Geoscience Reference
In-Depth Information
and abrupt (i.e. decadal scale) climate changes. Yet the observational and model under-
pinning of these hypotheses are at best sketchy making it very difficult to come to firm
conclusions. Reliable quantification of the variability and stability of the THC and its
atmospheric implications in the current and future climate are therefore a major challenge
in climate research.
Projects such as ''Thermohaline Overturning—at Risk?'' (THOR;
http://www.eu-thor.
eu/THOR-in-short.532.0.html
) and RAPID-WATCH: Monitoring the Atlantic Meridional
Overturning Circulation (
http://www.noc.soton.ac.uk/rapid/rw/index.php
)
aim to improve
our understanding of the Atlantic MOC. Areas of current work include quantification of the
risk of future shutdown of the MOC driven by salinity changes in the North Atlantic and
Arctic Oceans under climate change. Under climate change, there may be an increase in
the amount of freshwater input into the North Atlantic Ocean due to increased precipitation
and melt water from the Greenland ice sheets. By reducing the density of the surface
waters, this could stop the sinking of dense water in the North Atlantic and lead to a slow
down or even a shutdown of the MOC (e.g., Mignot et al.
2007
, and references therein).
Mechanisms of natural variability are also being investigated including interactions
between the MOC, extratropical salinity anomalies and the position of the inter-tropical
convergence zone on centennial timescales. As part of the RAPID-WATCH project, work
is underway using models to find variables that could be used to detect a significant trend in
the strength of the MOC earlier than is possible by observing it directly. Examples could
include temperatures and salinity in certain areas of the Nordic Seas.
2.3.4 The Role of the Cryosphere
The cryosphere encompasses the regions of the Earth's surface where water is in solid
form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps and ice sheets, and
frozen ground (which includes permafrost). Although the more spectacular parts of the
hydrological cycle may be apparent in the tropics and subtropics, the cryosphere is an
integral part of the global climate system, with important linkages and feedbacks generated
through its influence on surface energy and moisture fluxes, clouds, precipitation,
hydrology, atmospheric and oceanic circulation (see papers included in Bengtsson et al.
2011
).
Sea level rise is arguably the most certain consequence of a warming climate, due to
thermal expansion of water and the dependence of ice sheets and glaciers on low tem-
peratures. It is currently thought that melting of the major ice sheets contribute around
1.2 mm of the approx. 3 mm of annual sea level rise (Lemke et al.
2007
). An under-
standing of how this will change in the future is a policy driver. The key issue is not
whether sea level will rise, but by how much and how fast. The West Antarctic ice sheet
and Greenland ice sheets each contain enough land-based ice to raise sea level directly by
several metres. The rapid disintegration of either of these ice sheets could cause sea level
rise that is too great, or too fast for many coastal populations and ecosystems to adapt to. In
addition to sea level rise, fresh water from melting of the Greenland ice sheet may con-
tribute to a slowdown of the North Atlantic overturning circulation.
The IPCC Fourth Assessment Report (Meehl et al.
2007
) acknowledged that current
models do not adequately treat the dynamic response of ice sheets to climate change, and
that this is the largest uncertainty in assessing potential rapid sea level rise. Many current
climate models do not include an ice sheet model; in order to close the global water budget,
the accumulation of frozen water on the permanent ice sheets is often returned to the
