| 4.13 |
Most models project a weakening
of the ocean thermohaline circulation, which leads to a reduction of the
heat transport into high latitudes of Europe (see Figure
4-2). However, even in models where THC weakens, there is still
a warming over Europe due to increased concentrations of greenhouse gases.
The current projections do not exhibit a complete shutdown of THC by the
year 2100. Beyond the year 2100, some models suggest that THC could completely,
and possibly irreversibly, shut down in either hemisphere if the change
in radiative forcing is large enough and applied long enough. Models indicate
that a decrease in THC reduces its resilience to perturbations (i.e., a
once-reduced THC appears to be less stable and a shutdown can become more
likely).
|
 WGI
TAR SPM & WGI TAR Sections
7.3 & 9.3.4 |
| 4.14 |
The Antarctic ice sheet as
a whole is likely to increase in mass during the 21st century. However,
the West Antarctic ice sheet could lose mass over the next 1,000 years with
an associated sea-level rise of several meters, but there is an incomplete
understanding of some of the underlying processes. Concerns have
been expressed about the stability of the West Antarctic ice sheet because
it is grounded below sea level. However, loss of grounded ice leading to
substantial sea-level rise from this source is widely agreed to be very
unlikely during the 21st century. Current climate
and ice dynamic models project that over the next 100 years the Antarctic
ice sheet as a whole is likely to gain mass because of a projected increase
in precipitation, contributing to a relative decrease of several centimeters
to sea level. Over the next 1,000 years, these models project that the West
Antarctic ice sheet could contribute up to 3m to sea-level rise.
|
WGI
TAR Section 11.5.4 |
| 4.15 |
The Greenland ice sheet is
likely to lose mass during the 21st century and contribute a few centimeters
to sea-level rise. Over the 21st century, the Greenland ice sheet
is likely to lose mass because the projected increase in runoff will exceed
the increase in precipitation and contribute 10 cm maximum to the total
sea-level rise. The ice sheets will continue to react to climate warming
and contribute to sea-level rise for thousands of years after climate has
stabilized. Climate models indicate that the local warming over Greenland
is likely to be one to three times the global average. Ice sheet models
project that a local warming of larger than 3°C, if sustained for thousands
of years, would lead to virtually a complete melting of the Greenland ice
sheet with a resulting sea-level rise of about 7 m. A local warming of 5.5°C,
if sustained for 1,000 years, would likely result in a contribution from
Greenland of about 3 m to sea-level rise (see Question
3).
|
WGI
TAR Section 11.5.4 |
| 4.16 |
Pronounced changes in permafrost
temperature, surface morphology, and distribution are expected in the 21st
century. Permafrost currently underlies 24.5% of the exposed land
area of the Northern Hemisphere. Under climatic warming, much of this terrain
would be vulnerable to subsidence, particularly in areas of relatively warm,
discontinuous permafrost. The area of the Northern Hemisphere occupied by
permafrost could eventually be reduced by 12 to 22% of its current extent
and could eventually disappear from half the present-day Canadian permafrost
region. The changes on the southern limit may become obvious by the late
21st century, but some thick ice-rich permafrost could persist in relict
form for centuries or millennia. Thawing of ice-rich permafrost can be accompanied
by mass movements and subsidence of the surface, possibly increasing the
sediment loads in water courses and causing damage to the infrastructure
in developed regions. Depending on the precipitation regime and drainage
conditions, degradation of permafrost could lead to emission of greenhouse
gases, conversion of forest to bogs, grasslands, or wetland ecosystems and
could cause major erosion problems and landslides.
|
WGII
TAR Sections 16.1-2 |
|
|
