Sea level rise unevenly, which is bad news for some coastal regions

These weight-filled dents in the mantle do not make a permanent scar. When the extra weight lifts, the mantle rebounds. This does not just happen at the majestic pace of mountain ranges crumbling. It happens every day.

“The solid earth can respond very quickly — nearly instantaneously,” said Mark Tamisiea, a scientist at the National Oceanography Center, Liverpool, England, who studies the connection between sea levels and Earth processes. Tamisiea cited the example of solid-Earth tides, which pull the crust outward as much as a foot (thirty centimeters) toward the moon as it passes overhead. Similarly, Earth has an instant initial response to glaciers and ice sheets melting, called the elastic response.

Since NASA launched the Gravity Recovery and Climate Experiment (GRACE) twin satellites in 2002, scientists have had an extremely precise measurement of the contribution that ice sheets’ loss of mass contributes to changes in gravity and what it is adding to sea level rise. “Because of GRACE, we’ve had a pretty good idea of what’s happening since 2002,” said Steve Nerem of the University of Colorado, head of NASA’s Sea Level Change Team. “We know how much [of sea level rise] is from Greenland, how much is from Antarctica, how much is from glaciers.”

Because every ice sheet and glacier has a unique location and size, each one creates a pattern of response in the ocean as individual as a fingerprint. “The physics behind understanding these fingerprints is very well understood,” Tamisiea said. “It’s like the tides.” He and Jerry Mitrovica of Harvard University have calculated the fingerprints of East and West Antarctica and Greenland around the globe. “We do each ice sheet individually so we can use the latest GRACE analysis,” Tamisiea explained. “You can sort of add the effects up and see what the result is for any given location.”

As any ice sheet melts, sea levels along coastlines as much as 1,500 miles (2,000 kilometers) away will fall as seawater escapes from the reduced gravitational pull and the crust lifts. The escaping seawater flows clear across the equator: the melting of Antarctica affects the U.S. East and West coasts, and Greenland’s disappearance impacts the coastline of Brazil. These regional differences are significant — such as in the case of the East Coast of the United States.

The East Coast is also on the losing end of another important solid-Earth process that affects regional sea levels: post-glacial rebound. After the elastic response to a crustal weight loss, uplift continues more slowly for many millennia. North America is still responding to the massive melt-off at the end of the last ice age 6,000 years ago. The North American tectonic plate was not evenly loaded during that ice age: ice sheets were sitting on what is now Canada and Greenland, while most of today’s United States remained ice free. This ice load pushed the mantle out from under Canada and buoyed up the United States. Today, the U.S. side of the North American plate is sinking like the downhill end of a seesaw as the northern side continues to lift.

Greenland’s uplift from postglacial rebound means the island is gaining mass from below and its bedrock is continuously rising. At the same time, it is losing mass from above as its ice melts.

NASA notes that GRACE measures the net result of these opposing processes, not just the result of melting ice alone. A National Science Foundation- and NASA-funded program called the Greenland GPS Network is working to overcome this problem. Led by Michael Bevis of Ohio State University, Columbus, the program is using more than fifty GPS stations in Greenland to measure Greenland’s rise and fall. The network is dense enough, and the instruments record elevation precisely enough, to distinguish the steady, long-term rise caused by postglacial rebound from shorter-term changes in elevation caused by the weight of the winter snows and loss of weight in summer. The goal of the project is to provide a “correction factor” for postglacial rebound that can be applied to measurements by GRACE and succeeding missions so the remainder is an accurate measurement of the loss of mass from melting.

Scientists currently believe ice sheet fingerprints will be the major driver of future regional variations in sea levels. They are working on questions of how these solid-Earth processes interact with other global and local drivers of sea level rise. “We have to understand global and larger-scale regional changes to do localized impact studies,” Tamisiea explained. “In some places, it may very well be that regional processes will be the most important signal. There has to be a continuum of understanding of the global average, regional changes and more localized processes. We’ll need all of those layers to make viable predictions.”

The study of sea level rise may help in predicting flooding and emergencies, but currently there is no way to stop the oceans from rising at least three feet in the future.

“Given what we know now about how the ocean expands as it warms and how ice sheets and glaciers are adding water to the seas, it’s pretty certain we are locked into at least three feet of sea level rise, and probably more,” said Nerem told Motherboard.

“But we don’t know whether it will happen within a century or somewhat longer,” he said.