A 300-foot-wide, 70-mile-long rift in Antarctica's Larsen C Ice Shelf, as seen in November 2016. John Sonntag/IceBridge/NASA Goddard Space Flight Center

Antarctica is about to lose a very large iceberg that could calve "within days" or hours.

The iceberg will rival twice the volume of Lake Erie.

It may be the third-largest iceberg recorded since satellites began taking photos of Earth.

Human activity likely isn't responsible for this event, but carbon emissions are driving other worrisome changes to Antarctic ice.

A widening, meandering crack in an Antarctic ice shelf is about to birth a colossal iceberg, and new satellite imagery gives the best sense yet of the object's mind-boggling size.

Researchers noticed the distinctive rift in Antarctica's Larsen C ice shelf in 2010, but that fissure has been growing most rapidly since 2016.

Now just 2.8 miles of ice is keeping the iceberg connected to Larsen C, wrote Adrian Luckman, a glaciologist at Swansea University in the UK, in a July 6 tweet.

"It is remarkable how the moment of calving is still keeping us waiting," Luckman and his colleague Martin O'Leary wrote in a July 7 blog post for the Impact of Melt on Ice Shelf Dynamics and Stability project, known as Project MIDAS. "[T]his event will fundamentally change the landscape of the Antarctic Peninsula."

How big the new Antarctic iceberg will be

The ice block's area is roughly comparable to the US state of Delaware. But CryoSat — Europe's ice-monitoring satellite — recently took the most precise measurements to date of its thickness, allowing scientists to gauge its total volume.

When the crack splits open, the resulting iceberg entering the Southern Ocean will be about 620 feet (190 meters) thick and harbor some 277 cubic miles (1,155 cubic kilometers) of ice, Noel Gourmelen, a glaciologist at the University of Edinburgh, said in a European Space Agency press release.

That's big enough to fill more than 460 million Olympic-size swimming pools with ice, or more than twice the volume of Lake Erie — among the world's largest freshwater reservoirs.

Gourmelen and the ESA on Wednesday released this 3D animation that shows the iceberg's dimensions:

And here's Lake Erie for a size comparison:

The volume of Lake Erie is less than half that of the iceberg that's about to calve from Antarctica's Larsen C ice shelf. Google Maps; Business Insider

The iceberg could break off of Antarctica "within days" or even hours, researchers say. When it does, no one is sure what will happen.

"It could, in fact, even calve in pieces or break up shortly after. Whole or in pieces, ocean currents could drag it north, even as far as the Falkland Islands," Anna Hogg, a glaciologist at the University of Leeds, said in the ESA release. (Those islands lie more than 1,000 miles away from Larsen C in Antarctica.)

If the iceberg does stay intact, it could be the third-largest recorded since satellite measurements began, The Antarctic Report wrote in a July 6 tweet.

A block of ice thousands of years in the making

A map of Antarctica's biggest ice shelf systems. Larsen C was the fourth-largest — now the fifth-largest — until iceberg A68's calving between July 10 and July 12, 2017. Diti Torterat/Wikipedia (CC BY 2.0) Antarctica's Larsen C ice shelf is one of the largest such shelves in the southern continent.

According to a tweet from Project MIDAS, "most of the ice that calves off fell as snow on the ice shelf in the past few hundred years, but there's an inner core that's a bit older."

Project MIDAS announced in early June that satellite images showed the rift had split, turned north, and begun moving toward the Southern Ocean.

Luckman, who has closely monitored Larsen with his colleagues at Project MIDAS, just released an animation of the rift's rapid growth (below) that shows how it "jumps" as it slices through bands of weak ice. The ocean is shown in emerald green (top right), the Larsen C ice shelf is the light-blue patch, and the glacier behind it is depicted in white.



The final frame shows an image of the rift's tip breaking in multiple directions — a sign of imminent calving of the iceberg.

A close-up from the ESA's Copernicus satellite more clearly shows the chaos of the crack's tip:

ESA; CPOM News/Twitter

While it's impossible to say precisely when the rift will snap the ice off, this recent data has upped the stakes for the iceberg's eventual calving.

"New Sentinel-1 data today continues to show the rift opening more rapidly. We can't claim iceberg calving yet, but it won't be long now," O'Leary tweeted from the group's account on June 30.

Are humans behind this?

When the iceberg calves, it won't noticeably raise sea levels, since it's already floating in the ocean and displacing that water. But Luckman and O'Leary said that once Larsen C loses the iceberg, the rest of the shelf "will be less stable than it was prior to the rift."

Put another way: There's a very slim chance that this break could cause the entire Larsen C ice shelf, and an ancient glacier behind it, to slowly disintegrate and fall into the sea.

The chaos wouldn't be unprecedented. In 2002, a neighboring ice shelf called Larsen B collapsed and broke up in the Southern Ocean. This animation captures that event unfolding from January 31 through April 13, 2002:

If Larsen C and its accompanying glacial ice collapse, some scientists think sea levels may rise by up to 4 inches.

However, experts on Antarctic ice say that such a loss is exceedingly unlikely and would mostly be due to natural processes.

"Large calving events such as this are normal processes of a healthy ice sheet, ones that have occurred for decades, centuries, millennia — on cycles that are much longer than a human or satellite lifetime," Helen Amanda Fricker, a glaciologist who studies Antarctic ice for the Scripps Institution of Oceanography, wrote in The Guardian last month. "What looks like an enormous loss is just ordinary housekeeping for this part of Antarctica."

Buy Fricker warned that we shouldn't be complacent about climate change, which is mostly being driven by human activity.

"Antarctic ice shelves overall are seeing accelerated thinning, and the ice sheet is losing mass in key sectors of Antarctica," she said. "Continuing losses might soon lead to an irreversible decline."

Correction: In a previous version of this story, we misstated the iceberg's volume as comparable to that of Lake Michigan. The iceberg's size is closer to double the volume of Lake Erie.