Research on rising and sinking land shows that some past sea levels may not be as high as imagined

Research on rising and sinking land shows that some past sea levels may not be as high as imagined

But even the lowered estimates, if applied today, would be catastrophic

One of the mysteries of current climate science revolves around the widely accepted evidence that during the most recent period of natural warming on Earth, approximately 128,000 to 117,000 years ago, the global sea level Up to 6 to 9 meters (20 or 30 feet) higher than today. Moreover, during the so-called last interglacial period, the temperature was only 1 or 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) higher than the pre-industrial period—if not earlier, we might exceed this mark by the end of the century.This kind of flood could have been Only caused by collapse Greenland and/or Antarctic ice sheet. If this happens now, it will flood most of the human world. However, at least so far, the model of future sea level rise will usually hover around 1 meter in the next 100 years. What have we missed and how much should it scare us?

In a new study, a team from Columbia University Lamont-Dougherty Earth Observatory I believe they have an answer: They say that researchers examining past sea levels on different coasts may have failed to accurately correct the long-term ups and downs of the land itself. Based on the latest complex measurements in the Bahamas and new methods of analyzing the data, researchers have made a low estimate of the last interglacial period, although it is still daunting. They say that the peak of the sea is at least 1.2 meters (4 feet) higher than today—roughly consistent with most current models for the next 100 years. However, they said that the level could have been higher. They say that an unlikely upper limit is 5.3 meters (17 feet).The research was published this week in Proceedings of the National Academy of Sciences.

“To raise the sea level by 9 meters, you must melt most of Greenland and Antarctica,” said lead author Blake Dell. “This shows that that didn’t happen. So maybe we shouldn’t feel so bad about the future. On the other hand, our down-estimation is very poor, and our up-estimation is really bad.”

The key to the new research: As ice sheets form, they push down the ground beneath them. During the last glaciation about 15,000 years ago, the ice covering northern North America squashed the land hundreds of meters. But the earth is elastic: something that falls in one place will rise in other places, just like you squeeze a rubber ball or an inner tube.The inevitable deformation outside these icy areas Still little known; They may wriggle hundreds or thousands of miles over hundreds or thousands of years. They mainly move in the flexible mantle, down about 100 to 1,000 kilometers, and then return to the surface. Then, when the ice melts, the process is reversed; the previously covered area rebounds, while the edge area sinks in a slow seesaw manner.

Obviously, this movement, known as the glacial equilibrium rebound, can distort estimates of past water levels, and climate scientists have been trying to adjust them accurately. For example, previous research has shown that topographic ripples from glaciers in North America extend along the east coast of the United States to the Bahamas. When the ice is high, this pushes the island up, and when the ice melts, they slowly relax. But how much, and when, is unclear. To learn more, the researchers studied the coastal sediments of these islands in detail.

The Bahamas stretches 1,200 kilometers from north to south, and is very suitable for studying the isostatic rebound of glaciers. Theoretically, as evidenced by the island’s coastal sediments, the period of expansion and subsidence near the ice layer will be greater and will gradually disappear towards the south. This is exactly what the researchers discovered.

On Crooked Island, research co-author William DeAndria used GPS instruments to record the exact location and height of the rock. (Blake Dell)

The team hiked along the coast of seven islands and accurately measured the elevations of different types of geological formations, including fossil coral reefs; the fossil edges of ancient beaches and nearshore placer deposits; and fossil sand dunes. They found similar sequences of similar ages on each island-but their elevations vary by latitude. This means that it is impossible to change the water level alone. The movement of the land must be considered. Putting all the measurements together, they concluded that during the interglacial period, the northern island may sink as much as 10 meters, while the southern island only sinks about 6 meters. They combined these findings with hundreds of different models of how equilibrium bounces of glaciers travel across the earth, and converted the calculations into global sea levels. This produced a new, lower estimate.

“A lot of our estimates of the future are based on our observations from the past, so this directly affects our predictions,” co-author Jacqueline Osterman, a geodynamicist at Lamont-Doherty ( Jacqueline Austermann) said. “If our lower estimate is true, it means that the ice sheet will react to warming, but it may not be as dramatic as we thought.”

One takeaway from the study: evidence of higher estimates of sea level rise during the last interglacial period comes from many other places, including the Mediterranean, Indian Ocean, and Australia. The authors of the new study believe that the previous analysis may be based in part on a false premise that the ice sheet that covered North America 128,000 years ago was the same size as the most recent ice sheet. New research shows that the previous ice sheet was actually smaller, which may affect calculations in other locations. Previous estimates may also be affected by assumptions about the size of independent ice sheets covering Scandinavia, Northern Europe, and northwestern Russia.

For researchers William D’Andrea and Jacqueline Austermann, a small cave on Crooked Island can be sheltered from rain, and it is possible to observe the fossil ripples formed on ancient sand dunes up close. (Blake Dell)

In addition, the warm temperature of the last interglacial period was caused by the slow changes in the direction of the earth relative to the sun over thousands of years, and may not affect both poles at the same time.Research co-author Maureen Raymo, director of Lamont-Doherty and co-dean of the new school Columbia Climate School, Saying this may mean that while some polar regions are losing ice, others may increase. This will help balance things and limit global sea level rise. “This is still a problem. The ice sheet model is still in a toddler stage,” she said. She pointed out that compared with the last interglacial period, human carbon emissions are now making global warming much faster and more uniform, so there is no guarantee of any such buffer. “This makes it more difficult to apply the results to today,” she said. “It’s easy to say,’Oh, we proved that the sea level is not that bad. This is great.’ The harder it is to answer, the more honest the answer is, maybe the situation was different at the time, and we still don’t know.”

This study is not the first to propose a lower sea level estimate for the last interglacial period. Last year, a study Peter Clark of Oregon State University said the water level may have risen by about 4 meters. However, the research is strictly based on modeling, not new geological evidence from new research.

Robert Cope, a climate scientist at Rutgers University His works are widely cited As evidence of much higher sea levels during the last interglacial period, this study “should stimulate a substantial critical assessment of our understanding of the last interglacial period — or what we think we know about it. Given that it is based on a single Area, I think it should be seen as an alternative hypothesis for popular assessment [rather] Not as a new best estimate. He said that the next step will be to re-examine other regions.

Researchers plan to do this, and are already studying new assessments of sites in Denmark, France, England, and South Africa.

Robert DeConto, a scientist researching polar climate change at the University of Massachusetts Amherst, said of this research: [last interglacial] sea ​​level. Considerable uncertainty, the possibility that the sea level is much higher than 1.2 meters still exists. “

One thing is certain: the glacial equilibrium rebound is still working. In our current interglacial period, the Bahamas at this time seems to be sinking unconsciously, but this is not the case on the East Coast of the United States. In many regions, the Atlantic Ocean rises by 1 to 2 millimeters per year. At the same time, as the ice sheet further north melted thousands of years ago, the coast is sinking by the same amount. Floods caused by the resulting double blow can already be seen in many areas.

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

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