October 11, 2006

Antarctic Ice Sheet (and the Plot) Thickens

Filed under: Antarctic, Glaciers/Sea Ice, Polar

Glaciers the size of Rhode Island breaking off and floating away…grass growing along the periphery of Antarctica…coastal inundation from rising sea levels…even a forecast that Europe will freeze solid the “day after tomorrow.” These are a few of the doomsday portraits painted by a politicized few that are clamoring to have a full-throated voice in the great global climate change debate.

Evidence that lends support to some of the theories behind these supposedly dire portraits can be found throughout the scientific literature, as is the case with many of the issues in the debate about the warming of the global atmosphere. In reference to the scenes described above, many studies have presented measured and modeled data that suggest that the greatest amount of atmospheric warming is occurring or will occur across Earth’s polar regions. However, one can just as easily search the literature and find conflicting evidence.

In a recent issue of Science, Andrew Monaghan of Ohio State University’s Byrd Polar Research Center collaborated with fifteen colleagues from around the world on a piece describing their assemblage of a fifty-year time series of snowfall accumulation over Antarctica. To produce the time series the scientists combined climate model simulations, snowfall observations derived from a multitude of new ice core records, and snow pit and snow stake data. Antarctic ice sheets are of interest to climate change scientists because recent studies suggest a strong winter warming signal in the lower atmosphere over Antarctica since the early 1970s. An interesting twist to this is that future scenarios from global climate change models suggest that Antarctic snowfall should actually increase in a warming climate, as the warmer atmosphere comes with a greater capacity for moisture. Intuitively, increased snow accumulation supports ice sheet thickening.

However, Monaghan et al. note that there is evidence of conflicting trends in ice sheet thickness across Antarctica: the West Antarctic Ice Sheet has been thinning over the past decade, while the East Antarctic Ice Sheet became thicker over the period 1992 through 2003 (Davis et al. 2005). Previous work attributed the thickening of the East Ice Sheet to an increase in snowfall accumulation across that portion of the continent, following the logic of a warmer atmosphere and therefore greater moisture capacity. The thinning of the West Ice Sheet, however, is not well explained. As it turns out, Monaghan and his colleagues do not think that the thickening of the East Ice Sheet is well explained either!

The group of researchers reports that across both the East and West Ice Sheets “there are no statistically significant trends in snowfall accumulation over the past 5 decades, including recent years for which global mean temperatures have been warmest.” Clearly stated by Monaghan et al. in the article is that they do not dispute a clear thickening signal in the East Ice Sheet. However, based on their findings they do disagree with the idea that increased snowfall accumulation is the cause. Following the theory of increased snowfall with atmospheric warming, the conclusions of Monaghan and colleagues raises questions about the recent atmospheric warming over the East Antarctic Ice Sheet; and/or, the work raises questions about the validity of the relationship between air temperature and snowfall over Antarctica within general circulation models. Monaghan et al. go on to suggest that “atmospheric circulation variability, rather than thermodynamic moisture increases, may dominate recent Antarctic snowfall variability.”

To further thicken the plot, not to mention the mass of Antarctica, a recent article by Van de Berg et al. in the Journal of Geophysical Research describes a reassessment of the surface mass balance of Antarctica. The group compared results of the model-simulated surface mass balance for the continent to all available mass balance observations in a recalibration process that allowed them to construct a “best estimate” of contemporary Antarctic mass balance. What was found by the work is striking: the newly estimated surface mass balance “exceeds previous estimates by as much as 15%.” The largest differences between the findings of Van de Berg et al. and those of previous studies were up to one meter per year greater in the coastal zones of East and West Antarctica, which are “without exception in areas with few observations.” The coastal zones of the continent are areas most susceptible to melting and calving, or breaking apart, of ice (Monaghan et al. 2006).

The group of Dutch researchers contends that the new estimate of the mass balance of Antarctica is as reliable as the reliability given to current atmospheric models. The only improvement that could be offered would have to come from new surface mass balance observations from poorly covered high accumulation regions in coastal Antarctica. Until then, there is little evidence to disprove their conclusion that the mass of Antarctica’s grounded ice sheet steadily grew from 1980 to 2004.

The doomsday portraits of Antarctica’s glaciers reacting to a global climate change should be blurry at best. Consensus on changes in ice sheet thickness and their causes is difficult, and therefore of limited use on either side of the global warming debate. As Monaghan et al. (2006) state, “Vigorous efforts are needed to better understand this remote but important part of the planet and its role in global climate and sea level rise.”

References

Davis, C.H., Li, Y. McConnell, J.R., Frey, M.M. and Hanna, E. 2005. Snowfall-driven
growth in East Antarctic Ice Sheet mitigates recent sea-level rise. Science, 308, 1898-1901.

Monaghan, A.J., Bromwich, D.H., Fogt, R.L., Wang, S.-H., Mayeweski, P.A., Dixon,
D.A., Ekaykin, A., Frezzotti, M., Goodwin, I., Isaksson, E., Kaspari, S.D., Morgan, V.I., Oerter, H., Van Ommen, T.D., Van der Veen, C.J., and Wen, J. 2006. Insignificant change in Antarctic snowfall since the International Geophysical Year. Science, 313, 827-831.

Van de Berg, W.J., van den Broeke, M.R., Reijmer, C.H., and van Meijgaard, E. 2006.
Reassessment of the Antarctic surface mass balance using calibrated output of a regional amtospheric climate model. Journal of Geophysical Research, 111, 10.1029/2005JD006495.




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