Dating sediments using ragweed and nuclear fallout
The researchers determined the age of each chunk by carbon dating, possible because the sediment is rich in decomposed plant matter. They honed those estimates further by examining the chunks for the first signs of ragweed pollen, which settlers introduced to the area in 1720, and the cesium and lead fallout from atomic bomb testing in the 1950s.
Finally, Horton's team subtracted the annual rate of subsidence from the sea level rise pattern they reconstructed from the sediment samples. That left them with a record of the sea level caused by climate variations.
The technique isn't new, but the new study marks the longest continuous record produced from sediment samples, Horton said.
That understanding of the past will help scientists improve their models of future sea level rise, he said, by allowing them to test models' ability to simulate the known conditions of the past -- a technique known as "hindcasting."
"If you are tuning a model to just the 20th-century data, all you have is sea level rising," Horton said. "It's very hard to tune a model when it's just unidirectional."
Still, that doesn't mean the exact relationship between temperature and sea level rise evident in the salt marsh data will hold as climate change accelerates in the future, Miller cautioned.
The processes that drive the behavior of the world's ice sheets may be different in intensity or type than those that drove melting in the past, he said.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500