December 5, 2006

Sea Level Rise? - Not From Antarctic Melting

Filed under: Antarctic, Polar, Sea Level Rise

Earth’s polar regions have been loudly touted as evidencing the greatest response to global warming during the last two decades of the 20th century. Likewise, global climate models forecast that the high latitudes will experience the greatest change in temperature should greenhouse gas emissions increase through the 21st century. The warming supposedly has and will cause large-scale glacial melt and an input of fresh water that will produce global sea level rise and a breakdown of the fundamental worldwide oceanic circulation. These aspects of potential climate change serve as great talking points for alarmists, as they portray inundated coastal areas and a Europe subject to Arctic-like winters as in the movie Day After Tomorrow.

The scientific literature regarding future polar warming is rather one-sided, as theory seems to strongly suggest that significant warming will occur. However, confidence in the theory and its implementation within global climate models is less than might be suggested based on the recent history of Earth’s climate. The literature is now riddled with contradictory evidence, while there are studies that portray negative impacts of polar warming over the past two decades, there are also (less publicized) articles that show little evidence (especially over the South Polar regions). One such piece recently appeared in the British journal Philosophical Transactions of the Royal Society A. The work, authored by Duncan Wingham of the Centre for Polar Observation and Modeling of University College London and colleagues, analyzed recent changes in volume of the Antarctic ice sheet.

Wingham et al. set out to determine what fraction of the reported mean global sea level rise of 1.88 mm per yr over the 20th century could be attributed to the melting of the Antarctic ice sheet. As remote satellites offer the only tool with which changes in the volume of the ice sheet can be estimated, the group analyzed 1.2 x 10^8 European remote sensing satellite altimeter echoes to determine volume change over the period 1992 to 2003. Spatially, the survey covers 85% of the East Antarctic ice sheet and 51% of the West Antarctic ice sheet. In total, the area represents 72% of the grounded ice sheet, omitting only 6% of coastal areas where data are lost due to slope steepness and 22% of interior Antarctica that is outside the latitudinal limit of the satellite ground track.

The authors fitted a trend and annual cycle to the ice sheet elevation time series for 100 km^2 bins and averaged the time series over individual ice drainage basins and for the entire region of coverage. When corrected for isostatic rebound, their data “show the ice sheet growing at 5 ± 1 mm year-1”. The authors explain that to determine the change in mass the density at which the volume changes have occurred must be known. Using limited data to produce best estimates of differences in the density across the Antarctic ice sheet, Wingham et al. show that “72% of the Antarctic ice sheet is gaining 27 ± 29 gigatons per year.” To put this into perspective as it relates to global sea level, the authors indicate that this gain in mass is enough “to lower global sea levels by 0.08 mm year-1.” These results are very similar to the findings reported last year in Science magazine by Curt Davis and colleagues, who also used satellite altimetry measurements of Antarctica to calculate mass changes and estimated that over a similar period and found that snow and ice build-up of the Antarctic ice sheets contributed to a fall in sea level of about 0.12 mm per year (see here for our coverage of the Davis et al. results).


Figure 1. Elevation change of the Antarctic ice sheet, 1992-2003
(taken from Wingham et al., 2006).

The authors reference the third assessment of the Intergovernmental Panel on Climate Change (IPCC) (Church and Gregory, 2001) which provided a partial offset to the global sea level rise due to Antarctic melting since the last glacial maximum with a 20th century fall due to increased snowfall. The authors note, however, that the assessment relied on models that include “neither ice streams nor the Peninsula warming, and the data show that both have dominated at least the late twentieth century ice sheet.” Still, the authors reiterate that even in a worst case scenario “Antarctica has provided, at most, a negligible component of observed sea-level rise.” This, say the authors, places a greater burden on accounting for the reported 20th century sea level rise of 1.5-2.0 mm per year.

So, don’t sell the beachfront property just yet - the jury seems as if it will be out a tad bit longer on global sea level rise since the precise cause of a rise is questionable. Testimonials by climate change alarmists that coastal regions of the world will be inundated as glacial melt water streams into the world’s oceans have just been met with evidence that a large portion of the Antarctic ice sheet has thickened over the last of the 20th century. Evidence is scant that this observed sea level lowering from Anarctica won’t continue long into the future.

References:

Church, J.A. and J.M. Gregory. 2001: Changes in sea level, Intergovernmental Panel on Climate Change, pp. 641–693.,UK and New York, NY: Cambridge University Press.

Davis, C. H., et al., 2005. Snowfall-driven growth in East Antarctic ice sheet mitigates recent sea-level rise. Science, 308, 1898-1901.

Wingham, D.J., A. Shepherd, A. Muir, and G.J. Marshall. 2006: Mass balance of the
Antarctic ice sheet. Philosophical Transactions of the Royal Society A, 364, 1627-1635.




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