July 2, 2011 — andyextance

The US Atlantic coast is now experiencing sea-level rises that are faster than at any time in the past two millennia, microfossils preserved under coastal salt marshes have shown. Both sea-level and temperature changes had been “subtle” for most of that period, University of Pennsylvania’s Benjamin Horton said, but accelerated from the late 19th century. “The research shows a consistent link between global mean surface temperature and changes in sea-level for the past millennium – that is a big step forward in our understanding,” Horton told Simple Climate.

Although modern direct measurements have shown that sea-level rise has been speeding up in recent years, scientists needed more information to fully understand how it’s linked to climate. To add to the recent records, Horton, together with Yale University’s Andrew Kemp studied two North Carolina salt marsh sites to reconstruct past sea-level. “In regions where sea-level is rising, salt marshes build upward by accumulating sediment to maintain their position,” Horton explained. “The accumulated piles of sediment are archives of relative sea-level changes.”

Kemp, Horton and their colleagues therefore drilled narrow columns of sediment from the marshes to access archives covering the last 2,100 years. However, the position of the layers of sediment alone did not provide an accurate enough measure of sea-level, as the marsh beds do not lay at exactly the same height as the sea. So, to get a high-resolution sea-level reconstruction they were able to use their knowledge of micro-organisms called foraminifera. The scientists had previously calculated a “transfer function” relationship describing how many foraminifera live in the modern salt marsh at a particular height above sea-level. “In samples from each core we counted preserved foraminifera and used the transfer function to estimate how high above sea-level each sample formed,” Horton explained.

From North Carolina to the world

To precisely pin down the age of each layer, the scientists used a number of different methods, including radiocarbon dating. “To be confident in the validity of our technique, we compared our estimates of sea-level for the past 100 years to measurements made by tide gauges in North Carolina and found them to be the same,” Horton said. But the figures that this provided include changes in the Earth’s shape under the weight of ice lying on it in the form of glaciers. When the ice sheet thickens, the land underneath subsides, rebounding as the ice melts, a process which is occurring even today. So, to provide a measure of how sea-level had varied against a constant reference point, they also had to remove the impact of these changes. “To extract climate-related changes in sea-level we subtracted estimates of how much land subsidence had contributed over the last 2100 years,” Horton said.

This provided the first high resolution data for sea-level spanning the warm “Medieval Climate Anomaly” period between 950 AD and 1250 AD and the cold “Little Ice Age” that followed it. In a Proceedings of the National Academy of Sciences of the USA paper published last week, Horton and colleagues’ record showed sea-level was stable from at least 100 BC to 950 AD. It then rose at around 0.6 mm per year for 400 years, then remained stable, or fell slightly, until around 1900. Sea level has risen at an average of 2.1 mm per year since then, with the two sites the team studied showing close agreement. Although they are just 75 miles apart, Horton stressed that because they are in completely different environments, the close agreement shows how well their method works.

Horton also emphasised that the North Carolina measurements do relate directly to historical global sea-levels because any regional variations tend to smooth out over time. “There is close agreement between reconstructed sea level in North Carolina and compilations of global tide-gauge data,” Horton said. His team also showed that the data is consistent with past global temperatures using climate models. However, some of his colleagues are now looking to retrieve similar records elsewhere in the world. “The aim there is to address which areas are more susceptible to sea-level rise in the 21st century,” he said.