National Geographic : 2013 Sep
41 Intermediate low 1.7 feet Intermediate high 4.0 feet High 6.6 feet Low 0.6 feet A.D. 1 2100 500 1000 1500 2013 -1 1 0 Sea level (feet) Observed Projected Reconstructed from sediment samples Zero is set to sea level in 1992. 1880 SEA-LEVEL SCENARIOS, 2100 As the IPCC prepares to issue a new report this fall, in which the sea-level forecast is expect- ed to be slightly higher, gaps in ice-sheet science remain. But climate scientists now estimate that Greenland and Antarctica combined have lost on average about 50 cubic miles of ice each year since 1992—roughly 200 billion metric tons of ice annually. Many think sea level will be at least three feet higher than today by 2100. Even that figure might be too low. “In the last several years we’ve observed accel- erated melting of the ice sheets in Greenland and West Antarctica,” says Radley Horton, a research scientist at Columbia University’s Earth Institute in New York City. “ The concern is that if the ac- celeration continues, by the time we get to the end of the 21st century, we could see sea-level rise of as much as six feet globally instead of two to three feet.” Last year an expert panel convened by the National Oceanic and Atmospheric Ad- ministration adopted 6.6 feet (two meters) as its highest of four scenarios for 2100. The U.S. Army Corps of Engineers recommends that planners consider a high scenario of five feet. One of the biggest wild cards in all sea-level- rise scenarios is the massive Thwaites Glacier in West Antarctica. Four years ago NASA spon- sored a series of flights over the region that used ice-penetrating radar to map the seafloor topog- raphy. The flights revealed that a 2,000-foot-high undersea ridge holds the Thwaites Glacier in place, slowing its slide into the sea. A rising sea could allow more water to seep between ridge and glacier and eventually unmoor it. But no one knows when or if that will happen. “ That’s one place I’m really nervous about,” says Richard Alley, a glaciologist at Penn State University and an author of the last IPCC report. “It involves the physics of ice fracture that we really don’t understand.” If the Thwaites Glacier breaks free from its rocky berth, that would liberate enough ice to raise sea level by three meters—nearly ten feet. “ The odds are in our favor that it won’t put three meters in the ocean in the next century,” says Alley. “But we can’t absolutely guarantee that. There’s at least some laWSon paRkeR, ngm Staff. SouRceS: JoSh WilliS, naSa/Jpl; John chuRch and neil White, commonWealth Scientific and induStRial ReSeaRch oRganiSation; andReW kemp et al., 2011; R. Steven neRem et al., 2010; noaa local measurements of sea level with tide gauges became common after 1880; satellites began global mea- surements in 1992. they’ve shown a clear acceleration: at an eighth of an inch a year, sea level is rising twice as fast as it was a few decades ago.