March 4, 2020
During the 18th century, sea levels along a stretch of the Atlantic coast of North America were rising almost as fast as they were during the 20th Century, reveals a new study by researchers at the University of York, Woods Hole Oceanographic Institution (WHOI) and colleagues.
The study, published this week in the journal Geophysical Research Letters, found evidence for a multiple-decades-long period of enhanced pre-industrial sea-level rise of about two to three millimeters per year in Nova Scotia, Maine and Connecticut. This is only slightly slower than rates of change in these locations during the 20th century, which were faster than any other century in at least the last 3,000 years. The new findings are based on salt-marsh sediments from the Atlantic coast and from microscopic salt-marsh fossils, which act as “archives” of past sea levels going back hundreds of years.
The researchers say that the large rates of sea-level rise at these locations during the 18th century were natural, and partly related to the North Atlantic Oscillation—a large-scale atmospheric pressure see-saw over the North Atlantic region—and to periods of enhanced ice melt in the Arctic.
“One of the big goals in sea-level research is to say how much of the sea-level changes we’re seeing today are due to human actions or are caused by natural processes,” says Christopher Piecuch, a physical oceanographer at WHOI and a coauthor of the paper. “Our study takes an important step in that direction and helps put present-day sea-level change more clearly into a longer-term geological context.”
Previous studies showed that, since the 1950s, rates of sea-level rise along the Atlantic coast of North America have been increasing more than the global average, possibly related to weakening ocean currents predicted as a result of human-caused climate change. This region has since been called a sea-level rise “hotspot.” This new research shows that such periods of enhanced sea-level rise on the Atlantic coast, lasting for multiple decades at a time, can also occur for reasons unrelated to human influences.
“The likely future sea-level rise in places like New York City is expected to be considerably greater than the global average by the end of the 21st century,” says lead author Roland Gehrels from the University of York’s Department of Environment and Geography. “Our findings suggest that enhanced rates of sea-level rise along eastern North America are not only symptomatic of human activity, but might additionally arise from natural processes in the climate system.” The authors of the study say cities like New York and Boston should take this natural variability into account in planning for future sea-level rise.
This research was supported by the National Science Foundation and the Natural Environment Research Council. The research team included colleagues from University of Leeds; Durham University; Bangor University; the National Oceanography Centre, Liverpool; Woods Hole Oceanographic Institution, Massachusetts, USA; Old Dominion University, Virginia, USA; and the University of Siegen, Germany.
The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please visit www.whoi.edu.
- Past studies show that sea-level rise increased on the U.S. northeast coast more than the global average since 1950.
- This coastal sea-level acceleration has been tied to changes to ocean currents possibly related to human activity.
- Scientists discovered a period roughly 300 years ago when sea level accelerated by a similar amounts along this coastline.
- This pre-industrial sea-level acceleration is related to large-scale natural climate variability over the North Atlantic and Arctic regions.