West Antarctic ice sheet and sea level
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Many of the studies of the West Antarctic Ice Sheet (WAIS) cited in the previous sections of this report address its past and future effects on sea level. In this final section on the WAIS, we bring this body of research together in one place and add other research summaries.
Bindschadler (1998) analyzed the WAIS’s historical retreat in terms of its grounding line and ice front. This work revealed that from the time of the Last Glacial Maximum to the present, the retreat of the ice sheet’s grounding line had been faster than that of its ice front, which resulted in an expanding Ross Ice Shelf. Although Bindschadler wrote that “the ice front now appears to be nearly stable,” there were indications that its grounding line was retreating at a rate that suggested complete dissolution of the WAIS in another 4,000 to 7,000 years. Such a retreat was calculated to result in a sustained sea-level rise of 8-13 cm per century. However, even the smallest of these rates-of-rise would require, in Bindschadler’s words, “a large negative mass balance for all of West Antarctica,” and there were no broad-based data that supported that scenario.
A year later, Reeh (1999) reviewed what was known about the mass balances of both the Greenland and Antarctic ice sheets, concluding that the future contribution of the Greenland and Antarctic ice sheets to global sea level depends upon their past climate and dynamic histories as much as it does upon future climate. With respect to potential climate change, Reeh determined there was a broad consensus that the effect of a 1°C climatic warming on the Antarctic ice sheet would be a fall in global sea level on the order of 0.2 to 0.7 millimeters per year.
The following year, Cuffey and Marshall (2000) reevaluated previous model estimates of the Greenland ice sheet’s contribution to sea-level rise during the last interglacial, based on a recalibration of oxygen-isotope-derived temperatures from central Greenland ice cores. Their results suggested that the Greenland ice sheet was much smaller during the last interglacial than previously thought, with melting of the ice sheet contributing somewhere between four and five-and-a-half meters to sea-level rise. According to Hvidberg (2000), this finding suggests that “high sea levels during the last interglacial should not be interpreted as evidence for extensive melting of the West Antarctic Ice Sheet, and so challenges the hypothesis that the West Antarctic is particularly sensitive to climate change.”
Oppenheimer and Alley (2005) discussed “the degree to which warming can affect the rate of ice loss by altering the mass balance between precipitation rates on the one hand, and melting and ice discharge to the ocean through ice streams on the other,” with respect to both the West Antarctic and Greenland Ice Sheets. Their review of the subject led them to conclude that we simply do not know if these ice sheets had made a significant contribution to sea-level rise over the past several decades.
One year later, however, the world was exposed to a different view of the issue when Velicogna and Wahr (2006) used measurements of time-variable gravity from the Gravity Recovery and Climate Experiment (GRACE) satellites to determine mass variations of the Antarctic ice sheet for the 34 months between April 2002 and August 2005. The two researchers concluded that “the ice sheet mass decreased significantly, at a rate of 152 ± 80 km3/year of ice, equivalent to 0.4 ± 0.2 mm/year of global sea-level rise,” all of which mass loss came from the WAIS, since they calculated that the East Antarctic Ice Sheet mass balance was 0 ± 56 km3/year.
The many estimates and adjustments used by Velicogna and Wahr to reach this conclusion were described in Section 22.214.171.124. For example, the adjustment for post-glacial rebound alone exceeded the signal being sought by nearly a factor of five. Moreover, the study covers less than a three-year period, which compares poorly with the findings of Zwally et al. (2005), who determined Antarctica’s contribution to mean global sea level over a recent nine-year period to be only 0.08 mm/year.
Ramillien et al. (2006) also used GRACE data to derive estimates of the mass balances of the East and West Antarctic ice sheets for the period July 2002 to March 2005, obtaining a loss of 107 ± 23 km3/year for West Antarctica and a gain of 67 ± 28 km3/year for East Antarctica, which results yielded a net ice loss for the entire continent of only 40 km3/year (which translates to a mean sea-level rise of 0.11 mm/year), as opposed to the 152 km3/year ice loss calculated by Velicogna and Wahr (which translates to a nearly four times larger mean sea-level rise of 0.40 mm/year). Ramillien et al. note in their closing paragraph, “the GRACE data time series is still very short and these results must be considered as preliminary since we cannot exclude that the apparent trends discussed in this study only reflect interannual fluctuations.” That caveat also applies to the Velicogna and Wahr analysis.
About the same time, Wingham et al. (2006) analyzed European remote sensing satellite altimeter echoes to determine the changes in volume of the Antarctic ice sheet from 1992 to 2003. They found that “72% of the Antarctic ice sheet is gaining 27 ± 29 Gt per year, a sink of ocean mass sufficient to lower [their italics] global sea levels by 0.08 mm per year.” This net extraction of water from the global ocean, according to Wingham et al., occurs because “mass gains from accumulating snow, particularly on the Antarctic Peninsula and within East Antarctica, exceed the ice dynamic mass loss from West Antarctica.”
Remy and Frezzotti (2006) reviewed “the results given by three different ways of estimating mass balance, first by measuring the difference between mass input and output, second by monitoring the changing geometry of the continent, and third by modeling both the dynamic and climatic evolution of the continent.” They report that “the current response of the Antarctica ice sheet is dominated by the background trend due to the retreat of the grounding line, leading to a sea-level rise of 0.4 mm/yr over the short-time scale,” which they describe in terms of centuries. However, they note that “later, the precipitation increase will counterbalance this residual signal, leading to a thickening of the ice sheet and thus a decrease in sea level.”
Krinner et al. (2007), in a study summarized in Section 126.96.36.199., used the LMDZ4 atmospheric general circulation model of Hourdin et al. (2006) to simulate Antarctic climate for the periods 1981-2000 (to test the model’s ability to adequately simulate present conditions) and 2081-2100 (to see what the future might hold for the mass balance of the Antarctic Ice Sheet and its impact on global sea level). They determined that “the simulated Antarctic surface mass balance increases by 32 mm water equivalent per year,” which corresponds “to a sea-level decrease of 1.2 mm per year by the end of the twenty-first century,” which would in turn “lead to a cumulated sea-level decrease of about 6 cm.” This result occurs because the simulated temperature increase “leads to an increased moisture transport towards the interior of the continent because of the higher moisture holding capacity of warmer air,” where the extra moisture falls as precipitation, causing the continent’s ice sheet to grow.
There has been very little change in global sea level due to wastage of the WAIS over the past few decades, and there will probably be little change in both the near and far future. What wastage might occur along the coastal area of the ice sheet over the long term would likely be countered, or more than countered, by greater inland snowfall. In the case of the latter possibility, the entire Antarctic Ice Sheet could well compensate for any long-term wastage of the Greenland Ice Sheet that might occur.
Bindschadler, R. 1998. Future of the West Antarctic Ice Sheet. Science 282: 428-429.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Cuffey, K.M. and Marshall, S.J. 2000. Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet. Nature 404: 591-594.
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Krinner, G., Magand, O., Simmonds, I., Genthon, C. and Dufresne, J.-L. 2007. Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries. Climate Dynamics 28: 215-230.
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Oppenheimer, M. and Alley, R.B. 2005. Ice sheets, global warming, and article 2 of the UNFCCC. Climatic Change 68: 257-267.
Ramillien, G., Lombard, A., Cazenave, A., Ivins, E.R., Llubes, M., Remy, F. and Biancale, R. 2006. Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE. Global and Planetary Change 53: 198-208.
Reeh, N. 1999. Mass balance of the Greenland ice sheet: Can modern observation methods reduce the uncertainty? Geografiska Annaler 81A: 735-742.
Remy, F. and Frezzotti, M. 2006. Antarctica ice sheet mass balance. Comptes Rendus Geoscience 338: 1084-1097.
Velicogna, I. and Wahr, J. 2006. Measurements of time-variable gravity show mass loss in Antarctica. Sciencexpress: 10.1126science.1123785.
Wingham, D.J., Shepherd, A., Muir, A. and Marshall, G.J. 2006. Mass balance of the Antarctic ice sheet. Philosophical Transactions of the Royal Society A 364: 1627-1635.
Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. Journal of Glaciology 51: 509-527.