Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”
Whenever the topic of rising seas comes up, we point out that Antarctica is expected to gain mass through enhanced snowfall in a warmer climate, and therefore its contribution to global sea level rise should be negative—that is, the water locked up in the added snowfall there will act to reduce the level of the globe’s seas. The models used by the Intergovernmental Panel on Climate Change (IPCC) in their 2007 Fourth Assessment Report project the sea level reduction from this mechanism by the end of the 21st century to amount to somewhere between 2 cm and 14 cm (roughly 1 to 6 inches). While this is not a lot, the main point is that Antarctica is not expected to be a contributor to rising seas as the climate warms. Without a large contribution from Antarctica, we will not approach alarmist projections of a meter-plus of sea level rise by century’s end.
Up to now, though, Antarctica has not exactly been with the program.
Instead of gaining mass through increased snowfall, there have been indications that Antarctica is losing ice (contributing to sea level rise) as ice discharge from its coastal glaciers exceeds gains from snow increases (which have been hard to find). One has to wonder whether Antarctica, contrary to expectations, will continue to lose mass and become an important contributor sea level rise, or whether the projected increases in snowfall have just not yet reached a magnitude sufficient to offset the loss from glacial discharge.
Things are starting to change down there.
The research that has gotten the most attention on the subject of Antarctic mass balance has been based on observations made by the Gravity Recovery And Climate Experiment (GRACE) satellite. This orbiter senses changes in gravity (i.e., mass) which can be caused by increasing snow and ice loads over the continent. One key piece of information which must be factored into the calculations of ice mass change is the change in the underlying geologic formations, which are still rebounding from enormous amounts of ice lost after the end of the last ice age. This geologic motion, known as the glacial isostatic adjustment (GIA), is largely modeled rather than directly observed. Our level of knowledge (or lack thereof) of the true GIA adds a sizable amount of uncertainty to GRACE-based estimates of the ice mass changes over time in Antarctica (and Greenland, the northern hemisphere’s cheap imitation of Antarctica).
In a widely cited finding, Velicogna (2009) reported that Antarctica was losing ice at a rate of about 104 gigatons per year (Gt/yr) during the period 2002–2006, increasing to a loss rate of 246 Gt/yr during 2006–2009 (about 374 Gt of ice are equivalent to 1 mm of sea level). Rignot et al. (2011) also found an acceleration of ice loss there, increasing from a loss of about 209 Gt/yr (in 2003-2007) to about 265 Gt/yr from 2007 to 2010. However, Wu et al. (2010) argued that the GIA model used in these previous studies is incorrect, and that when a more accurate GIA model is incorporated in the GRACE-based ice mass change calculations, Antarctica was only losing about 87 Gt/yr during the period 2002–2008.
Support for the GRACE-based calculations comes from the general agreement between the GRACE numbers and those calculated from studies of changes in the grounding lines of coastal glaciers and the ice flow across those grounding lines in association with the other aspects of the mass balance. This method is known as the Input-minus-Output Method (IOM). The IOM estimates of the average ice loss from Antarctica over the past several decades (1992–2007) lie somewhere around 136 Gt/yr, in rough agreement with the GRACE-based estimates. However, the IOM is also subject to a lot of uncertainty. An attempt by Zwally and Giovinetto (2011) to reduce the uncertainty and increase the accuracy resulted in an IOM-based estimate of a loss of only 13 Gt/yr over the same 18-yr period and led the researchers to conclude that:
Although recent reports of large and increasing rates of mass loss with time from GRACE-based studies cite agreement with IOM results, our evaluation does not support that conclusion.
It seems that as the calculations and derivations are improved, the amount of ice mass that Antarctica is supposedly losing gets less and less.
Or perhaps it isn’t losing any mass.
Using a set of observations from a series of satellites that have been in orbit since 1992 and that measure changes in the height of the surface of the ice (ICESat), NASA’s Jay Zwally and colleagues (2012) report that Antarctica is gaining mass. Zwally recently presented his findings to a workshop of the Ice-Sheet Mass Balance and Sea Level expert group of the Scientific Committee on Antarctic Research and the International Arctic Science Committee. According to his abstract, Zwally reported that “During 2003 to 2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 Gt/yr (2.5% of input), as derived from ICESat laser measurements of elevation change.”
Zwally further added, "A slow increase in snowfall with climate warming, consistent with model predictions, may be offsetting increased dynamic losses."
So the "global warming, leading to increased snowfall, leading to a drawdown of global sea level" mechanism may be operating after all.
A paper to soon appear in Geophysical Research Letters give us another enticing look at recent snowfall changes in Antarctica. In “Snowfall driven mass change on the East Antarctic ice sheet,” Carmen Boening and colleagues from NASA’s Jet Propulsion Laboratory report that extreme precipitation (snowfall) events in recent years (beginning in 2009) have led to a dramatic gain in the ice mass in the coastal portions of East Antarctica amounting to about 350 Gt in total (Figure 1).
Figure 1. Timeseries of snow accumulation in coastal East Antarctica (shaded region in inset).
(Source: Boening et al., 2012)Boening et al. reported that the increase in ice mass in East Antarctica has not completely offset the loss of ice mass during the same time in West Antarctica, but as this comparison is made using GRACE data, it is hard to know just how accurate it is.
Also note that a few years with a lot of snowfall does not mean that a change in the long-term snowfall rate has occurred. Nevertheless, the situation bears careful watching.
Putting everything together, we conclude that many of the claims that Antarctica is rapidly losing ice and increasingly contributing to a rise in global sea levels must now be, at the very least, tempered, if not overturned entirely. Time will certainly tell. And time will also tell just how much we need to worry about future sea level rise. Currently, the answer seems to be “not overly much.”
References:
Boening, C. et al., 2012. Snowfall-drive mass change on the East Antarctic ice sheet. Geophysical Research Letters, in press, DOI:10.1029/2012GL053316.
Rignot, E., et al., 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, L05503, DOI:10.1029/2011GL046583
Velicogna, I., 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, 36, L19503, DOI: 10.1029/2009GL040222.
Wu, X., et al., 2010. Simultaneous estimation of global present-day water transport and glacial isostatic adjustment. Nature Geoscience, 3, DOI: 10.1038/NGEO938.
Zwally, H.J., and M.B. Giovinetto, 2011. Overview and assessment of Antarctic ice-sheet mass balance estimates: 1992-2009. Surveys in Geophysics, 32, 351-376, DOI: 10.1007/s10712-011-9123-5.
Zwally, H.J., et al., 2012. Mass gains of the Antarctic ice sheet exceed losses. Presentation to the SCAR ISMAA Workshop, July 14, 2012, Portland Oregon.