When did sea levels reach their highest point in history?

About 117 million years ago, sea levels were about 210 m higher than they are today, but this may not have been the highest level in history.

Icebergs on the southwest coast of Greenland. Photo: NASA/JPL-Caltech — Icebergs on the southwest coast of Greenland. Photo: *NASA/JPL-Caltech*

Sea levels are rising as climate change causes glaciers and ice sheets to melt rapidly, and the amount of water in the oceans to increase as the world warms. So have sea levels ever been higher than they are now, and when were they highest? According to scientists, sea levels were much higher in the past than they are today, but it is unclear exactly when they were at their highest.

Considering the past half-billion years, sea levels likely peaked 117 million years ago, during the Aptian period of the Cretaceous. During this period, sea levels were about 210 meters higher than they are today, according to a 2022 study in the journal Gondwana Research .

"Over the past 540 million years, the highest sea levels occurred during the Cretaceous period, when dinosaurs roamed the Earth," said geologist Douwe Van der Meer at Utrecht University, lead author of the study.

“For more distant times, it’s basically speculation,” said Jun Korenaga, a professor of earth and planetary sciences at Yale University. According to Korenaga’s research, sea levels were higher early in Earth’s 4.5-billion-year history, when the first continents were forming and the Earth’s surface was largely devoid of dry land.

In the short term, sea level rise is closely linked to ice melt. For example, if the Thwaites Glacier in Antarctica melts, the entire West Antarctic Ice Sheet could collapse, raising global average sea levels by about 11 feet (3.4 m). In the long term, shifting continents and expanding seafloors also affect sea levels. Korenaga also believes that the early oceans contained more water than they do today. Since the Earth’s formation, the oceans may have gradually percolated down into the planet’s mantle.

The last time the seas were higher than their present levels was about 120,000 years ago, during the Last Interglacial Period. During this period, warmer climates melted the Antarctic ice, causing peak sea levels to be about 6 meters higher than their current average.

When Earth was completely or almost ice-free, sea levels could have been 10 times higher than during the Last Interglacial. "If you go back about 50 million years ago, when there was no ice in Greenland and Antarctica, sea levels would have been about 70 meters higher," says Van der Meer.

Sea levels were highest when ice was at its lowest, but that is not a complete explanation for the high sea levels during the Cretaceous, when 30% of what is now dry land was underwater. Tectonic plates also played a role.

Van der Meer estimates that sea levels peaked around the time the South American plate moved away from Africa, about 200 million to 100 million years ago. The plates were pushed apart as the South Atlantic Ocean formed in between. The new oceans tended to be shallower than the ones they replaced, Van der Meer says. During the Cretaceous, the combination of a lack of polar ice and shallow oceans led to the highest sea levels in about half a billion years.

Going back more than half a billion years, to a time when there is little geological evidence and scientific data, sea levels may have been very high. In a study in the journal Philosophical Transactions of the Royal Society A , Korenaga and his colleagues estimate that Earth’s early surface contained twice as much water as it does today.

Like ocean plates, water can circulate in and out of magma beneath the Earth’s crust. Korenaga’s calculations suggest that some of the water in Earth’s surface oceans has been lost over billions of years. If his calculations are correct, even though sea levels continue to rise today, the peak may already be in the past. Earth’s earliest seas were higher simply because there was more water.

Thu Thao (According to Live Science )

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