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This story originally appeared on BusinessInsider.com.

The first summer without an Arctic ice sheet is already on the horizon. The massive chunk of frozen ocean has capped the northern pole of our planet year-round for millennia, but it's now at risk of receding until it disappears entirely. 

Unlike its northern sibling, the kilometers-thick Antarctic ice cap in the south is seated on a buried continent rather than on water. It's bigger and older than the Arctic ice sheet, and less vulnerable to threats of a warming climate.

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Researchers generally agree, however, that the Antarctic will also lose significant amounts of ice mass as the Earth's temperature rises. The timeline and extent of that loss is just less clear. Unlike the charts of the Arctic's annual ice, which seem to have taken a plunge toward zero over the last decade, the Antarctic's process has been more wobbly. As recently as 2014, the southern ice cap reached is largest extent on record.

Here's what we know about Antarctica's strange, ancient ice, and what could happen in its future.

Most Antarctic ice sits on land, not open ocean.

That means the possibility of it melting poses a bigger threat to the planet — when Arctic ocean ice melts, sea level stays the same. But when Antarctic ice drains into the water, sea levels rise all over the world.

In early April, Antarctica's ice broke records for daily lows that had been set in 1980. As of May 10, the ice is still low but not record-breaking — there's 8.078 million square miles now, compared to 1980's 7.958 square miles.

There are immediate signs of trouble, though.

A wide crack in the Larson C ice shelf is drawing attention and concern.The Larson C ice shelf is a colossal block of sea ice off Antarctica's coast. It covers about 2,000 square miles of open ocean, and is 1,100 feet thick at the edge (and thicker toward the middle).

Getty Images/Science Source

The huge crack isn't new — it was first spotted in 2010 and has been fairly stable for the last few months.

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But a new crack has appeared at its leading edge, forking away from the existing rift. Researchers say this split signals that the iceberg — nearly the size of Delaware — is at risk of calving off.

The good news is that Larsen C already floats on open ocean, so it won't contribute to sea level rise if it breaks off. But this split is part of a larger story in the threatened Antarctic.

Larsen and other ocean ice shelves like it ring the continent, creating natural barriers that keep massive amounts of ice from sliding off the land into the ocean.

If Larsen C breaks off, some researchers worry it would be like removing a cork from a bottle of champagne. Once it pops off and into the sea, some land-based glaciers could flow out after it.

Related: Antarctica is melting more than scientists thought, and it’s creating giant waterfalls

Those blocks of ice would raise sea levels if they entered the ocean. In fact, if all of Larsen C were to fall off, the masses of landlocked ice it holds back could raise sea levels by four inches all over the world.

But this type of process is difficult to model and predict, because melted landlocked ice can slide into the ocean in hidden rivers far below the ice surface.

That means that not all ice loss is immediately noticeable — researchers need specialized equipment to track it. In Greenland, data revealed that summer melts have already increased 30% from 1979 to 2006.

Melting has been slower in Antarctica, however, and no one can say for certain whether the Larsen C ice shelf will calve, or if other similar ice blocks elsewhere in Antarctica might do the same.

NASA's Earth Science program is currently leading the effort to learn more about the frozen continent, though its funding may be threatened. The answers NASA finds, however, will help us understand the future of the planet.

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