst if overfilled or displaced by earthquakes, landslides or avalanches. So-called 'glacial lake outbursts' have occurred in every region of the world where glaciers are located. Continued glacier retreat is expected to create and expand glacial lakes, increasing the risk to infrastructure, property and life relating to glacial lake failures.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate Change 2007: The Physical Science Basis, Chapter 4 Observations—changes in snow, ice and frozen ground.