Even more stunning, it was only in the mid-2010s that Antarctic sea ice was at record highs—at least highs since satellite observations began—having increased slightly but steadily in the years since 1979.

That recent growth of Antarctic sea ice has been in stark contrast with that of the Arctic, a region that is now warming up to four times faster than the rest of the planet and has been steadily losing ice for decades. That’s due to a phenomenon called Arctic amplification: Melting ice exposes darker ocean water or land, which absorbs more of the sun’s energy than white ice, which in turn leads to more warming. 

The Antarctic is a different beast: It’s a frozen continent surrounded by open ocean, whereas the Arctic is an ocean of floating ice enclosed by land, like Russia, Alaska, and northern Canada. Antarctica’s ice is insulated, in a sense, by strong, cold ocean currents that swirl around the continent. Plus, Antarctica’s elevation is quite high, providing additional cooling.

Antarctica’s sea ice—which forms when seawater freezes—is distinct from the continent’s ice sheets and shelves. An ice sheet rests on the land, and can be thousands of feet thick. It becomes an ice shelf when it begins floating on coastal waters. While Antarctica’s ice sheets and shelves have indeed been deteriorating as the planet warms, the continent’s sea ice is much more seasonal, waxing and waning dramatically between winter and summer. 

Losing that sea ice won’t add to sea levels, just as melting ice cubes floating in a glass of water won’t cause the glass to overflow. (The ice is already displacing the water.) But sea ice plays a critical role in protecting Antarctica’s colossal ice shelves from deteriorating, and those could dramatically raise ocean levels if they break apart. If it totally melts, the Thwaites Glacier, aka the Doomsday Glacier, could add 10 feet...