Fossil coral exposed in a limestone outcrop above present sea level in the Seychelles.
Belinda Dechnik/University of Wisconsin-Madison
Newly uncovered evidence from fossil corals suggests that sea levels could rise even more steeply in our warming world than previously thought.
“This is not good news for us as we head into the future,” says Andrea Dutton, a professor of geoscience at the University of Wisconsin–Madison. Dutton and her PhD student Karen Vyverberg at the University of Florida led an international collaboration that included researchers from University of Sydney, University of Minnesota Twin Cities, Victoria University of Wellington and University of Massachusetts Amherst who analyzed fossilized corals discovered in the Seychelles islands.
Models of future sea level rise generally hover around a meter (3 feet) within the next 100 years, but factors like how much of the ice caps will melt, water temperatures, oceanic currents, tidal range, coastal geomorphology and land subsidence can affect local sea levels.
Based on an analysis published by NASA in 2021, global mean sea levels rose by about 20 centimeters (around eight inches) between 1901 and 2018.
Fossils provide an opportunity to reconstruct sea level change over a longer time span. The researchers used remnants of coral species that only live in shallows very near the sea surface. Their tropical location also means they were far away from any past ice sheets, which have a more pronounced effect on local sea levels.
By determining the ages of two dozen fossil corals from various elevations on the islands and analyzing the sediments around the fossils, the team reconstructed the relationship between global climate and sea levels between 122,000 and 123,000 years ago. That was during a period known as the Eemian Interglacial, when global temperatures were actually very similar to what they are now.
Perhaps more importantly, the researchers noted that sea levels didn’t rise at a constant rate, but there were periods of stagnation followed by abrupt pulses. Likely the ice sheets in Greenland and Antarctica — thousands of kilometers away from the Seychelles islands — didn’t melt simultaneously as it is happening today.
“These swings suggest that the polar ice sheets were growing and shrinking out of phase with each other as a result of temperature changes in the two hemispheres that were also not aligned,” explains Dutton.
At the beginning of the Eemian, sea levels rose over 6,000 years leading up to a peak that was 5 to 7 meters (16 to 23 feet) higher globally than it is today. Such high levels are unlikely in the near future, however, today’s sea level rise could happen much faster.
“So even though sea level rose at least several meters higher than present during this past warm period, if temperature rises simultaneously in both hemispheres as it is today, then we can expect future sea level rise to be even greater than it was back then.”
“This is hugely important for coastal planners, policy makers and those in the business of risk management,” concludes Dutton.
Over 600 million people (around 10 percent of the world’s population) live in coastal areas that are less than 10 meters (32 feet) above sea level.
Rising sea levels not only will displace an estimated 267 million people worldwide, but contaminate groundwater, increase the risk for floods, cause beach erosion and habitat loss for animals and plants living on or near the shoreline.
The good news, as Dutton sees it, is that society has the means to blunt the impact of climate change on sea levels.
“The more we do to draw down our greenhouse gas emissions, and the faster we do so, could prevent the worst scenarios from becoming our lived reality.”
NASA created a sea level projection tool that visualizes the impacts of future sea level rise on coastlines and communities. This online tool can display possible future sea levels under several greenhouse gas emission and socioeconomic scenarios. The worst case scenario, with rapidly increasing emissions and warming of over 2 degrees, could be enough for Greenland’s ice sheet to melt, which would cause sea levels to rise globally by more than 2 meters (6 feet).
The study,”Episodic reef growth in the Last Interglacial driven by competing influence of polar ice sheets to sea-level rise,” was published in the journal Science Advances.
Additional material and interviews provided by University of Wisconsin-Madison.
