In 2002, scientists at NASA were tracking daily satellite images of Antarctica when they watched in astonishment as a huge chunk of the Larsen B Ice Shelf splintered and collapsed in just a month.
This was an area of ice the size of Rhode Island — 1,250 square miles — and it just disintegrated before their eyes:
"This ice shelf has existed for at least 10,000 years, and soon it will be gone," said Ala Khazendar of NASA's Jet Propulsion Laboratory, who led the study. The video below gives an overview:
While Larsen B is a particularly dramatic case, it's not an isolated one. As the planet warms up, a number of Antarctica's ice shelves have been melting dramatically in recent decades — with serious consequences for rising sea levels around the world.
Why collapsing ice shelves are a big deal
First, some terminology. Antarctica is covered by vast ice sheets that sit atop the continent. Many of these ice sheets flow, gradually, toward the ocean, where they form ice shelves that float on the water. Over time, chunks of these ice shelves break off — or "calve" — to form icebergs.
These ice shelves essentially keep the ice sheets behind them hemmed in — think of the shelves as a pie crust, preventing the filling from leaking out:
For thousands of years, this system was roughly stable. Ice would flow gradually into the oceans. But this was all counterbalanced by snow that was falling back on top of Antarctica and replenishing the ice sheets.
Over the last two decades, however, that's changed. As global warming has progressed, Antarctica's ice sheets are now losing 147 gigatons of ice each year, mainly from the Amundsen Sea sector of West Antarctica. The system is no longer in balance, and Antarctica's ice is flowing more quickly into the ocean than before.
One reason for that? Those floating ice shelves that keep the land ice hemmed in have been thinning and at times collapsing. That's partly because water beneath the shelves has gotten warmer, eating away at the ice from below. Stronger winds have also pushed more of that warm water into the shelves.
A big 2015 study in Science estimated that Antarctica's ice shelves have thinned by up to 18 percent in the last two decades — mainly in West Antarctica but also around the bigger East Antarctica region. Meanwhile, some shelves have been collapsing entirely. Scientists watched the Larsen A Ice Shelf collapse in 1995. Then the Larsen B Ice Shelf in 2002. Then the Wilkins Ice Shelf in 2008.
When ice shelves collapse, they don't immediately affect sea-level rise much, since their ice was already floating in the water anyway. But the disintegration of these shelves allows the ice behind them to flow more rapidly into the sea — and that does have the potential to significantly speed up sea-level rise.
After Larsen A and Larsen B collapsed, the glaciers behind them began losing ice into the sea 300 percent faster than before. Overall, studies have found that the loss of ice shelves has contributed to a 59 percent increase in ice discharge from regions of West Antarctica. That's all helping to push up global sea levels.
So how high will sea levels rise?
This is the big question — and the one most relevant to coastal cities around the world.
For now, the Intergovernmental Panel on Climate Change (IPCC) projects that global warming will cause worldwide sea levels to rise between 1 foot and 3 feet, on average, by 2100. That's partly because ocean water expands as it warms. But it's partly because more ice from Antarctica, Greenland, and other glaciers is melting and flowing into the oceans, pushing up sea levels:
Yet some scientists have argued that the IPCC's projections actually underestimated the possibility of major ice loss from Antarctica. If so, it's possible that future sea-level rise could be on the higher end of projections.
The ultimate amount of sea-level rise will depend partly on how much the Earth actually warms in the coming decades, and partly on local conditions in Antarctica. Increased snowfall in East Antarctica, for instance, could partially counteract the increased loss of land ice.
Over the longer term, however, Antarctica has the capability of raising sea levels much, much higher. West Antarctica's ice sheet contains enough ice to raise sea levels worldwide by 10 to 14 feet, on average. In 2014, two studies argued that a major portion of West Antarctica's ice sheet has now been destabilized irreversibly, and its slide into the sea over the next 200 to 900 years is all but inevitable. The only real question is how fast it will go.
Then there's the even more massive East Antarctica ice sheet, which contains enough ice to raise global sea levels by another 190 feet or so. Thankfully, this sheet appears far more stable, and it would take many, many centuries for all that ice to melt. Yet if even a fraction of that ice slid into the sea, it would alter coastlines and cities dramatically for generations to come.
By the way, there's another aspect of Antarctica's climate that wasn't discussed above — namely, the floating sea ice that forms in the ocean around the continent during the winter.
This sea ice has actually been expanding to record highs of late, even as the planet has warmed, and scientists are still debating why. It's a fascinating story. But it's also not very relevant to global sea levels, since this ice was already floating in the sea. When it comes to sea-level rise, it's land ice that matters most — and Antarctica is losing its land ice, on net.