When were sea levels highest?

Sea levels are rising as climate change rapidly melts glaciers and ice sheets and the water within the oceans expands in a warming world. But have sea levels ever been higher than they are today? And when were they the highest?

In short, sea levels have easily been higher than they are today. But it's still unclear exactly when they were at their highest, although scientists have a few ideas.

Within the past half-billion years, sea levels likely peaked 117 million years ago, during the Aptian age. At this time, which was part of the Cretaceous period (145 million to 66 million years ago), sea levels were around 700 feet (200 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 were in the Cretaceous, at the time when the dinosaurs were walking the Earth," Douwe van der Meer, the study's lead author, an exploration geoscientist in the oil and gas industry and a guest researcher at Utrecht University in the Netherlands, told Live Science. "Beyond that, it's basically speculation," Jun Korenaga, a professor of Earth and planetary sciences at Yale University, told Live Science.

Korenaga's research suggests that sea levels were higher much earlier in Earth's roughly 4.5 billion-year-old history, when the first continents were still forming and Earth's surface was nearly devoid of dry land.

In the short term, sea level is a function of melting ice. For instance, when Antarctica's "Doomsday" Thwaites Glacier melts, the entire West Antarctic Ice Sheet may collapse, increasing the average global sea levels by around 11 feet (3.4 m). In the long term, shifting continents and a stretching seafloor also come into play. And then there's the curveball: Korenaga believes the early oceans held more water than they do today. Since the planet's genesis, oceans may have been slowly draining into the Earth's mantle.

Related: How will sea levels change with climate change?