In the past two decades, we’ve seen dramatic images of ice shelves and the floating tongues of glaciers crumble into the ocean. The summer of 2012 saw a huge chunk of ice–two times the size of Manhattan–snap off of Greenland’s Petermann Glacier. Two years earlier, a piece of ice twice as big as that one split from the glacier’s front. In early 2002, ice covering an area the greater than the size of Rhode Island sloughed into the ocean from a lobe of the Antarctic Peninsula’s Larsen Ice Shelf, releasing into the ocean three-quarters of a trillion tons of ice. Seven years before that, the northernmost sector of the same ice sheet completely collapsed and an area of ice roughly the size of Hawaii’s Oahu island dissolved into the sea.

Scientists have long thought that sudden and dramatic ice calving events like these, along with more moderate episodes of calving that occur daily, were the main mechanisms for how polar ice gets lost to the sea. New research, however, shows that calving icebergs is only the tip of the iceberg–seawater bathing the undersides of ice shelves contributes most to ice loss even before calving begins, at least in Antarctica.

The discovery, published in the journal Science, shows that interactions with the ocean underneath floating ice account for 55 percent ice lost from Antarctic ice shelves between 2003 and 2008. The researchers arrived at their findings by studying airborne measurements of ice thicknesses from radar sounders and the rates of change in ice thickness based off of satellite data. Combining these data allowed them to calculate the rates of bottom melting.

Given that thick platforms of floating ice surround nearly 75 percent of Earth’s southernmost continent, covering nearly 580 million square miles, ice melted in this fashion may well be the main contributor to sea level rise. “This has profound implications for our understanding of interactions between Antarctica and climate change.” said lead author Eric Rignot a researcher at UC Irvine and NASA’s Jet Propulsion Laboratory, in a statement. “It basically puts the Southern Ocean up front as the most significant control on the evolution of the polar ice sheet.”

Interestingly, the big ice shelves–Ross, Ronne and Filchner, which cover about 61 of Antarctica’s total ice shelf area–only contribute a small fraction meltwater through their bases. Instead, less than a dozen small ice shelves, particularly those on the Antarctic Peninsula, are responsible for most–nearly 85 percent–of the basal melting observed by the authors during their study period. These shelves not only float in warmer water, relatively, but their small sizes may mean that their interiors are less sheltered from already warmer ocean waters that creep underneath the ice.

The findings reveal a lot about the vulnerability of polar ice in a warming world. Ice sheets ooze through glaciers to the sea, where they interlace and form ice shelves. These shelves are akin to a cork that keeps the contents inside from spewing out–when ice sheets collapse, the glaciers that feed them thin and accelerate, helping to drain the interior ice sheet. Polar ice sheets already are losing at least three times as much ice each year as they were in the 1990s, and the findings released today may give a mechanism for this frantic pace.

In fact, the major ice calving events of the last two decades on the Petermann Glacier and Larsen Ice Shelf may have started with the fact that melting from underneath was weakening the ability of ice to coalesce into a solid mass.

“Ice shelf melt can be compensated by ice flow from the continent,” Rignot added. “But in a number of places around Antarctica, they are melting too fast, and as a consequence, glaciers and the entire continent are changing.”