V. Antarctica Ice Mass: Growing into the Future

Both models and observations indicate that the mass of Antarctica’s ice sheets should continue to increase into the future as warming temperatures lead to enhanced atmospheric moisture and increased snowfall. The net result is a negative contribution to future global sea level.

It has been long held that a warming climate in the regions surrounding Antarctica would enhance snowfall there, lead to snow and ice accumulation, and ultimately result in a negative contribution to sea level rise (in other words, continued snow and ice build up on the Antarctic continent would result in a lowering of sea levels). This idea has been reflected in each and every report of the Intergovernmental Panel on Climate Change (IPCC). For instance, in the IPCC’s First Assessment Report, published in 1995, they summarize the expectations from Antarctica in a warming climate as “On the whole, the sensitivity of Antarctica to climate change is such that a future warming should lead to increased accumulation and thus a negative contribution to sea level change.” In the IPCC’s Fourth Assessment Report, released in 2007, 12 years after their initial report, they state that “General Circulation Models indicate that the Antarctic Ice Sheet will receive increased snowfall without experiencing substantial surface melting, thus gaining mass and contributing negatively to sea level.” In fact, under every IPCC future emissions scenario, the expected Antarctic contribution to sea level rise in the 21^st century is negative.

However, with the March 2006 Science article by researchers Isabella Velicogna and John Wahl, this idea was (purportedly) turned on its head.

The [IPCC] report predicted that the Antarctic ice sheet will probably gain mass during the 21^st century because of increased precipitation in a warming global climate. Recent radar altimeter measurements have shown an increase in the overall thickness of the East Antarctic Ice Sheet’s (EAIS’s) interior during 1992–2003. However, the IPCC prediction does not consider possible dynamic changes in coastal regions, and radar altimetry provides only sparse coverage of those areas. Detailed interferometric synthetic-aperture radar and airborne laser altimeter surveys of glaciers along the edge of the West Antarctic Ice Sheet (WAIS) show rapid increases in near-coastal discharge during the past few years. The overall contribution of the Antarctic ice sheet to global sea-level change thus depends on the balance between mass changes in the interior and those in coastal areas. The gravitational survey of Antarctica provided by the Gravity Recovery and Climate Experiment (GRACE) satellites and discussed in this paper is a comprehensive survey of the entire ice sheet and is thus able to overcome the issue of limited sampling….

Using measurements of time-variable gravity from the [GRACE] satellites, we determined mass variations of the Antarctic ice sheet during 2002–2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 ± 80 cubic kilometers of ice per year, which is equivalent to 0.4 ± 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.

This result, reported by Velicogna and Wahl, indicated that instead of gaining ice, Antarctic was instead rapidly losing ice and contributing to sea level rise. However, while big attention was being paid to its suggested implications (a future warming would lead to a sea level rise much greater than expected), little attention was being paid to the fact that the study only covered a period of three years (2002-2005)—a period far too short to reliably determine long-term tendency or behavior of the balance of Antarctica’s mass of snow and ice.

In fact, longer-term studies that better illustrate the trend and variability of Antarctica’s amount of snow and ice, find overall increases in ice mass as well as multi-year variations in ice accumulation that could explain Velicogna and Wahl’s results.

For instance, over the short term, variations in snowfall amounts from year-to-year can explain the reported mass loss from 2002-2005. Andrew Monaghan and colleagues, made this point abundantly clear in a paper they published in Science less than six months after the Velicogna and Wahl study was published. Monaghan et al. investigated the behavior of snowfall over the Antarctic continent and examined both its long-term and short-term variability. They found that a short-term decrease in snowfall during the time period examined by Velicogna and Wahl could explain their results. Monaghan et al. wrote:

Interannual and interdecadal snowfall variability must be more seriously considered when assessing the rapid ice volume changes that are occurring over Antarctica. With regard to interannual variability, consider a recent estimate of Antarctic ice sheet mass loss that is the equivalent of 0.4 ± 0.2 mm year^-1 GSL [global sea level rise] rise for 3 years (2002–2005) from satellite-derived time-variable gravity measurements [made by Velicogna and Wahl]. Antarctic-wide annual snowfall accumulation decreased by about 25 mm y^-1 WEQ [water equivalent], or about 0.86 mm year^-1 GSL rise, between calendar year 2002 and 2003, suggesting that the gravity fluctuations could be heavily influenced by interannual snowfall variations.

Over the longer term, recent papers continue to show overall snow and ice mass gain in Antarctica. Davis et al., 2005 reported that ice mass gain over the East Antarctic Ice Sheet likely exceeded ice mass loss from the West Antarctic Ice Sheet, indicating a net gain in mass over the entire Antarctic continent. Wingham et al. reported an overall increase of 27 ± 29 Gt per year during the same time period. Wingham et al. explained “Mass gains from accumulating snow, particularly on the Antarctic Peninsula and within East Antarctica, exceed the ice dynamic mass loss from West Antarctica.”

Elevation change in the Antarctic Ice Sheet, 1992-2003, as derived by Wingham et al., 2006.