Scientists have been probing the connection between rising sea levels and global warming for decades. As temperatures rise, water expands and its density decreases. And warmer air is causing glaciers to melt more rapidly, further swelling the planet’s oceans.

Understanding the timing and magnitude of this apparent acceleration in the rate of relative sea-level rise is critical for testing models of global climate change. To answer the question of how high and how fast the seas might rise in the future, scientists at Penn’s Sea Level Research Laboratory in the Department of Earth and Environmental Science are looking to the past for answers.

By studying sediment levels in salt marshes off the North Carolina coast dating back 500 years, the Penn team has found sea-level rise in the 20th century occurred at a rate three times higher than normal. Furthermore, they discovered another accelerated rise in sea level occurred between the years 1879 and 1915, a time of industrial progress that may provide a direct link to human-induced climate change.

Lead author Andrew Kemp, a post-doctoral researcher in Penn’s Department of Earth and Environmental Science, says the rate of rise they recorded is unprecedented over the last 2,500 years, and the timing of the acceleration is consistent with other studies done along the north Atlantic coast. But much more research is needed on the link between warming and sea-level rise.

“We’ve been very hesitant to attribute it to a particular mechanism. Just because the timing is concurrent with warming doesn’t necessarily mean it’s caused by it. The relationship between climate and sea level is one that is not particularly well understood on these short timescales,” Kemp says.

For five years, Kemp and Benjamin P. Horton, assistant professor in Penn’s Department of Earth and Environmental Science, have been studying the evolution of coastal North Carolina as part of an international team of researchers. The team used a variety of scientific techniques to date sediment samples and develop retrospective sea-level records from the last millennia. They validated their data using available historical documentation and more recent satellite observational data, giving them a baseline for assessing recent and future sea level and coastal changes.

“We’re interested in trying to develop models of how climate and sea level are related, which has big implications,” Kemp says. “All the climate predictions you see for the future give quite a broad range. If we can understand the relationship between climate and sea level in the past, then we can ... get a better hold on what sea level might do in the future.”

By continuing to study the evolution of coastal North Carolina and locations farther south, ultimately the research team hopes to gain a better understanding of how the current melting of the Greenland ice sheet may be affecting climate change. Coastal sediment samples taken in New England also indicate sea levels in that region were rising during the heart of the Industrial Revolution in America, but at a much slower rate than the Penn team observed in North Carolina.

One school of thought is that as the Greenland ice sheet melts, the effect of the gravitational pull it exerts on the Atlantic is lessened. Places near the ice sheet see little rise, while sites farther away see greater rises. But more data is needed at other locations along the coast to confirm those models, which means many more years of trudging through salt marshes for Kemp, Horton and their team.

“[Coastal sea rise] is important in terms of coastal management and what people need to do in terms of sustaining property,” Kemp says. “But there are a lot of geological reasons we’re interested as well—it’s the baseline for driving changes in island migration, beach formation and more. These geologic records provide us with a means to better understand what happened to the solid Earth when great ice sheets [developed] in North America and northern Europe in the past, which is a whole more complicated issue.”

The paper was published in the journal Geology. The study was funded by the National Oceanic and Atmospheric Administration Coastal Ocean Program, North Carolina Coastal Geology Cooperative Program, and the U.S. Geological Survey and National Science Foundation.