What are we missing and how much should it scare us?

A new study of the Bahamian shores could change estimates of past sea level. Here, some of the authors study the archipelago’s Crooked Island coastal rocks formed when the sea level was higher. Credit: Blake Dyer

Some past sea levels may not have been as high as thought, according to a study of the rise and fall of land masses. But even reduced estimates, if applied to today, would be catastrophic.

One of the current mysteries of climate science surrounds the widely accepted evidence that during the planet’s most recent natural warm period, approximately 128,000 to 117,000 years ago, global sea level rose peaked up to 6 to 9 meters (20 or 30 feet) higher than today. And, during this so-called last interglacial, temperatures were only 1 or 2 degrees C (1.8 to 3.6 F) warmer than those in pre-industrial times – marks we could surpass by the end of the century, if not sooner. Such a deluge could only have been produced by the collapse of the Greenland and / or Antarctic ice caps. If this happens now, it will drown much of the human world. Yet at least so far, patterns of future sea level rise typically hover around a meter or so over the next 100 years. What do we miss, and how much should it scare us?

In a new study, a team of Columbia universityThe Lamont-Doherty Earth Observatory thinks it has an answer: They say researchers examining signs of past sea level along various coasts may have failed to accurately correct the ups and downs at long term of the earth itself. Based on newly sophisticated measurements made across the Bahamas along with new methods of data analysis, the researchers produced lower, though still daunting, estimates for the last interglacial. They say the seas peaked at least 1.2 meters (4 feet) higher than they are today, which roughly matches most current models for the next 100 years. However, they say, the levels could have been higher. An unlikely upper limit, they say, is 5.3 meters (17 feet). The study appears this week in the Proceedings of the National Academy of Sciences.

On Crooked Island, study co-author William D’Andrea uses a GPS instrument that records the exact location and elevation of rocks. Credit: Blake Dyer

“To reach 9 meters of sea level rise, large parts of Greenland and Antarctica would have to be melted,” said lead author Blake Dyer. “This suggests that it didn’t happen. So maybe we should feel less bad about the future. On the other hand, our lower estimate is bad and our upper estimate is really bad.

Key to the new study: the fact that as ice caps form, they drive the earth beneath them. The ice that covered northern North America during the last ice age, about 15,000 years ago, crushed the land for hundreds of meters. But the Earth is elastic: what goes down in one place goes up in another, like when you squeeze a rubber ball or an inner tube. These corollary deformations outside the frozen regions are still poorly understood; they can crawl hundreds or thousands of kilometers over hundreds or thousands of years. They move mainly in the soft mantle, some 100 to 1000 kilometers deep, before coming to the surface. Then, when the ice melts, the process is reversed; regions previously covered with ice bounce back, while those on the fringes sink, slowly rocking.

For researchers William D’Andrea and Jacqueline Austermann, a small cave on Crooked Island offers shelter from the rain and a close examination of the fossilized ripples formed on an ancient sand dune. Credit: Blake Dyer

Clearly, such movements, known as glacial isostatic rebound, can skew estimates of past water levels, and climatologists have difficulty adjusting them precisely. For example, previous studies have suggested that topographic ripples from the North American glaciations spread along the eastern seaboard of the United States and reached the Bahamas archipelago. This pushed the islands up when the ice was high and slowly lowered them when the ice melted. But exactly how much, and when, was unclear. To find out more, the researchers studied the islands’ coastal deposits in detail.

The Bahamas stretch north to south for 1,200 kilometers, making them ideal for studying glacial isostatic rebound. Theoretically, the swelling and subsidence cycles would be greatest closer to the ice and stop further south, as evidenced by the coastal deposits of the islands. This is exactly what the researchers found.

Walking along the coasts of seven islands, the team accurately measured the elevations of different types of geological formations, including fossil coral reefs; fossilized edges of ancient beaches and coastal sand deposits; and fossil sand dunes. They found similar sequences of similar ages on each island, but their elevations varied with latitude. This meant that the variations could not have been produced by water levels alone; ground movements had to be taken into account. Putting all the measurements together, they concluded that the islands to the north probably sank up to 10 meters during the interglacial, while those to the south only sank about 6 meters. They combined these results with hundreds of different models of how glacial isostatic rebound could have moved through Earth and converted the calculations to global sea levels. This produced the new lower estimates.

“Much of what we estimate for the future is based on what we observe in the past, so this directly affects our projections,” said co-author Jacqueline Austermann, a geodynamicist at Lamont-Doherty. “If our lower estimates are true, that implies that the ice caps will react to warming, but maybe not as dramatically as we thought. “

Take under consideration: Evidence for much higher estimates of sea level rise during the last interglacial comes from many other places, including the Mediterranean, Indian Ocean and Australia. The authors of the new study argue that previous analyzes could be based in part on a flawed premise that the ice sheet covering North America before 128,000 years ago was the same size as the most recent one. The new study suggests that in fact the previous ice sheet was smaller, which could have affected calculations at other sites. Previous estimates could also be affected by assumptions about the size of the separate ice sheet that covered Scandinavia, northern Europe and northwest Russia.

In addition, the warm temperatures of the last interglacial were slowly produced by variations in the orientation of the Earth relative to the sun over thousands of years, and may not have affected both poles simultaneously. Study co-author Maureen Raymo, principal of Lamont-Doherty and co-dean of the new Columbia Climate School, said this could mean that when some polar regions lost ice, others could have lost ice. to win. It would have balanced things out and limited the rise in global sea level. “That’s still a question. The models of ice caps are still in their infancy, ”she said. She points out that human carbon emissions are now heating the globe much more quickly and evenly than during the last interglacial, so there is no guarantee of such a buffer. “This makes it more difficult to apply the results to today,” she said. “The easy thing to say would be, ‘Oh, we’ve shown the sea level isn’t that bad, and that’s great.’ The hardest answer, the most honest answer, is that maybe things were different then, and we’re not clear. “

The study is not the first to provide estimates of lower sea level for the last interglacial. Last year, a study by Peter Clark of Oregon State University suggested that the levels may have risen by about 4 meters. However, this study was strictly based on modeling, and not on new geological evidence, as in the new study.

Robert Kopp, a climatologist at Rutgers University whose work is widely cited as evidence of the much higher sea level of the last interglacial period, said the study “should inspire a substantial critical appraisal of what we understand – or what we think we understand – about the last interglacial. Given that it is based on a single region, I think it should be seen as an alternative hypothesis to the prevailing valuation. [rather] that as the new best guess. He said the next step would be to re-examine other regions.

Researchers plan to do so and are already studying new site assessments in Denmark, France, England and South Africa.

Robert DeConto, a scientist at the University of Massachusetts at Amherst who studies polar climate change, said of the study: “I guess that’s not the last word on [last interglacial] sea ​​level. Considerable uncertainty and the possibility of a sea level much higher than 1.2 meters remains. “

One thing is certain: the glacial isostatic rebound is still at work. In our current interglacial period, the Bahamas at this point appear to be sinking imperceptibly, but this is not the case along the eastern seaboard of the United States. In many areas, the Atlantic Ocean rises 1 to 2 millimeters per year. At the same time, the coast is sinking by an equivalent amount, in response to the melting of the ice cap further north thousands of years ago. The flooding caused by the resulting double whammy can already be seen in many areas.

Reference: “Sea level trends across the Bahamas constrain peak of last interglacial ice melt” by Blake Dyer, Jacqueline Austermann, William J. D’Andrea, Roger C. Creel, Michael R. Sandstrom, Miranda Cashman, Alessio Rovere and Maureen E. Raymo, August 9, 2021, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2026839118

The other authors of the study are William D’Andrea, Roger Creel, Michael Sandstrom and Miranda Cashman, all of the Lamont-Doherty Earth Observatory; and Alessio Rovere from the University of Bremen. Blake Dyer is now based at the University of Victoria, British Columbia, Canada. The study was supported by the National Science Foundation of the United States.