Sea Level Projections

Thanks to the heat-trapping effects of greenhouses gases, climate scientists project that if emissions continue to grow unabated — and there’s currently no reason to expect that they won’t — the temperature will likely rise by between three and seven more degrees Fahrenheit by 2100. These numbers come from global climate models, which are sophisticated computer simulations of the Earth’s climate that respond, roughly like the Earth will, to changes in the concentrations of greenhouse gases in the atmosphere.

Rising temperatures mean rising seas, as seawater expands and land-based ice continues to melt. But ice also drops into the ocean directly, in the form of icebergs, as glaciers flow into the sea. This raises sea level too, just as dropping ice cubes into a drinking glass raises its level of water. Until recently, scientists didn’t have a good handle on how glacier flow might change in a warming word. The major 2007 report from the IPCC underscored that uncertainty: the report projected a sea level rise of between 7 inches and 2 feet by the end of the century — but added a note saying the projections “[excluded] future rapid dynamical changes in ice flow.” In short, the projections were incomplete, and had to be taken with a grain of salt.

Since then, scientists have observed an acceleration in ice flow, mostly in Greenland. They’ve also learned more about how ice responded to warming episodes in the distant past. With this new information, they now agree that rather than rising between 7 inches and 2 feet, the seas are likely to rise between two and seven feet by century’s end, with about three feet being the most popular estimate.

Local increases, however, will vary to some degree from the global change.

Next section: Local Sea Level