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West Antarctic Ice Sheet (Sea Level) -- Summary
Although we all well know what Al Gore and James Hansen have to say about global warming and the West Antarctic Ice Sheet's influence on global sea level, it behooves everyone to keep abreast of what is reported in the peer-reviewed scientific literature, which is often quite different from what the two icons unabashedly declare with great confidence. Hence, we here review what has been learned over the past decade by scientists who specialize in this particular field of research.

In a review of what was known about the West Antarctic Ice Sheet (WAIS) that was published in Science back in 1998, Bindschadler analyzed its 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; and 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, in fact, Reeh determined there was a broad consensus that the effect of a 1C 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."

Jumping ahead five years, 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 far 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. When all was said and done -- which included a lot of dubious approximations -- 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.

What these results imply about the real world is highly dependent upon their ability to truly represent what they presume to describe; and in this regard Velicogna and Wahr say there is "geophysical contamination ... caused by signals outside Antarctica," including "continental hydrology ... and ocean mass variability." The first of these confounding factors, according to them, "is estimated [our italics] using monthly, global water storage fields from the Global Land Data Assimilation system," while "the ocean contamination is estimated [our italics] using a version of the Estimating Circulation and Climate of the Ocean (ECCO) general circulation model [our italics]."

In addition to these problems, the two researchers note that the GRACE mass solutions "do not reveal whether a gravity variation over Antarctica is caused by a change in snow and ice on the surface, a change in atmospheric mass above Antarctica, or post-glacial rebound (PGR: the viscoelastic response of the solid Earth to glacial unloading over the last several thousand years)."

To adjust for the confounding effect of the variable atmospheric mass above Antarctica, Velicogna and Wahr utilized European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological fields, but they acknowledge "there are errors in those fields," so they "estimate [our italics] the secular component of those errors by finding monthly differences between meteorological fields from ECMWF and from the National Centers for Environmental Prediction."

With respect to post-glacial rebound, the two researchers say "there are two important sources of error in PGR estimates: the ice history and Earth's viscosity profile." To deal with this problem, they "estimate [our italics] the PGR contribution and its uncertainties using two ice history models [our italics]."

All of these estimates and adjustments are convoluted and complex, as well as highly dependent upon various models. In addition, the estimates and adjustments do not deal with miniscule effects, as Velicogna and Wahr acknowledge that "the PGR contribution is much larger than the uncorrected GRACE trend." In fact, their calculations indicate that the PGR contribution exceeds that of the signal being sought by nearly a factor of five!!! And they are forced to admit that "a significant ice mass trend does not appear until the PGR contribution is removed."

In light of the latter humungous confounding problem, Velicogna and Wahr rightly state in their concluding paragraph that "the main disadvantage of GRACE is that it is more sensitive than other techniques to PGR." In fact, considering the many other adjustments they had to make, based upon estimations utilizing multiple models and databases with errors that had to be further estimated, we are led to totally discount the significance of their final result, particularly in light of the additional fact that it did not even cover a full three-year period. Much more likely to be much more representative of the truth with respect to the WAIS's mass balance are 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 compared to the five-times-greater value of 0.4 mm/year calculated by Velcogna and Wahr.

Skipping ahead one more 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). Clearly, Ramillien et al.'s mean sea level rise is much less ominous than the much larger value calculated by Velicogna and Wahr; and it is of the same order of magnitude as the 0.08 mm/year Antarctic-induced mean sea level rise calculated by Zwally et al. (2005), which was derived from elevation changes based on nine years of satellite radar altimetry data obtained from the European Remote-sensing Satellites ERS-1 and -2. Even at that, the GRACE approach is still laden with a host of potential errors, as we noted in our discussion of the Velicogna and Wahr paper. In addition, as 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," which caveat also applies to the Velicogna and Wahr analysis.

About the same time, Wingham et al. (2006) "analyzed 1.2 x 108 European remote sensing satellite altimeter echoes to determine the changes in volume of the Antarctic ice sheet from 1992 to 2003," which survey, in their words, "covers 85% of the East Antarctic ice sheet and 51% of the West Antarctic ice sheet," which together comprise "72% of the grounded ice sheet." In doing so, they found that "overall, the data, corrected for isostatic rebound, show the ice sheet growing [our italics] at 5 1 mm per year." To calculate the ice sheet's change in mass, however, "requires knowledge of the density at which the volume changes have occurred," and when the researchers' best estimates of regional differences in this parameter were used, 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."

In yet another contemporary study, 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." In describing their findings, the two researchers state that "the East Antarctica ice sheet is nowadays more or less in balance, while the West Antarctica ice sheet exhibits some changes likely to be related to climate change and is in negative balance." In addition, 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 [our italics]."

Last of all, Krinner et al. (2007) 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). This work revealed, first of all, that "the simulated present-day surface mass balance is skilful on continental scales," which gave them confidence that their results for the end of the 21st century would be reasonably skilful as well. Of that latter period some 90 years from now, they determined that "the simulated Antarctic surface mass balance increases by 32 mm water equivalent per year," which corresponds "to a sea level decrease [our italics] 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, in their words, 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.

The results of this study -- based on sea surface boundary conditions taken from IPCC Fourth Assessment Report simulations (Dufresne et al., 2005) that were carried out with the IPSL-CM4 coupled atmosphere-ocean general circulation model (Marti et al., 2005), of which the LMDZ4 model is the atmospheric component -- argue strongly against climate-alarmist predictions of future catastrophic sea level rise due to mass wastage of the Antarctic Ice Sheet caused by CO2-induced global warming. In fact, they suggest just the opposite, i.e., that CO2-induced global warming would tend to buffer the world against such an outcome.

All in all, it would appear there has been very little change in global sea level due to wastage of the WAIS over the past few decades, and that there will probably be little change in both the near and far future. In fact, what wastage might occur along the coastal area of the ice sheet over the long term would likely be countered, or even more than countered, by greater inland snowfall. And in the case of the latter possibility, or probability, the entire Antarctic Ice Sheet could well compensate for any long-term wastage of the Greenland Ice Sheet that might occur, in terms of how these phenomena impact global sea level.

References
Bindschadler, R. 1998. Future of the West Antarctic Ice Sheet. Science 282: 428-429.

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.

Dufresne, J.L., Quaas, J., Boucher, O., Denvil, S. and Fairhead, L. 2005. Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century. Geophysical Research Letters 32: 10.1029/2005GL023619.

Hourdin, F., Musat, I., Bony, S., Braconnot, P., Codron, F., Dufresne, J.L., Fairhead, L., Filiberti, M.A., Friedlingstein, P., Grandpeix, J.Y., Krinner, G., Le Van, P., Li, Z.X. and Lott, F. 2006. The LMDZ4 general circulation model: climate performance and sensitivity to parameterized physics with emphasis on tropical convection. Climate Dynamics 27: 787-813.

Hvidberg, C.S. 2000. When Greenland ice melts. Nature 404: 551-552.

Marti, O., Braconnot, P., Bellier, J., Benshila, R., Bony, S., Brockmann, P., Cadule, P., Caubel, A., Denvil, S., Dufresne, J.L., Fairhead, L., Filiberti, M.A., Foujols, M.A., Fichefet, T., Friedlingstein, P., Grandpeix, J.Y., Hourdin, F., Krinner, G., Levy, C., Madec, G., Musat, I., de Noblet-Ducoudre, N., Polcher, J. and Talandier, C. 2005. The new IPSL climate system model: IPSL-CM4. Note du Pole de Modelisation n. 26, IPSL, ISSN 1288-1619.

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.

Last updated 15 October 2008