When did sea level reach its highest level in history?

About 117 million years ago, sea level was about 210 m higher than today, but this may still not be the highest level in history .

Sea levels are rising due to climate change, causing glaciers and ice sheets to melt rapidly, and the amount of water in the oceans increases as the world warms. So was sea level ever higher than it is today and when was it highest? According to scientists, sea levels in the past were much higher than today , but it is unclear exactly when they reached their highest level.

If we consider the past half billion years, sea level most likely peaked 117 million years ago , during the Aptian period of the Cretaceous period. According to a study in the journal Gondwana Research in 2022, during this period, sea level was about 210m higher than today.

Ice sheets on the southwest coast of Greenland. (Photo: NASA/JPL-Caltech).

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

"For more distant times, it's basically just speculation ," said Jun Korenaga, a professor of Earth and planetary sciences at Yale University. According to research conducted by Korenaga, sea levels were higher early in Earth's 4.5 billion-year-old history, when the first continents were forming and the Earth's surface was almost devoid of dry land. .

In the short term, sea level is associated with ice melt. For example, when the Thwaites glacier in Antarctica melts, the entire West Antarctic ice sheet could collapse, causing the global average sea level to rise by about 3.4 m. In the long term, the movement of the continents and the expansion of the sea floor also affect sea levels. In addition, Korenaga also believes that early oceans contained more water than today. Since Earth formed, oceans may have gradually seeped down into the planet's mantle.

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

When Earth is completely or almost ice-free, sea levels can be 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 level would be about 70 meters higher ," Van der Meer said.

Sea levels were highest when ice was least, but this is not a full explanation for high sea levels during the Cretaceous period, when 30% of today's dry land was underwater. Tectonic plates are also a cause.

Van der Meer estimates that sea levels were highest around the time the South American plate moved away from Africa, about 200 million to 100 million years ago. These plates were pushed apart as the South Atlantic Ocean formed in between. According to Van der Meer, the new oceans tend to be shallower than the oceans they replace. During the Cretaceous period, 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, a time when there was little geological evidence and scientific data, sea levels could also have been very high. In a study in the journal Philosophical Transactions of the Royal Society A , Korenaga and colleagues estimate that the Earth's surface originally contained twice as much water as it does today.

Like ocean plates, water can circulate in and out of the magma layer beneath the Earth's crust. Korenaga's calculations show that some of the water of Earth's surface oceans has been lost over billions of years. If this calculation is correct, even though sea levels continue to rise today, the peak is probably in the past. The first seas on Earth had higher water levels simply because there was more water then.