Expert Commentary

A reconciled estimate of ice-sheet mass balance

2012 study in Science on the true nature and extent of melting of the Antarctic and Greenland ice masses.

The melting of land-based ice masses as a consequence of climate change has significant implications for sea level rise. The shrinking of the ice sheets in the Earth’s polar regions has been the subject of intense study for many years now, but scientists have been sharpening the satellite tools and techniques they use to analyze the true mass of ice and its rate of decline.

A 2012 study from an international research team — 47 scientists from 26 research labs, including NASA — published in the journal Science, “A Reconciled Estimate of Ice-Sheet Mass Balance,” analyzes vast datasets compiled from three different satellite imaging methods and data-gathering techniques to produce a comprehensive picture of ice-sheet mass balance. (The accompanying NASA Web feature and Science news analysis article help explain the data’s significance.)

The data relate to the period between 1992 and 2011 and focus on the land-based ice masses in Greenland and West Antarctica. Though the satellite radar methods, for example, have been around for decades, data derived from the satellite use of lasers is only available for more recent years. However, by looking at the period between 2003 and 2008 — when all the satellite techniques were operating — the researchers were able to ensure rigorous comparisons of data methods and to quantify the strengths and weaknesses of each technique. The researchers state that their findings are twice as accurate as those produced by previous studies, and they bring much greater precision to estimates included in the landmark 2007 U.N. Intergovernmental Panel on Climate Change report.

The study’s findings include:

  • Between 1992 and 2011, the melting of the ice sheets in Antarctica and Greenland produced an estimated 0.44 inches, or 11.1 millimeters, of global sea level rise. This accounts for one-fifth of all sea level rise over the past two decades. On average, this means the melting of these ice sheets has contributed about 0.6 millimeters a year to sea level rise (the observed rate now is about 3 millimeters per year globally.)
  • The rate of loss of ice mass in these regions is now three times as fast as it was in the 1990s.
  • The ice mass in Greenland, in particular, shows worrisome patterns: Its melting contributed two-thirds of the sea level rise attributed to these regions; and the rate of its decline approaches five times the rate evidenced in the 1990s.
  • Currently about 344 billion tons of glacial ice are being lost per year, with Greenland’s melting making up about 263 billion tons (76%) of that figure.
  • There have been some snowfall-driven increases in the ice sheets of East Antarctica, which have reduced the overall Antarctic ice losses, but the scientific record is “too short to determine whether this is a long-term trend.”

Other contributors to rising ocean levels include the expansion of seawater due to warming seas and the decline of other land-based glaciers. The study provides the best picture yet of the ice sheets and their decline, and reconciles data from numerous past studies. But the future of these ice sheets is of even greater importance: Scientists estimate that completely melting the polar ice sheets would cause about 60 meters of sea level rise. At the current annual rate of sea rise, 3 mm, such dramatic effects would take millennia, but scientists fear the rate could be “non-linear” and may rise substantially in the coming decades.

Tags: global warming

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