Sea level is rising faster along the U.S. East Coast than it has for at least 2,000 years, according to new research.
The ocean began rising an average of 2.1 millimeters per year some time between 1865 and 1892 and hasn't stopped, the study concludes. The current rate of sea level rise is about 3.2 mm per year.
That trend, gleaned from muck collected in North Carolina salt marshes, is a direct consequence of increasing temperatures, said co-author Benjamin Horton, a coastal geologist at the University of Pennsylvania.
"We can see climate-related patterns," he said. "We can see our hypothesis that as temperature goes up, sea level goes up."
The reverse is also true, according to the research, which shows temperatures and sea levels rising and falling in lockstep for at least the last 1,000 years. The findings will be published online this week by the Proceedings of the National Academy of Sciences.
Peter Clark, a geologist at Oregon State University, said the new study represents a "significant advance" because it extends the record of sea level rise back two millennia, giving scientists a better context for current sea level rise.
"It's important to learn about past climate (and sea level) because we want to know how it varies naturally (i.e., when there was no possible human influence) so as to evaluate whether the current recent changes are unusual," he said in an email.
The new paper does "firmly establish" that current sea level rise is unprecedented for the recent past, said Ken Miller, a geologist at Rutgers University. The work was warmly received last year when the authors presented it at a sea level-rise workshop sponsored by the Intergovernmental Panel on Climate Change, he said.
Period of stability to A.D. 950
In addition to the recent sea level rise, the record pieced together by Horton's team shows a long period of stable sea level from 100 B.C. to A.D. 950. That marked the beginning of a warm period that lasted roughly four centuries, a time when seas rose 0.6 mm per year. Sea level then held steady, or slightly fell, until the late 19th century, when the current sea level spike began.
"When temperatures really rocketed as they did in the 20th century," Horton said, "our sea levels did the same."
Scientists have good measurements of recent sea level rise from a combination of tide gauges, which began collecting data in the early 20th century, and satellites, which started tracking sea level in 1992.
To go even further back in time and put recent conditions in context, the authors of the new study examined sediment samples from North Carolina salt marshes.
Those marshes are "an ideal natural laboratory," Horton said, because they are always slowly sinking and being rebuilt as tides wash in new sediment. That steady accumulation of muck traps tiny organisms called foraminifera, plant matter and other substances that form a natural record of sea level rise.
The scientists began decoding that record by mapping the distribution of different foraminifera species -- or "forams" -- at 10 North Carolina marsh sites.
A marsh is typically home to several species of forams, which each prefer slightly different conditions. Some thrive in the relatively fresh water found in areas that aren't often inundated by ocean tides, while others seek out a saltier brew.
Understanding which foram species were found in areas often overtopped by salt water and which weren't allowed scientists to construct a model they used to analyze sediment cores from two marshes and estimate sea level rise over time.
Peering back two millennia meant collecting cores 3 to 4 meters long. Extracting information about sea level required cutting those cores into 1-centimeter chunks and analyzing the foram fossils found in those chunks.