Three million years ago when temperatures were a few degrees warmer than they are now, the seas were 16m higher.
That's the conclusion of a new study which looked at deposits left in coastal caves inundated during the Pliocene.
"Constraining models for sea level rise due to increased warming critically depends on actual measurements of past sea level," said researcher Victor Polyak of the University of New Mexico.
"This study provides very robust measurements of sea-level heights during the Pliocene."
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Between 3 million and 3.2 million years ago, temperatures were about 2C to 3C warmer than they were before the Industrial Revolution.
"The interval also marks the last time the Earth's atmospheric CO2 was as high as today, providing important clues about what the future holds in the face of current anthropogenic warming," said University of South Florida geoscience professor Bogdan Onac.
Since the revolution, the Earth's atmosphere has risen about 1C. If it keeps rising, scientists predict there will be stronger storms, increasingly erratic weather and more droughts.
Sea levels will rise as the ice sheets melt, and due to thermal expansion - warm water takes up more room than cold.
Even if humanity managed to halt the rising temperatures where they are now, the scientists behind the new research say a sea level rise between 5.6m and 19.2m could still happen, based on their measurements.
"Considering the present-day melt patterns, this extent of sea level rise would most likely be caused by a collapse of both Greenland and the West Antarctic ice sheets," said PhD student Oana Dumitru of the University of South Florida, who led the research.
Four million years ago, when temperatures were 4C above where they are now, the scientists measured sea levels more than 23m above where they are now.
"This is a possible scenario, if active and aggressive reduction in green house gases into the atmosphere is not undertaken," said Professor Yemane Asmerom of the University of New Mexico.
The UN estimates the seas will rise only about 65cm by the end of this century, but admits that is a "conservative" estimate.
The latest research was published in journal Nature.