Sea level rise in two millennia

Showing a consistent link between changes in global mean surface temperature and sea level, the rate of sea level rise along the U.S. Atlantic coast is greater now than at any time in the past 2,000 years.

That conclusion comes from research conducted by Andrew Kemp of Yale University and others from various institutions.

The findings are published in the journal

Proceedings of the National Academy of Sciences (PNAS).

Important context

“Having a detailed picture of rates of sea level change over the past two millennia provides an important context for understanding current and potential future changes,” says Paul Cutler, program director in NSF's Division of Earth Sciences.

“It's especially valuable for anticipating the evolution of coastal systems,” he says, “in which more than half the world's population now lives.”

Kemp and colleagues developed the first continuous sea-level reconstruction for the past 2,000 years, and compared variations in global temperature to changes in sea level over that time period, according to a National Science Foundation press release.

Sea-level stable

The team found that sea level was relatively stable from 200 BC to 1,000 AD.

Then in the 11th century, sea level rose by about half a millimetre each year for 400 years, linked with a warm climate period known as the Medieval Climate Anomaly.

Then there was a second period of stable sea level during a cooler period called the Little Ice Age. It persisted until the late 19th century.

Since the late 19th century, sea level has risen by more than 2 millimetres per year on average, the steepest rate for more than 2,100 years.

Disastrous outcome

“Sea-level rise is a potentially disastrous outcome of climate change,” says Benjamin Horton, of University of Pennsylvania “as rising temperatures melt land-based ice, and warm ocean waters.”

To reconstruct sea level, the scientists used microfossils called foraminifera preserved in sediment cores extracted from coastal salt marshes in North Carolina. The age of the cores was estimated using radiocarbon dating and other techniques.

Findings confirmed

To test the validity of their approach, the team compared its reconstructions with tide-gauge measurements from North Carolina for the past 80 years, and global tide-gauge records for the past 300 years.

A second reconstruction from Massachusetts confirmed their findings.

The records were corrected for contributions to sea-level rise made by vertical land movements.

The reconstructed changes in sea level over the past millennium are consistent with past global temperatures, the researchers say, and can be determined using a model relating the rate of sea level rise to global temperature.

“Data from the past helped calibrate our model, and will improve sea level rise projections under scenarios of future temperature increases,” says Stefan Rahmstorf of Potsdam Institute for Climate Impact Research, Germany.