February 22, 2016 โ 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 planetโs oceans. They also underscore the effects that rising greenhouse gases and global temperatures could have on future sea level.
โThe big takeaway is that the modern rate of sea level rise in the 20th century is faster than anything weโve seen in the previous two millennia,โ says Benjamin Horton, a Rutgers University, New Brunswick, in New Jersey geologist who helped direct one of the studies. โThis isnโt a model. This is data.โ
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 oceanโs 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 0.2ยฐC between 1000 and 1400 C.E., for example, the worldโs 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 studyโs 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 โforams,โ which dwell in the muck of saltwater marshes. The marshesโ location at the border between land and seaโalong with their relatively flat topographyโmake them a handy yardstick of sea level changes. In the new study, geologists extracted sediment cores from marshes and then painstakingly recorded the numberโand typesโof different foram species in different layers. The numbers indicated the mix of saltwater and freshwater, which correlates with sea level, Horton says.