For now, the best estimates suggest that Antarctica will sweat off enough ice to raise global sea levels by 1.5 to 3.5 feet [0.5-1.1m] by 2100, depending on how quickly humans continue to pump out greenhouse gases. Throw in Greenland and other rapidly melting glaciers around the world, and sea level could plausibly rise three to seven feet [1.0-2.4m] by 2100.
But that’s not the worst case: Sea level won’t stop rising in 2100. Earth’s past offers worrisome clues to what the more distant future might bring. Geologists studying ancient shorelines have concluded that 125,000 years ago, when the Earth was only slightly warmer than today, sea levels were 20 to 30 feet higher [6-9m]. Some three million years ago, the last time atmospheric carbon dioxide was as high as it is today, and the temperature was about what it’s expected to be in 2050, sea levels were up to 70 feet [21m] higher than today. Yet a collapse of the Greenland and West Antarctic Ice Sheets would raise sea level only about 35 feet [10m].