The vast ice sheets of Greenland and Antarctica are melting faster than previously thought, and that melting is accelerating, according to a new report that verifies 18 years of melting via two independent techniques.

Left unchecked, the extra water dumped into the oceans could push average global sea level 6 inches higher by 2050, the report finds. That would mark the ice sheets as the largest contributors to sea level rise, outstripping melting from Earth’s two other huge, frozen reservoirs, mountain glaciers and polar ice caps.

A new study finds the melting of ice sheets in Antarctica and Greenland is accelerating, outstripping previous estimates of their contribution to sea level rise. (Eric Rignot)

“It’s going to be a concern for people in coastal areas,” said Isabella Velicogna of the California Institute of Technology and NASA’s Jet Propulsion Laboratory, a co-author of the report online at Geophysical Research Letters. “It looks like [the IPCC estimate] will easily be an underestimate of the sea level rise.”

While six inches of additional sea height may sound small, the increase will distribute unevenly across the globe, Velicogna said, and disproportionately impact low-lying countries like Bangladesh.

The study used two techniques to measure the melting of the ice sheets. The most thorough data set, from 1992 through the present, employed satellite radar readings of ice movement, soundings of ice thickness, and other ground-based observations to build a complete picture of the size of the ice sheets from month to month.

The second technique drew on unique twin satellites, together called Grace, which measure minute differences in gravity over the entire Earth. Because the density of the ice sheets differs from the density of surrounding areas, the ice sheets present a distinct gravity signature in Grace’s readings.

The Grace satellites, an acronym for Gravity Recovery and Climate Experiment, were launched by NASA and the German Aerospace Center in 2002.

Eric Rignot, a co-author on the report also from Cal Tech and JPL, said the independent techniques provided very similar figures for the acceleration of the melting. He added that year-to-year variability in snowfall, which can increase ice sheet mass, requires a long-term record to assess melting trends.

“We can’t really pinpoint the start point for the acceleration” in melting, Rignot said. “We looked at 18 years. If we had 30 years [of data], we would have looked at that.”

Combined, the two ice sheets dumped 475 gigatonnes of ice (which then melted) into the ocean each year. (A gigatonne is one billion metric tons.) Averaged over the 18 years of the study, the ice sheets lost a combined 36 gigatonnes more each year than they had the year before.

A 2006 study found that the melting of mountain glaciers and the polar ice caps was also accelerating, but at a rate about three times slower than that of the ice sheets.

With the ice sheets disappearing more rapidly than previously thought, estimating the magnitude of future sea level rise is less important than acknowledging its quickening pace, Velicogna said. “The point is, it’s happening and we can’t deny it. Eventually it’s going to have an impact.”