Sea Level Rise
Sea level rise is widely cited as an imminent and detrimental consequence of climate change; however, predicting the rate and extent of the threat has been a complicated task for climate modelers. Thermal expansion – the expansion of water molecules as water temperature rises — and glacier and ice sheet mass loss through rapid surface melting and accelerating ice flows are the two predominant forces in sea level rise.
The earth's two largest ice sheets, in Greenland and Antarctica, represent the largest unknowns in predictions of global sea-level rise.[1] Flows from outlet glaciers and ice streams that drain the massive ice sheets have accelerated dramatically in recent years[2], and scientists are only just beginning to report findings in relation to this phenomenon. In addition, a number of newly discovered processes, including the lubrication of the ice sheet base by melt-water trickling down from the surface through glacial cracks, make ice sheet flow dynamic and difficult to predict.[3]
Because of these complexities and rapid growth of knowledge in the field, models of ice flow dynamics are not yet as accurate as those in other areas of climate science. Thus the contribution of ice flow is included as a constant in the recent sea level rise estimates of the Intergovernmental Panel on Climate Change's (IPCC). As a result, the estimate of sea level rise (18-59 cm for 2090-2099) published in the Physical Science Basis of the IPCC's Fourth Assessment Report (AR4) does not tell the full story about and may underestimate future sea level rise.
The sea level rise figures in the AR4 have generated confusion and misinterpretation because they appear to represent a downward revision of the estimated range (9-88 cm by 2100) published in IPCC's 2001 Third Assessment Report (TAR). The IPCC explains that the new figures are not directly comparable to the older ones because they work under different timescales (projections were made for 2100 vs. the period of 2090-2099) and many major uncertainties (including those about changes in ice sheet flow) were treated differently. The reason for the much narrower range in sea level rise seen in the AR4 is mainly a result of improved information and more conservative assumptions made about some large unknowns.
For more information please visit the following links:
Intergovernmental Panel on Climate Change's Climate Change 2007: The Physical Science Basis Summary for Policymakers (Contribution of Working Group I to the Fourth Assessment Report)
The IPCC sea level numbers. An analysis by Dr. Stefan Rahmstorf found at RealClimate.
[1] Dowdeswell, J. (2006) The
Greenland Ice Sheet and Global Sea-Level Rise. Science. 311(5763): 963-964.
[2] Shepherd, A. and D. Wingham. (2007) Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets. Science. 315(5818): 1529-1532; Peters, L., S. Anandakrishnan, R. Alley, and A. Smith. (2007) Extensive storage of basal meltwater in the onset region of a major West Antarctic ice stream. Geology. 35(3):251-254; Howat, I., I. Joughin, and T. Scambos. (2007) Rapid Changes in Ice Discharge from Greenland Outlet Glaciers. Science. 315(5818): 1559-1561; More information on the Ice Streams of the West Antarctic Ice Sheet (WAIS) can be found at: http://www.ldeo.columbia.edu/~mstuding/wais.html.
[3] Zwally, H. et al. (2002) Surface
Melt-Induced Acceleration of Greenland Ice-Sheet Flow. Science. 297:218-222; Rignot, E. and P. Kanagaratnam. (2006) Changes
in the Velocity Structure of the Greenland Ice Sheet. Science. 311(5763): 986-990.
