February 22, 2016 — Global sea levels appear exquisitely sensitive to changes in temperature and greenhouse gas levels, according to a set of new studies that examines up to 6 million years of climate change data. The four papers, published today in the Proceedings of the National Academy of Sciences(PNAS), illustrate the growing power of computers to simulate complex interactions between climate, polar ice, and the planet’s oceans. They also underscore the effects that rising greenhouse gases and global temperatures could have on future sea level.
“The big takeaway is that the modern rate of sea level rise in the 20th century is faster than anything we’ve seen in the previous two millennia,” says Benjamin Horton, a Rutgers University, New Brunswick, in New Jersey geologist who helped direct one of the studies. “This isn’t a model. This is data.”
Some of the studies provide a detailed account of changes in sea level and the Antarctic ice sheet, a story embedded in fossils and rocks at the ocean’s edge. They also add to a growing body of research that suggests sea level can change more dramatically over a short time than previously suspected, says Andrea Dutton, a University of Florida in Gainesville geologist and a leading expert on reconstructing ancient sea levels.
The first study found that small temperature fluctuations have led to measurable changes in ocean levels over the past 3000 years. As the global thermostat turned down just 0.2°C between 1000 and 1400 C.E., for example, the world’s seas dropped an estimated 8 centimeters. By contrast, they have risen about 14 centimeters in the 20th century. At least half of that increase is due to human-induced climate change, say the researchers, who add that sea levels are very likely to rise another 0.24 to 1.3 meters during this century.
The study’s results come in part from measurements of past sea levels gathered at 24 sites around the world. When there were no good written records from tide gauges, scientists relied chiefly on the shells of single-celled creatures called foraminifera or “forams,” which dwell in the muck of saltwater marshes. The marshes’ location at the border between land and sea—along with their relatively flat topography—make them a handy yardstick of sea level changes. In the new study, geologists extracted sediment cores from marshes and then painstakingly recorded the number—and types—of different foram species in different layers. The numbers indicated the mix of saltwater and freshwater, which correlates with sea level, Horton says.