Thermal expansion raises sea levels because the ocean, like all water, expands as it warms up and thus takes up more space. Warming-driven expansion is responsible for approximately half of the sea level rise in the past hundred years. In addition, around the globe, large amounts of land-based water, especially groundwater (such as is found in underground aquifers), is pumped out for farming and drinking. Because more groundwater is extracted than returns to the ground, that water also ends up in the world’s oceans, which contributes to sea level rise.

Melting mountain glaciers also contribute to sea level rise, because frozen water that was trapped on land flows to the sea. Globally, some 90% of glaciers are shrinking in size. The previously landlocked ice ends up in the oceans, which boosts sea level rise. The cumulative volume of global glaciers began to decrease sharply in the mid-1990s. This coincides with a more than doubling of the rate of sea level rise.

Greenland and Antarctica are both covered with two enormous ice sheets. The Greenland ice sheet is nearly 2 miles (3 kilometers) thick at its thickest point and extends over an area almost as large as Mexico. If it completely melts, Greenland, by itself, would raise sea levels more than 20 feet. In 2012, a team of international experts backed by NASA and the European Space Agency put together data from satellites and aircraft to produce “the most comprehensive and accurate assessment to date of ice sheet losses in Greenland and Australia.” They found that the Greenland ice sheet saw “nearly a five-fold increase” in its melt rate between the mid-1990s and 2011. The year 2012 in particular saw unusually high spring and summer temperatures in Greenland. NASA reported that year, “According to satellite data, an estimated 97% of the ice sheet surface thawed at some point in mid-July.” Scientists told ABC News they had never seen anything like this before. In the summer of 2012, the Jakobshavn Glacier, Greenland’s largest, moved ice from land into the ocean at “more than 10.5 miles (17 kilometers) per year, or more than 150 feet (46 meters) per day,” another study found. The researchers pointed out, “These appear to be the fastest flow rates recorded for any glacier or ice stream in Greenland or Antarctica.” By 2014, researchers were able to map Greenland’s ice sheets using the European Space Agency satellite CryoSat-2, which can measure the changing height of an ice sheet over time. They found that since 2009, Greenland had doubled its annual rate of ice loss, to some 375 cubic kilometers per year.

The Antarctic ice sheet is vastly larger than Greenland— bigger than either the United States or Europe—and its average thickness is 1.2 miles (2 kilometers). The Antarctic ice sheet contains some 90% of all the Earth’s ice. It would raise sea levels 200 feet if it completely melts. The West Antarctic ice sheet (WAIS) in particular has long been considered unstable because most of the ice sheet is grounded far below sea level—on bedrock as deep as 1.2 miles (two kilometers) underwater. The WAIS is melting from underneath. As it warms, the WAIS outlet glaciers become more unstable. In the future, rising sea levels themselves may lift the ice, thereby letting more warm water underneath it, which would lead to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, in an ongoing vicious cycle. A 2012 study found that Antarctica’s rate of ice loss rose 50% in the decade of the 2000s. In 2014, researchers looked at measurements by the European Space Agency’s CryoSat-2 satellite “to develop the first comprehensive assessment of Antarctic ice sheet elevation change.” They concluded: “Three years of observations show that the Antarctic ice sheet is now losing 159 billion tonnes of ice each year—twice as much as when it was last surveyed.” Two major studies from 2014 found that some WAIS glaciers have begun the process of irreversible collapse. One of the authors explains, “The fact that the retreat is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the oating sections of the glaciers.”

In late 2014, researchers reported the results of a comprehensive, 21-year analysis of the fastest-melting region of Antarctica, the Amundsen Sea Embayment. This region is approximately the size of Texas, and its glaciers are “the most significant Antarctic contributors to sea level rise.” During those two decades, the total amount of ice loss “averaged 83 gigatons per year (91.5 billion U.S. tons).” This is equivalent to losing a Mount Everest’s worth of ice (by weight) every 2 years. Coauthor Isabella Velicogna said, “The mass loss of these glaciers is increasing at an amazing rate.”