Understanding the long-term impact of a warming climate is vexing for scientists and citizens alike; there are many variables and obscure, complex relationships between them. But one such relationship stands out, both in terms of directness and consequences: the relationship between rising temperatures and rising seas.

“Sea-level rise is a potentially disastrous outcome of climate change, as rising temperatures melt land-based ice and warm ocean waters,” says Ben Horton, an associate professor in the Department of Earth and Environmental Science and director of its Sea Level Research Laboratory. Along with postdoctoral fellow Andrew Kemp, now at Yale University’s Climate and Energy Institute, and an international research team, Horton has shown that the rate of sea-level rise along the Atlantic Coast in the United States is greater now than at any time in the past 2,000 years. Researchers have also shown that there is a consistent link between changes in global mean surface temperature and sea level.

To reconstruct historic sea levels, the research team analyzed microfossils called foraminifera preserved in sediment cores from coastal salt marshes in North Carolina. The team members then confirmed their reconstructions against tide-gauge measurements from North Carolina for the past 80 years and global tide-gauge records for the past 300 years. The records were also corrected for contributions to sea-level rise made by vertical land movements.

The group found that sea level was relatively stable from 200 B.C.E. to 1,000 C.E. During a warm climate period beginning in the 11th century, known as the Medieval Climate Anomaly, sea level rose by about half a millimeter per year for 400 years. There was then a second period of stable sea level associated with a cooler period, known as the Little Ice Age, which persisted until the late 19th century. Since the late 19th century, however, sea level has risen by more than 2 millimeters per year on average, the steepest rate for more than 2,100 years.

The team’s research shows that the reconstructed changes in sea level during the past millennium are consistent with past global temperatures and can be described using a model relating the rate of sea-level rise to global temperature.

"Scenarios of future rise are dependent upon understanding the response of sea level to climate changes. Accurate estimates of past sea-level variability provide a context for such projections,” Kemp says.