The ground there is lifting upwards, in a slow-motion rebound that has been going on for 10,000 years, since the glacial ice sheet that once weighed down the continent receded at the end of the last ice age. Gravitational influences on the oceans are also at work: as local glaciers recede and the Greenland ice sheet melts, their gravitational pull is subtly reduced, allowing more ocean water to slop southwards.
Trends in local sea level can differ strongly from the global average, which is increasing by around 3.2 millimeters per year. “Some places, sea-level rise is ten times faster than the average,” says Jerry Mitrovica, a geophysicist at Harvard University in Cambridge, Massachusetts.
One side of this equation is the movement of the land. Canada's Hudson Bay, for example, was once buried under more than 3 kilometers of ice, and the release from that load is now causing the land to rise at about 1 centimeter per year. As that part of North America moves upwards, land to the south is being levered down: the US east coast is dropping by millimeters per year.
Subsidence can cause some areas to sink much faster. Compaction of river sediments and hollowing out of the earth by groundwater extraction, for example, are causing parts of China's Yellow River delta to sink at up to 25 centimeters per year.
Adding to the complexity, the oceans do not rise evenly all over the world as water is poured in. Air pressure, winds and currents can shove water in a given ocean to one side: since 1950, for example, a 1,000-kilometer stretch of the US Atlantic coast north of Cape Hatteras in North Carolina has seen the sea rise at 3–4 times the global average rate. In large part, this is because the Gulf Stream and the North Atlantic current, which normally push waters away from that coast, have been weakening, allowing water to slop back onto US shores.
Finally, waters near big chunks of land and ice are literally pulled up onto shores by gravity. As ice sheets melt, the gravitational field weakens and alters the sea level. If Greenland melted enough to raise global seas by an average of 1 meter, for example, the gravitational effect would lower water levels near Greenland by 2.5 meters and raise them by as much as 1.3 meters far away.
Scientists and engineers are only just starting to wrangle all these effects into local projections. In June, the New York City Panel on Climate Change updated its estimates of sea-level rise by including the local effects of gravitational shifts. Panel members concluded that they expect to see 30–60 centimeters of rise by 2050. Finding and combining the right data sets took about six months; the exercise should pave the way for other cities to do the same, says Cynthia Rosenzweig, a climate-impact researcher at NASA's Goddard Institute for Space Studies in New York City. “We really are working to get the best science.”
Aimée Slangen, now a member of Church's group in Australia, last year put out one of the first global maps of regional sea-level change that takes all these factors into account, but it had only rough resolution, with pixels of more than 100 kilometres each. Researchers want to provide city-level predictions, but are hampered because these depend heavily on decadal shifts in winds and ocean currents. Predicting such changes is “very problematic”, says Chambers.
Regional figures for sea-level rise are of interest not only to people trying to plan for local impacts, but also to those trying to model global effects. For the latter, the numbers bring some good news. Gravitational shifts caused by melting ice in the Antarctic should actually help to prevent catastrophic collapse of the West Antarctic ice sheet: as Antarctica loses some of its ice, local sea levels will fall, which will cause some floating edges of the ice sheet to come to rest on the sea floor. Firmly grounded ice is less susceptible to runaway melting than floating ice. “That's going to stabilize the ice sheet,” says Mitrovica.