The earth's CRYOSPHERE
is showing evidence of global warming. The cryosphere includes
the sea ice that covers the Arctic and surrounds Antarctica; the massive
ice sheets that cover Antarctica and Greenland and associated ice
shelves that extend from the Antarctic ice sheets over the surrounding
ocean; and the world’s alpine and high-latitude glaciers and
ice caps. Excepting Antarctic sea ice, all these components of the
cryosphere exhibit decline in ice mass consistent with the effects
of surface warming, and most are projected to undergo further melting
at rates that may be accelerating.
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.
Glaciers
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.
Further reading:
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.
