|
GCOS Terrestrial ECV
Ice Sheets
Introduction: Our understanding of the time scale of ice sheet response to climate change has changed dramatically over the last decade. Rapid changes in ice-sheet mass have surely contributed to abrupt changes in climate and sea level in the past. The mass balance loss of the Greenland Ice Sheet increased in the late 1990s to 100 gigatonnes per year (Gt a-1) or to even more than 200 Gt a-1 for the most recent observations in 2006. It is extremely likely that the Greenland Ice Sheet has been losing mass, and very likely on an accelerated path, since the mid-1990s. The mass balance for Antarctica as a whole is close to equilibrium, but with a likely net loss since 2000 at rates of a few tens of gigatonnes per year. The largest losses are concentrated along the Amundsen and Bellinghausen sectors of West Antarctica and the northern tip of the Antarctic Peninsula. The potentially sensitive regions for rapid changes in ice volume are those with ice masses grounded below sea level, such as the West Antarctic Ice Sheet which, if it melted, would raise sea level by 7 m, or large glaciers in Greenland like the Jakobshavn, also known as Jakobshavn Isbræ and Sermeq Kujalleq (in Greenlandic), with an over-deepened channel reaching far inland. There are large mass-budget uncertainties from errors in both snow accumulation and calculated ice losses for Antarctica (~±160 Gt a-1) and for Greenland (~±35 Gt a-1). Most climate models suggest that climate warming would cause increased melting from coastal regions in Greenland and an overall increase in snowfall. However, they do not predict the substantial acceleration of some outlet glaciers that we are now observing. This results from a fundamental weakness in the existing models, which are incapable of realistically simulating the outlet glaciers that discharge ice into the ocean.
Observations show that Greenland is thickening at high elevations due to the increase in snowfall, which has been predicted, but that this gain is more than offset by an accelerating mass loss, with a large component from rapidly thinning and accelerating outlet glaciers. Although there is no evidence for increasing snowfall over Antarctica, observations show that some higher elevation regions are also thickening, likely as a result of high interannual variability in snowfall. There is little surface melting in Antarctica, and the substantial ice losses from West Antarctica and the Antarctic Peninsula are very likely caused by increased ice discharge as the velocities of some glaciers increase. This is of particular concern in West Antarctica, where bedrock beneath the ice sheet is deep below sea level, and outlet glaciers are to some extent “contained” by the ice shelves into which they flow. Some of these ice shelves are thinning, and some have totally broken up. These are the regions where the glaciers are accelerating and thinning most rapidly.
Recent observations show a high correlation between periods of heavy surface melting and increase in glacier velocity. A possible cause is rapid meltwater drainage to the base of the glacier, where it enhances basal sliding. An increase in meltwater production in a warmer climate will likely have major consequences on ice-flow rate and mass loss. Recent rapid changes in marginal regions of the Greenland and West Antarctic ice sheets show mainly acceleration and thinning, with some glacier velocities increasing more than twofold. Many of these glacier accelerations have closely followed reduction or loss of their floating extensions known as ice shelves.
Efforts should be made to (a) reduce uncertainties in estimates of mass balance and (b) derive better measurements of ice-sheet topography and velocity through improved observation of ice sheets and outlet glaciers. This includes utilizing existing satellite interferometric synthetic aperture radar (InSAR) data to measure ice velocity, using observations of the time-varying gravity field from satellites to estimate changes in ice sheet mass, and monitoring changes in ice sheet topography using tools, such as satellite radar (e.g., Envisat and Cryosat-2), lasers (e.g., ICESat-1/2), and wide-swath altimeters.
Monitoring the polar regions with numerous satellites at various wavelengths is essential to detect change (i.e., melt area) and to understand processes responsible for the accelerated loss of ice sheet ice and the disintegration of ice shelves in order to estimate future sea level rise. Further, aircraft observations of surface elevation, ice thickness, and basal characteristics should be utilised to ensure that such information is acquired at high spatial resolution along specific routes, such as glacier flow lines, and along transects close to the grounding lines. In situ measurements (e.g., of firn temperature profile and surface climate) are equally important in assessing surface mass balance and understanding recent increases in mass loss.
(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)
References:
Data, Product, Metadata and Information Access
[ECV Matrix Main Page] [About the ECV Matrix] [Reference Documents] [Contact] [Updated June 6, 2010]
| Non-satellite or in-situ
|
Satellite
|
|
|
|
|
