In a recent post
titled “Is the Planet Earth Drowning?” dated June 15, 2015, I cited sea level
rise as one of the causes of coastal erosion. One phenomenon that feeds rise in
sea level is melting ice.
A committee, the
International Artic Science Committee, has published a report on the topic,
“Artic Climate Impact Assessment”.
I want to share with
you a paraphrased extract from the January 2013 edition of the report, as
follows:
Dowdeswell and Hagen (International
Artic Science Committee) estimated that the total volume of land ice in the Arctic is about 3.1 million cubic-kilometers
(km3), which corresponds to a sea-level equivalent of about 8 meters
(m). In terms of volume and area, the largest feature is the Greenland Ice
Sheet, which covers about four times the combined area of the glaciers and ice caps of Alaska, the Canadian
Arctic, Iceland, Svalbard, Franz Josef Land, Novaya Zemlya, Severnaya Zemlya,
and northern Scandinavia. However, unlike most small glaciers and ice caps,
more than half the surface of the Greenland Ice Sheet is at altitudes that
remain well below freezing throughout the year. Hence, relative to the
Greenland Ice Sheet, the smaller ice caps and glaciers are susceptible to
greater percentage changes of mass and area in response to changes in temperature and precipitation.
The arctic glaciers
and ice caps are irregularly distributed in space, and are located in very
different climatic regimes. The glaciers in southern Alaska and Iceland are
subject to a maritime climate with a relatively small annual temperature range
and high precipitation rates (a few meters per year). Conversely, the glaciers
in the Canadian High Arctic are in a very continental climate. The summer is
short, the annual temperature range is very large, and precipitation is about
0.25 m/yr. The conditions on Svalbard and the Russian Arctic islands fall
between these two climatic regimes.
The Greenland Ice
Sheet covers a wide latitude belt. The climate is dry and cold in the north,
although summer temperatures
can be high, with mean July temperatures of up to 5–6°C. The North Atlantic
storm track directly influences the southeastern part of the ice sheet.
Maritime air masses are pushed onto the ice sheet and release large amounts of
moisture. The accumulation rates are greatest in this part of the Greenland Ice
Sheet.
Glaciers, ice caps, and ice sheets respond to
climate changes over very different timescales depending on their size, shape,
and temperature
condition. The smaller glaciers are likely to respond quickly, with shape,
flow, and front position changing over a few years or a few decades, while the
Greenland Ice Sheet responds to climate changes over timescales of up to
millennia. Parts of the Greenland Ice Sheet may still be responding to climate
variations that occurred thousands of years ago.
Many glaciers in dry
regions have low accumulation rates. Consequently, it takes a long time before
the climate signal penetrates into these glaciers, and over a 100-year
timescale, the effects are unlikely to be very large. However, in areas where
meltwater penetration increases, the effect of latent heat release is likely to
cause a faster response in the thermal regime.
RECENT AND ONGOING CHANGES
The general pattern
of glacier and ice-cap variations in the Arctic (apart from the Greenland Ice Sheet)
is a retreat of glacier fronts, indicating a volume decrease since about 1920
that follows a period of general temperature increase throughout the Arctic.
However, there are large regional variations in the magnitude of this retreat,
and it is not known whether thickening in the accumulation areas may be
compensating for some or all of the frontal retreat. Long-term mass-balance
investigations have been conducted for only a few glaciers, which occupy less
than 0.1% of the total glaciated area in the Arctic. For the measured glaciers,
no clear trends are discernible in the mass-balance parameters, winter
accumulation, or summer melting prior to 1990. Several of the glaciers had a
negative mass balance, but with no acceleration in the melt rate. However,
changes in these trends since 1990 have been observed. Arendt et al. observed
increased and accelerating melting of Alaskan glaciers, and the same trend has
been reported for the Devon Ice Cap in northern Canada. In other parts of the
Arctic (e.g. Svalbard), no accelerated melting has been observed. In subarctic
areas (i.e., Scandinavia), increased precipitation and positive mass balance
were observed from 1988 to 1998, although the mass balances have generally been
negative since 1998.
The Greenland Ice
Sheet (1,640,000 km2) is the largest ice mass in the Arctic. Two factors contribute to the
difficulty of measuring the total mass balance of the Greenland Ice Sheet:
short-term (interannual to decadal) fluctuations in accumulation and melt rate
cause variations in surface elevation that mask the long-term trend; and
climate changes that occurred hundreds or even thousands of years ago still
influence ice flow, as do changes that are more recent.
The geological and
historical records show that the marginal zone of the Greenland Ice Sheet has
thinned and retreated over the past hundred years. Whether this mass loss was
compensated, partly or fully, by thickening in the interior is unknown.
Although several expeditions have crossed the ice sheet since the late 19th
century, the earliest measurements of sufficient precision to permit
calculation of surface-elevation change are those made by the British North
Greenland Expedition (BNGE), which crossed the ice sheet during 1953 and 1954.
Height measurements repeated in 1959, 1968, and 1992 along a profile across the
ice sheet in central Greenland showed thickening on the western slope between
1959 and 1968, but subsequent thinning between 1968 and 1992, probably
reflecting decadal-scale fluctuations in accumulation rates.
The extent of surface
melt over Greenland increased between 1979 and 2002, although large interannual
variations are superimposed on this increase.
In view of the
limited knowledge of arctic glaciers and the uncertainties discussed in Recent
and ongoing changes section above, a mass-balance sensitivity approach was
used to project future change in glaciers and ice sheets. Projected regional
changes were extrapolated from the sensitivities of glaciers for which
mass-balance data exist. These projections assume that the glaciers are in
balance with the baseline climates (temperature and precipitation) simulated by
the models, although this assumption is unlikely to be correct.
This approach to
projecting changes in mass balance does not include glacier or ice-sheet
dynamics, calving, or an explicit treatment of internal accumulation
(refreezing of meltwater that percolates into the glacier); other types of
mass-balance models would provide different results.
The Greenland Ice
Sheet is projected to make the largest contribution, which is a direct
consequence of its size. Although the glaciers in Alaska cover a much smaller
area, they are also projected to make a large contribution, in agreement with
recent analyses. For Alaskan glaciers, the relatively large sensitivity to
temperature change drives the regional changes. Glaciers and ice caps in the
Eurasian Arctic Ocean
(Svalbard, Franz Josef Land, Severnaya Zemlya, and Novaya Zemlya) are projected
to contribute about the same amount as those in the Canadian Arctic.
IMPACTS OF PROJECTED CHANGES
On other parts of the
physical system
The greatest impact
of changes in the mass of arctic land ice over
decade-to-century timescales is likely to be a change in the freshwater input to the high-latitude oceans,
which will change ocean stratification in sensitive areas such as the Greenland
and Labrador Seas. Sea-ice production and export are also likely to be affected
if more freshwater goes into the oceans and stabilizes the water column. In
some areas, increased freshwater flux is likely to increase the formation of
sea ice. Over longer timescales, changes in glacial ice (especially the
Greenland Ice Sheet) may affect the geoid and the rotation rate of the earth.
On ecosystems
To the extent that
changes in freshwater influx affect upper-ocean stratification (and possibly
sea ice), impacts on marine ecosystems are likely. Riparian ecosystems are
also likely to be affected by changes in river
flow and aufeis (ice formed when water from a stream emerges and freezes on top
of existing ice) production. Any significant change in sea level will have
impacts on ecosystems in low-lying coastal areas.
On people
The greatest direct
impacts on humans from changes in arctic land ice are likely to result from
changes in global sea level, which will affect coastal communities in many
parts of the Arctic. Other possible
impacts include changes in hydropower production and water supply from glacier-fed lakes and reservoirs. Changes in
iceberg production will increase or decrease hazards to shipping and
navigation.
The compilation of an
up-to-date global glacier inventory is a critical research need. For some
regions, existing inventories are sparse; inventories also need to be updated
where glacier areas have changed. A global satellite-derived dataset of exposed
ice areas is a minimum requirement. Ideally, a complete glacier database
describing individual glacier locations, areas, and geometries should be
compiled, so that mass-balance measurements on individual benchmark glaciers
can be extrapolated to unmeasured glaciers with greater certainty.
For future
projections, it would be useful to develop additional mass-balance models so
that spatial variations can be better depicted and so that the records can be
extended back in time at locations for which atmospheric data are available.
For this purpose, additional mass-balance observations should be obtained in
regions where existing data are particularly sparse, in order to provide
credibility and a sense of the uncertainties in model projections of future
trends. It is also important to continue the ongoing monitoring of glacier mass balance with in situ
measurements on selected glaciers in order to improve understanding of the
response of glaciers to climate change, improve model projections of future
change, and calibrate remote-sensing data.
In order to improve
projections of future mass-balance changes, the following studies should be
given high priority:
- improving understanding of albedo changes and
feedback mechanisms;
- studies of outlet glacier dynamics with
emphasis on their potential for triggering persistent, rapid changes in
ice-sheet volume;
- improving ice-dynamic models for
determining the long-term response of the ice sheet to past climate
change;
- improving parameterization and
verification of internal-accumulation models; and
- improving understanding of the relationships
between climate change, meltwater penetration to the bed, and changes in
iceberg production.
Future changes in
mass balance are strongly dependent on future changes in climate. Consequently,
the ability to project changes in the mass balance of the Greenland Ice Sheet
is linked closely to the ability of atmosphere–ocean general circulation models
(AOGCMs) to project changes in regional climate over Greenland. For example,
recent AOGCM model runs project a greater increase in the accumulation rate
over Greenland associated with a temperature increase than did previous studies. If
the latest projections prove to be accurate, increases in accumulation would
largely compensate for the increased runoff resulting from projected
temperature increases.
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