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.
These tiny organisms are hard evidence that the past 100 years were record-setting for recent times, he says.
The study—the most complete accounting of global sea level for the past 3000 years—echoes the most recent findings of the Intergovernmental Panel on Climate Change (IPCC). Combining past sea level and temperature records with statistical calculations, the team arrives at sea level projections for 2100 that are very close to IPCC estimates. A second, separate paper in the same edition of PNAS also produced nearly identical results using historic measurements. The convergence of different methods adds confidence that they are fairly accurate—unless they share flawed assumptions, says Robert Kopp, a Rutgers scientist who did much of the statistical work on the first paper. “I certainly wouldn’t say the problem is settled because of the agreement,” Kopp says. “It’s possible there are some common biases.”
The third paper, based on a 1.1-kilometer-long plug pulled from an Antarctic seabed, sheds light on the ebb and flow of the continent’s ice sheets much further in the past: between 14 million and 20 million years ago, when carbon dioxide levels in the atmosphere are thought to have risen only slightly higher than today’s levels. The lengthy cross-section of fossils, mud, and rock—which came from the ice sheet sitting over the waters of McMurdo Sound—tell a tale of massive shifts in the ice sheets that were in sync with changes in atmospheric CO2. When CO2 was at its lowest, the ice sheets extended so far into the ocean that they dug into the sea floor at the drill site, leaving gaps like an eraser dragged across a pencil line. At other times, ice sheets retreated far enough onto land that scallops and pollen from shore plants showed up in the sediment. That happened at times when CO2 is thought to have been above 500 parts per million (ppm)—about 100 ppm higher than today.
“It suggests that land-based ice sheets are vulnerable to relatively small changes in CO2. I think that’s the realization that we had coming out of this study,” says Richard Levy, a New Zealand government scientist and the paper’s lead author.
Those results are in line with a new computer simulation of how ice sheets grow and shrink in Antarctica—one that might overcome problems vexing earlier models. Those didn’t produce the dramatic changes in ice sheets found in the geologic record, says Rob DeConto, a climate modeler at the University of Massachusetts, Amherst. But the new model, developed by DeConto and colleagues, and described in a fourth paper, closely matches the results emerging from Levy’s sediment core. The new simulations incorporated recent insights into how ice shelves can collapse, ice sheets erode, and shrinking ice sheets influence the surrounding climate, DeConto says. The two groups of scientists started working independently, but they collaborated in the final stages when they discovered the overlap in their work.
Looking to the future, the results of the new model suggest that changes in Antarctic ice might cause sea level to rise even more rapidly than current studies indicate, DeConto says. “It may be that the Kopp approach and the IPCC are both wrong,” he says. “Once we start seeing a lot of meltwater on the ice shelves around Antarctica, what’s that going to do?”