Ocean Water Melting Antarctic Ice From Bottom Up

In the first comprehensive study of all Antarctic ice shelves - thick, floating platforms of ice that act as extensions of glaciers on land -- researchers have discovered that ocean water is melting the ice shelves from the bottom up.

Antarctic researchers have long known that basal melt - the melting ice at the boundary where ice meets sea - occurred, but the generally accepted explanation for ice shelf mass loss was that the shelves were shedding icebergs. Known as calving, chunks of ice will break off the seaward front of the ice shelf. An ice shelf can extend for decades between major calving events.

The new study turns the conventional belief on its head, saying that the majority of mass lost on ice shelves came not from the top down, but from the bottom up.

The study found that basal melt accounted for 55 percent of all Antarctic ice shelf mass loss from 2003 to 2008, an amount much higher than previously thought.

"The traditional view on Antarctic mass loss is it is almost entirely controlled by iceberg calving," said Eric Rignot, a glacier expert with a dual appointment at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the University of California, Irvine.

"Our study shows melting from below by the ocean waters is larger, and this should change our perspective on the evolution of the ice sheet in a warming climate."

In total, Antarctic ice shelves lost 2,921 trillion pounds (1,325 trillion kilograms) of ice per year in 2003 to 2008 through basal melt, while iceberg formation through calving accounted for 2,400 trillion pounds (1,089 trillion kilograms) of mass loss each year.

Another surprising find by the researcher is that the largest glaciers -- four of which account for 61 percent of Antarctic ice shelf cover -- only accounted for 15 percent of the meltwater.

Rignot said that if the ocean melts the ice shelves, then it will also affect ice sheets on land.

"Ice shelf melt doesn't necessarily mean an ice shelf is decaying; it can be compensated by the ice flow from the continent," Rignot said in a statement. "But in a number of places around Antarctica, ice shelves are melting too fast, and a consequence of that is glaciers and the entire continent are changing as well."

Rignot and his colleagues' study is published in the journal Science.