Even if governments across the globe somehow manage to overcome their intractable differences over how to deal with climate change, and succeed in the near-impossible task of limiting global warming to below 2 degrees Celsius (3.6 degrees Fahrenheit) above the pre-industrial levels, it may already be too late to prevent a drastic rise in global sea levels.

A team of scientists analyzing data from the interglacial period about 3 million years ago found that when average global temperatures were between 1.8 degrees Fahrenheit and 5.4 degrees Fahrenheit higher than pre-industrial levels, sea levels stood 20 feet higher than they are today. Sea level peaked somewhere between 20 feet and 40 feet above the present about 400,000 years ago, when global average temperatures are estimated to have been up to 3.6 degrees Fahrenheit warmer than the pre-industrial average. 

This rise in sea level can be attributed to the melting of ice sheets in Greenland and Antarctica, the scientists who conducted the study said, in a statement.

“This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate,” Andrea Dutton from the University of Florida, who led the study, said in the statement released Thursday. “As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond.”

Already, sea levels have risen nearly seven inches over the past 100 years. Correspondingly, several recent studies have spotted an uptick in the melting of Antarctica’s floating ice shelves, which act as doorstops and hold back its glaciers and ice sheets from spreading outward into oceans.

In some regions, the thickness of these shelves has fallen by as much as 18 percent over the past 18 years -- a process that has accelerated over the last decade. According to some estimates, if Antarctica’s ice sheet melts completely, it would raise sea levels by over 200 feet -- enough to flood the planet's land masses.

At the opposite end of the globe, the Arctic has also witnessed wide swings in its ice cover. According to recent observations, the annual winter maximum of ice cover in the North Pole this year was the lowest on satellite records, which date back to 1979.

So how would a sea-level rise of 20 feet impact the world? As this analysis by Climate Central points out, it would mean a loss of over 444,000 square miles of land, displacing 375 million people.

While Asian cities like Shanghai, Hong Kong, Dhaka, Mumbai and Calcutta -- where huge areas lie below the 20-feet mark -- would be the worst affected, coastal cities in the U.S., including much of the state of Florida, would also be submerged. Miami, which has an average elevation of about 6 feet, would probably be completely underwater.

“While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades,” Dutton said in the statement.