Fossil corals point to possibly steeper sea level rise under a warming world

SEYCHELLES — Coastal planners take heed: Newly uncovered evidence from fossil corals found on an island chain in the Indian Ocean 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. 

These particular fossils provided an exceptional opportunity for the researchers to reconstruct past sea levels. That's in part because they're 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 gathered a wealth of insights. The findings will be published June 13 in the journal Science Advances.

First, the team was able to confirm the timing of peak global sea levels to between 122 and 123,000 years ago. That was during a period known as the Last Interglacial, when global temperatures were actually very similar to what they are now. Such a precise date gives us a better understanding of the relationship between global climate and sea levels.

Perhaps more importantly, though, the researchers discovered that there were three distinct periods of sudden and sharp sea-level rise over the 6,000 years leading up to peak sea levels during the Last Interglacial. These abrupt pulses of sea-level rise were punctuated by periods of falling seas, and Dutton says they point to times when the polar ice sheets in Greenland and Antarctica — thousands of miles away from the Seychelles islands — were changing rapidly. 

"That says there's potential for this very rapid, dynamic change in both ice sheet volume and sea level change," says Dutton. "This is hugely important for coastal planners, policy makers and those in the business of risk management." 

These rises and falls in sea level that the team documented also point to a key difference between the present and the Last Interglacial, which is sometimes used as a model for understanding how the current and future climate could affect ice sheets and sea levels due to the similar temperatures between the two time periods. 

"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," says Dutton. "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." 

The researchers made one more sobering observation: One of the sharp pulses of sea-level rise they identified occurred at about the same time that the last remnants of a massive ice sheet in North America were likely collapsing, according to evidence collected by other teams working in the Atlantic Ocean.

While there's no large North American ice sheet today, Dutton says this finding has important implications for understanding the dynamics of other present day ice sheets. That's because most scientists have not previously considered a North American ice sheet as a major factor in sea-level dynamics during the Last Interglacial.

"But if ice was still present in North America several thousand years into this past warm period, then some of the rise we've documented would have required more meltwater from another ice sheet, such as Antarctica," says Dutton. "This would suggest that Antarctica was even more sensitive to warming than we previously recognized, because the full extent of sea-level rise flowing from the continent was masked by a remnant ice sheet in North America."

In its totality, Dutton says the new evidence, thanks to fossilized corals from thousands of years ago suggests that sea levels could rise even faster and higher thanks to climate change than current projections indicate.

"We could be looking at upward of 10 meters of global average sea-level rise in the future just based on the amount of warming that has already occurred," she says. 

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," Dutton says.

This research received funding from the National Science Foundation (grant awards 1155495, 1159040, 1934477, 2035080 and 2202913).

END