But even that worst-case scenario may not capture the risk, Rignot said. That's because the IPCC models only take into account temperature changes at the surface of glaciers, but not the rapid melting that occurs when glaciers calve and break up into the ocean, Rignot said.
In addition, much of glaciers' melting occurs at deep, undersea ice canyons. Warmer water is salter and therefore heavier. That means it sinks into the deeper layers of the ocean, and the contrast between this warm water and the undersea ice canyons contributes an unknown but substantial amount of sea level rise, said Josh Willis, an oceanographer at JPL in Pasadena, California.
When the massive Jakobshavn glacier in Greenland calved earlier this summer, it moved 4.6 square miles (12 square kilometers) of ice in one day, Rignot said. [See Photos of Greenland's Melting Glaciers]
If the Jakobshavn glacier had melted completely, "it contains enough ice to raise global sea level by half a meter — just this one glacier in Greenland," Rignot said. If all the land ice on the planet were to melt, it would raise sea levels about 197 feet (60 m), he added.