New Study Reinforces Worries About Pulses of Rapid Sea Level Rise

An analysis of peat layers at the bottom of the North Sea shows how fast sea level rose during the end of the last ice age, when Earth was warming at a similar rate as today.

A new analysis of ancient layers of peat at the bottom of the North Sea will help scientists more accurately project how much sea level will rise in the coming decades and centuries. The research shows how fast sea level rose about 11,000 years ago, at the end of the last ice age, the last time Earth warmed as fast as it is warming now.

Current estimates for sea level rise in the next 75 years range between 1 and 4 feet. The new study, published today in Nature, affirms that projections for 3 feet of sea level rise by 2100 are not unrealistic, and “help[s] unravel the complex interaction between ice sheets, climate, and sea level,” said co-author Marc Hijma, a geologist with Deltares, a nonprofit research institute in The Netherlands.

The findings suggest sea level jumped by as much 3.3 feet per century during at least two separate periods between about 8,300 and 10,300 years ago, as the North American and Eurasian ice sheets melted away early in the Holocene geologic era.

Previous estimates for the total amount of sea level rise in the early Holocene varied by as much as 50 feet. 

“The uncertainty was just enormous for this time period … because of a lack of good data,” said Hijma, adding that pinpointing future sea level rise is critical for coastal communities trying to prepare themselves. “This provides insights for both scientists and policymakers, so that we can prepare better for the impacts of current climate change, for example by focusing on climate adaptation,” he said.

In the study, the scientists drilled into the seafloor to extract submerged peat from an area of the North Sea called Doggerland, which was part of a land bridge between Great Britain and mainland Europe during the peak of the last ice age, when global sea level was more than 390 feet lower than today. The area flooded at the end of the ice age and recent improvements in seabed mapping help show scientists where to look for layers of organic peat, which scientists can date accurately.

“The North Sea area is a laboratory for sea level rise,” Hijma said. “It’s quite flat and slowly got submerged during the early Holocene.” Analyzing the peat layers from different levels made it possible to digitally reconstruct the course of sea level rise during the study period, he added.

“The biggest struggle for us was that there was also a lot of vertical land motion involved because of the disappearance of the Eurasian ice sheet,” he said, describing how land areas can rise upward as they rebound like a mattress after being compressed by heavy ice for thousands of years. 

There is also uncertainty about the exact location and configuration of the continental ice sheets of that ice age. That matters because those masses of ice were so big that they exerted a gravitational pull on the ocean that raised sea level nearby, relative to more distant areas. As they shrink, that pull decreases and the water sloshes away, raising sea level in faraway regions. 

Warnings Pile Up

It took about seven years to work through the data and disentangle the effects of rebounding land and changing gravity as the ice sheets melted, but at the end of that, Hijma said, a clear global sea level signal emerged for the early Holocene, along with renewed warnings for the present that are echoed by other recent studies—all of which point to rates of sea level rise accelerating beyond what was expected just a few decades ago.

In a 2023 study, scientists found sea levels along the southeastern coast of the U.S. and in the Gulf of Mexico were rising much faster than the global average, accelerating up to a rate of nearly 0.4 inches per year. A 2024 paper found that the rate of sea level rise globally has doubled from 0.8 inches to 1.7 inches per decade since 1993. And in mid-March, NASA scientists wrote that 2024 showed an “unexpected” amount of sea level rise.

Sea level rise could wash away some low-lying island states. It has already displaced tens of thousands of people, and by one estimate could displace 13 million people in Bangladesh alone by 2050. Salt water is also infiltrating fresh water supplies on low-lying coastal communities and worsens the impacts of property destruction by coastal flooding from tropical storms. Rising oceans also threaten coastal ecosystems, including habitat for economically and ecologically valuable species.

Understanding more precisely how, and where, sea level rose at the end of the last ice age is crucial to making better projections going forward, said Aimée Slangen, a researcher with the Royal Netherlands Institute for Sea Research, who was not involved in the new study.

“Having a better constraint of past sea level changes, and on the locations of those changes, which is done in this paper helps to get better projections for the future,” she said.

The new findings are “valuable because the early Holocene period serves as a valuable analogue for future sea-level rise due to the similar rates of change as those expected in the next 50-100 years,” said Roland Gehrels, a geologist and geographer at the University of York, in the United Kingdom.

The peat samples used in the new study offer a much higher level of precision for estimating sea level rise at the end of the ice age than the fossil coral records from which most data have been derived so far, he said.

Isolating the sources of ice-sheet melting and their contribution to sea level rise provides “essential insights into the mechanisms behind such changes that helps predict how current and future ice-sheet dynamics will affect future sea levels,” said Gehrels, who was not involved in the new research.

For planners in The Netherlands, Hijma added, whether sea level will increase at the rate suggested by middle-ground scenarios or at the rate projected by more extreme scenarios is critical information.

“That makes a huge difference for the things that you have to do for flood safety,” he said. He also said he was a bit worried that the study’s findings could be misused to minimize human influence on the climate because it rose in the past due solely to natural causes.

The study helps show that ice sheets can react relatively fast, within a century. “It doesn’t take thousands of years for them to respond,” he said.

Cover photo:  An aerial view of the Kalabogi village in Khulna, Bangladesh during the high tide on March 10, 2023. The village has been facing frequent cyclones and floods since the late 1990s. Credit: Kazi Salahuddin/NurPhoto via Getty Images