New Climate Study Highlights Dire Sea Level Warnings

To learn about how polar ice sheets melted during an ancient era, scientists examined fossil coral reefs in the tropics.

A new set of detailed clues gleaned from ancient fossil reefs on the Seychelle Islands shows an increasing likelihood that human-caused warming will raise the global average sea level at least 3 feet by 2100, at the high end of the projections by the Intergovernmental Panel on Climate Change. 

Due to regional variations, sea level would rise twice that much in some tropical areas, causing misery for millions of people living in low-lying coastal zones, including islands like the Maldives, in the Indian Ocean, which would be completely swamped by 6 feet of sea level rise.

University of Wisconsin-Madison climate scientist Andrea Dutton, one of the authors of the paper published today in Science Advances, said experts in this field know that there’s a much higher possibility that sea level will exceed projections, rather than fall short. Nearly every study published in the last 20 years suggests that large tracts of ice in Antarctica and Greenland will melt at the current level of warming, she added.

“We used to think those ice sheets were too big to be influenced by small amounts of warming,” she said “But everything we’re learning from the paleoclimate record, and with modern observations, is telling us, oh no, we’re waking up what we thought were sleeping giants. This is not good news for us as we head into the future.”

The fossilized reefs that Dutton and the other scientists studied lived about 120,000 years ago and were shallow-water species growing near the surface of the ocean in similar combinations to today. The global average temperature was similar to now, but sea levels were 16 feet to 33 feet higher.

By determining the ages of two dozen fossil corals from various elevations on the islands and analyzing the sediments around the fossils, the team was able to precisely confirm the timing of peak global sea level to between 122,000 and 123,000 years ago. They also found that sea level rose sharply in three distinct pulses in the 6,000 years leading up to the peak.

Dutton said identifying those swings was crucial, especially in connection with atmospheric data from bubbles trapped in ice cores about the same time, because they point to times when the polar ice sheets in Greenland and Antarctica were changing rapidly. 

During the Last Interglacial—the warmest time between the last two ice ages—the climate changes were driven by Earth’s orbital changes, with different effects on the Northern and Southern Hemispheres. The swings in sea level rise “suggest 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,” Dutton said. 

Despite the differences between the polar regions, global sea level was “at least several meters higher than present during this past warm period,” she said. “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 evidence they uncovered from the fossil reef also suggests that the varying hemispheric responses also had the effect of masking the true extent of Antarctica’s contribution, via melting ice, to sea level rise during the Last Interglacial. And that, Dutton said, means we could be underestimating how much of Antarctica’s ice will melt from the heating that humans have already caused in the last 100 years.

No In-Between

Several other recent international research efforts with U.S. participation have included similar warnings, including a May study showing that the Paris Agreement goal of limiting warming to 2.7 degrees Fahrenheit above the pre-industrial level isn’t ambitious enough to save polar ice sheets and stave off sea level rise of three feet or more by the end of this century. 

Those findings were affirmed in early June, when the Potsdam Institute for Climate Impact Research (PIK) announced a study showing that human-caused warming has already brought the West Antarctic Ice Sheet to the brink of collapse, which would devastate many coastal areas with more than 12 feet of sea level rise over the course of several hundred years. 

That research showed that, for the last 80,000 years, the West Antarctic Ice Sheet basically has had a temperature-driven on and off switch—either it’s there or it’s not, said  lead author David Chandler, with the Norwegian Research Center NORCE. 

“Once a tipping has been triggered it is self-sustaining and seems very unlikely to be stopped before contributing to about four meters of sea-level rise. And this would be practically irreversible,” Chandler said.

Co-author Torsten Albrecht, a senior ice dynamics researcher at PIK, said there is mounting proxy climate evidence, from marine fossils and other sources, that the West Antarctic Ice Sheet did indeed collapse during perhaps several of the interglacial periods during the past 800,000 years, but that it’s still inconclusive. 

But all the evidence has to be considered together, like a 2020 study that modeled a very steady warming rate that enabled researchers to separate human-caused warming from natural variations. In that smoothed line, the tipping points became clear.

There are no steady states in reality, but using that approach helps understand the system, and then, he said, “You can really find the  threshold where a little bit extra warming tips the system into a different state, and this happens very clearly in the West Antarctic Ice Sheet.”

The ice sheet can persist within a certain temperature range, but, he said, “If you add a little bit more of warming, the whole process of self-amplification kicks in, and you end up with a collapsed state.”

The speed of change caused by human activities like burning fossil fuels is astounding, added co-author Julius Garbe, also a climate researcher at PIK. 

“It takes tens of thousands of years for an ice sheet to grow, but just decades to destabilize it by burning fossil fuels,” he said. “Now we only have a narrow window to act.”

Cover photo:  A crew works to construc a sea wall to reduce the risk of coastal flooding and erosion due to sea level rise on March 4 in La Baule, France. Credit: Loic Venance/AFP via Getty Images