Antarctica is melting and one of its largest glaciers is collapsing from underneath. This is Thwaites glacier, sometimes called the doomsday glacier – and for good reason. If it destabilises, it could trigger a wider collapse of the West Antarctic ice sheet, raising sea levels by up to 5 metres. Scientists believe we may be approaching a tipping point that could completely redraw the world’s coastlines, putting cities such as Kolkata, New York and London at risk of severe flooding, displacing millions of people.

In our latest video, we speak directly to scientists involved in an international effort to better understand what is happening with Thwaites, what the risks involved are and why it’s potentially much more devastating than we previously thought.

Read more: The ‘doomsday’ glacier’s giant ice shelf is about to break away

Scientists have been debating why Antarctic sea ice, which once seemed impervious to climate change, has shrunk dramatically in the past decade. Now research suggests stronger winds have churned up warming water from the deep ocean, breaking through upper water layers that were protecting the ice from melt.

While Arctic sea ice has declined about 40 per cent over four decades, until recently the sea ice around Antarctica was slightly expanding, confounding most climate models. Then after 2015 ice extent fell from a record high to several record lows, losing an the size of Greenland.

Some has suggested the sea ice may be melting largely due to air temperatures, which have been so high in recent years that Antarctic researchers have posed for in swimwear. Two new studies make the case that ocean warming played a bigger role in this “regime shift”.

“Plenty of people will say… that it was atmospheric warming which melted the sea ice from above,” says at the National Oceanography Centre in Southampton, UK, who wasn’t involved in the research. “Now these scientists have done the thorough analysis and have got a plausible chain of events, which says that the ocean is the key player in that 2016 melt. Nobody’s put that argument together before.”

As part of global ocean circulation, a mass of warm, salty water called circumpolar deep water flows southward from the tropics and circles Antarctica at depths below 200 metres. But it’s increasingly coming to the surface where it can melt sea ice, two decades of temperature and salinity measurements from several hundred drifting buoys suggest.

Antarctica is surrounded by a belt of strong winds and storms in the latitudes of the “roaring forties”, “furious fifties” and “screaming sixties”. Climate change has shifted this storm track southward, bringing more precipitation into the sea ice zone, according to a study by at Stanford University and his colleagues. Initially, the precipitation created a layer of fresh surface water that better insulated the bottom of the sea ice from warm deep water, allowing it to expand to its 2014 record extent.

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But the southward-shifted storm track also delivers stronger winds that blow surface water and ice forward. Due to the spinning of the Earth, water moves 90 degrees to the left of the wind direction, generating spirals like the Weddell Sea gyre. As surface water is flung to the edges, deep water wells up from below to fill the void at the centre.

Between 2014 and 2016, this wind-driven upwelling began to win the “tug-of-war” against the protective layer of increased precipitation, and the sea ice began to melt away in the Weddell Sea. When the researchers plugged the observed changes in temperature and salinity into a simple computer model, it projected sea ice would expand and then contract, as it did in the real world.

“Most signs point to a persistent and sustained decline in sea ice, because even with the precipitation potentially suppressing the deep ocean heat… the heat is still there,” Wilson says. “All it would take is a sudden reversal of conditions for that heat to come back up.”

That reversal began with a string of wind storms, according to the second study by at the Alfred Wegener Institute in Bremerhaven, Germany, and his colleagues.

Even before the additional precipitation of recent years, the warm circumpolar deep water had been kept away from the surface layer by winter water, a layer of cold, salty water created when the atmosphere cools the upper ocean in winter and sea ice forms, rejecting salt ions from its new crystalline structure.

But the deep water has been getting hotter due to global warming. Because water expands when it’s warmer, the deep water has been taking up more space, thinning the winter water. In 2015 and 2016, stronger-than-average winds brought up more deep water across the winter water barrier. The layering hasn’t recovered since.

The finding suggests that even if the strong winds were a natural fluctuation, the stage had been set by global warming.

“It’s the wind that pushes [sea ice] over into these rapid declines, but it’s the ocean that really keeps it low,” Spira says. “There’s definitely evidence that we’re in a new regime.”

While sea ice melt doesn’t raise sea levels, it could harm species that spend part of their lives on this ice, like krill or penguins. And if sea ice recedes near key ice shelves it could impact global ocean currents, including the Atlantic Meridional Overturning Circulation that keeps Europe warm. That’s because salt rejection from ice formation close to shore helps form Antarctic bottom water, which is more dense than winter water and circumpolar deep water. The bottom water cascades off the continental shelf and flows north along the seabed.

“If you were to reduce sea ice production in those regions… you’ll have less bottom water and potentially a slowdown of the meridional overturning circulation,” Wilson says, although he notes that freshwater from glacier melt has a larger effect on bottom water.

Meltwater on a city‑sized iceberg in the Southern Ocean is rapidly forming a giant pool on its surface – possibly a sign that it is close to breaking apart.

Scientists are captivated by the frozen colossus, known as A23a, because meltwater is collecting and being held on its surface in an unusual way.

Satellite images reveal a raised rim of ice running around the entire cliff edge of the tabular Antarctic iceberg, giving it the appearance of an oversized children’s play pool — except this one spans about 800 square kilometres, an area larger than Chicago.

In places, the ponded water appears a deep, vivid blue, suggesting depths of several metres. Across the whole of A23a, the water volume probably runs into billions of litres – enough to fill thousands of Olympic‑sized swimming pools.

at the University of Chicago says the rim effect is typical of the world’s largest icebergs.

“My theory is that the edges are bent, nose‑down, creating an arch‑like dam on the top surface that keeps the meltwater inside,” he says. “The bending is probably a combination of cliff-face undercutting by waves and melting, and the natural tendency for ice cliffs to bend over even if they would be perfectly vertical otherwise.”

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The streaks of surface water visible in the satellite imagery are a relic of the way the ice once flowed when the iceberg was still attached to Antarctica’s coastline, he says.

A23a is an old iceberg. It calved from the Filchner–Ronne ice shelf in 1986 and was then more than five times its current size. For a while, it held the title of the world’s largest iceberg.

In recent years, however, it has drifted north into warmer waters and air, and is now undergoing relentless fragmentation. The sheer volume of meltwater pooling on its surface may finally break it apart. “If that water drains into cracks and refreezes, it will prise the berg open,” says at the British Antarctic Survey.

It could, he says, turn to mush almost overnight.