A few thousand years into the last interglacial, the sea level was ten meters higher than today. A study published in Nature Communications recently shows that the extra water must have come from Antarctic ice melt. Only later in the interglacial did meltwater from Greenland contribute to sea level rise.

The last interglacial lasted from 130,000 to 118,000 years ago. That was the last time the ocean stood higher than now. The sea level varied, but was for longer periods 6–9 meters higher than present. The increase occurred in three leaps. In one period the global sea level increased by 3 meters per century.

«We have known for half a century that the sea level was higher during the last interglacial”, says Eirik Vinje Galaasen from the Bjerknes Centre for Climate Research and the Department of Earth Science at the University of Bergen, one of the researchers behind the study. “But we have not known which ice sheet contributed the most, and when».

There were two obvious candidates: Antarctica and Greenland. For one of them, data existed.

The chemistry of the ocean floor reflects the sea water of the past

The scientists that led the study, from The Australian National University in Canberra, had samples that showed the development of the global sea level during the last interglacial. But these data showed only how high the sea level was and not where the extra water had come from.

For Antarctica, information was sparse. But in Bergen, there were data from the bottom of the sea outside Greenland. These could show how much meltwater that had poured into the ocean there.

“We cannot explain a 6–9 meters’ sea level rise with contributions from Greenland”, says Nil Irvali, also from the Bjerknes Centre and the University of Bergen.

Nil Irvali has worked on data that show the amount of meltwater coming from Greenland. She collected fossilized plankton, foraminifera, from sediment samples from the ocean floor. These tiny organisms live in the upper layers of the ocean, and when they die, they sink and become part of the sediments that layer by layer builds up the ocean floor. The sediments were collected at 3400 meters’ depth southwest of Greenland, and from 23 to 28 meters down into the sediments, the last interglacial was neatly stacked.

The chemical composition of the fossils depends on the temperature and other conditions in the water when the organisms lived. By analyzing the fossils, the researchers could tell how much meltwater from Greenland had been in the ocean during different parts of the last interglacial. Similar data from the Red Sea were the basis for the estimates of the global sea level.

That made the equation simple. Data from the Red Sea showed how much higher the sea level was globally. Data from the ocean floor outside Greenland showed how much of this water that came from Greenland. The rest must have come from Antarctica.

“This is one of the first attempts to separate Greenland from the Antarctic ice sheet with empirical data, not just inferences”, says Eirik Vinje Galaasen.

The results suggest that Antarctica dominated completely.

The sea level increased step-wise

The sea level increased in three separate leaps. At the most, about 129,000 years ago, the sea level increased with almost three meters per century. It then decreased a bit before again increasing abruptly around 1500 years later. This brought it to about ten meters above present-day levels.

In both these leaps, the meltwater came from Antarctica. Only in the third and smallest, about 124,000 years ago, meltwater from Greenland contributed.

The first leap came early in the interglacial. The ice age was over, and the sea level had increased 120 meters as the continental ice sheets melted. The freshwater released during this deglacial phase stabilized the ocean, halting the overturning circulation that today brings water around the world through currents both at the surface and in the deep ocean.

The first rapid melting occurred at a time when the ocean circulation was restored. More warm deep water flowed southwards through the Atlantic to Antarctica. Data from ice cores and the ocean outside the continent have shown that temperatures also were higher at the sea surface and on land.

Antarctica is still influenced by the sea

Today, increasing greenhouse gas concentrations in the atmosphere lead to higher temperatures in both polar regions at the same time. This uniform warming may cause the ice sheets to retreat simultaneously in both hemispheres, as opposed to the asynchronous melting of the last interglacial.

“The current interglacial is different from the last, and so is the current warming”, says Eirik Vinje Galaasen.

Galaasen and Nil Irvali still warn against thinking that similar abrupt changes could not happen today.

During the last interglacial, the ice in Antarctica melted mainly because it was warmer there. Why it was warmer, the scientists still do not know, but ocean currents may have contributed. Neither do they know what happened when so much ice melted and poured into the ocean. But they do know that more ice melts when warm sea water flows under the ice shelfs around Antarctica. They also know that the amount of ice reaching the ocean depends on the dynamics of the ice streams transporting ice from the Antarctic ice sheet.

It was like that during the last interglacial, and it still is. Antarctic ice melt depends on more than the global mean temperature.

«If the same processes kick in again, and the northern and southern ice sheets now melt in phase, there is a possibility for even higher sea level increases in the coming centuries than in the last interglacial”, says Eirik Vinje Galaasen.

About the research The work was part of the THRESHOLDS project funded by the Research Council of Norway.



Sediment samples were analyzed at FARLAB at the Department of Earth Science at the University of Bergen, and Rutgers University.

Reference:

Rohling, E.J., Hibbert, F.D., Grant, K.M. et al. Asynchronous Antarctic and Greenland ice-volume contributions to the last interglacial sea-level highstand. Nat Commun 10, 5040 (2019) doi:10.1038/s41467-019-12874-3