Impacts of a melting cryosphere – ice loss around the world
© Carsten Egevang/ARC-PIC.COM
With delicately balanced ecosystems, weather patterns and a lot
of ice, the planet's frozen areas (collectively known as the
'cryosphere') are some of the most sensitive to climate change. As
the cryosphere warms, we can expect to see a range of knock-on
effects. This briefing runs through what we know about how the
cryosphere is reacting to climate change, and what the likely
consequences of continued warming are.
The warming cryosphere
The cryosphere comprises of the Earth's ice - notably the
Arctic, the Antarctic, and the world's glaciers.
Historical records suggest that Arctic sea ice
began to decline around 1900, with a more accelerated ice loss
since the 1950s. Since 1979, satellite records have confirmed an
overall decline in summer sea ice coverage of around 13%
per decade. Although fears that Arctic sea ice would reach a
'tipping point' and rapidly disappear currently seem to be unfounded, the
mainstream view of scientists is that we will see ice-free summers
in the Arctic ocean within the next few decades.
The major body of land ice in the Arctic - the Greenland
ice sheet - has been found to be thickening inland. Nearer
the coast, however, the ice sheet is thinning and shrinking, at an
accelerating rate. Overall, the Greenland ice sheet has been
found to be losing
ice mass over the last twenty years, and this ice loss is
becoming faster.
Antarctic ice consists of a land mass topped by
an ice sheet and surrounded by sea ice. Antarctic sea
ice extentis increasing at around 1% per
decade, thought
to be caused by shifting weather patterns reacting to changes
in ozone, high in the atmosphere.
The 2007 IPCC report
concluded that the Antarctic ice sheet was
most likely losing
ice overall - with the Antarctic Peninsula (in the west of the
continent) warming and the eastern continental interior cooling
slightly. The range of uncertainty in this assessment was large,
with ice loss assessed as 24 gigatonnes per year, plus or minus 25
gigatonnes - i.e. somewhere between gaining 1 gigatonne and losing
49.
This considerable uncertainty is due in large part to the
difficulty of obtaining measurements in such a harsh environment.
However,
recent research suggesting the whole of Western Antarctic is
warming in winter and spring, while controversial in the media,
has been supported by a subsequent
borehole study.
Around the world, mountain glaciers are also
losing ice mass. For example, a study
of the Patagonian ice fields found that the rate of ice loss in
the area has increased over the 20th century. Another group of
scientists modelled
around 120,000 glaciers around the world, concluding that they are
likely to lose around a fifth of their ice volume over the next
century.
The impacts of melting
The picture emerging from the scientific research is one of
accelerated ice melting in every part of the cryosphere. Research
has found that the environmental changes at both poles cannot
result from natural climate change alone, and is directly
attributable to man-made climate change.
This has knock-on effects. The cryosphere is an important part
of our water cycle. Ice locks water out of the
cycle for long periods of time - with large-scale melting, a vast
amount of water can be released back into the water cycle in a
relatively short amount of time.
Unsurprisingly, water from the current cryosphere melt is
contributing to global sea level rise. As shown in
the graph below, the observed increase in global sea level is
currently at the upper limit of projections made by the IPCC in
1990.
Credit: The
Copenhagen Diagnosis Report
As ice loss from both the polar ice sheets and mountain glaciers
accelerates, global sea level rise is also set to increase. One recent
study suggests that the Greenland and Antarctic ice sheets are
likely to be the dominant contributor to sea level rise in the 21st
century, providing around half of all sea level rise over the next
40 years. Ice melt from glaciers is also likely to play an important
role.
Given current melting rates, and taking thermal heat expansion
of the oceans into account, total sea level rise is estimated to be
around
32 cm by 2050.
Melting ice also has implications for water
supply, with many millions of people around the globe
dependent on rivers fed by mountain glaciers. For example, in the
Indus and Brahmaputra basins alone around 60
million people rely on glacial meltwater for their water
supply.
With glaciers melting faster, the
IPCC has suggested that in the short term summer river flow is
likely to increase - but further into the future river flow is
likely to decrease as the ice feeding rivers disappears.
Other impacts
Permafrost - ground frozen for at least two
years - contains large amounts of carbon, stored as frozen methane
or organic material unable to decay in its frozen state.
Researchers
from the National Snow and Ice Data Center (NSIDC) estimate
that if permafrost melt continues, around 190 gigatonnes of carbon
could be released into the atmosphere by 2200, further warming the
planet. To put this figure in context, in the year 2010 manmade
greenhouse gas emissions were at around 30
gigatonnes.
Scientists are also warning of a
more immediate potential threat from melting permafrost,
suggesting that methane released from seafloor permafrost in the
Arctic Ocean could enhance ocean acidification in that region over
the next century.
For more information about the melting cryosphere & sea
level variability:
This blog is also posted at Skeptical
Science.