Ice sheets hold back Antarctica’s glaciers from adding to sea level – but they’re collapsing – The Conversation

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When Antarctica’s slow-moving ice rivers hit the sea, they float, forming ice sheets. These shelves extend the glaciers into the ocean until they calve into icebergs.

But they also play a crucial role in preserving the world as we know it, by acting as a brake on how fast the glaciers can flow into the ocean. If they weren’t there, the glaciers would flow into the sea faster and melt, so the sea level would rise.
Unfortunately, the Antarctic ice sheets are not what they used to be. In research published today in Nature, we show that these ice sheets have significantly decreased in area in the last 25 years due to more and more icebergs breaking off. In total, the net loss of ice is about 6,000 billion tons since 1997.

Previous estimates of ice sheet loss come from satellite measurements, which show that the ice sheets have been gradually disappearing in recent years. We tracked how much additional ice was lost as icebergs calved away from the retreating edge of the continent. We found that the Antarctic ice sheets have lost twice as much mass as previous studies suggested.

Ice sheets are weaker now than at any time since at least the 1990s. This has led the glaciers of Antarctica to start adding more water to the oceans, and faster. Until now, much of the concern about the cryosphere – the frozen parts of the world – has focused on the rapidly melting Arctic sea ice. But as climate change intensifies, Antarctica will begin to melt seriously, contributing more to sea level rise.

Sørsdal Glacier in East Antarctica, where meltwater lakes appeared.
Sarah Thompson, Author provided

What we measured

We built numerical models to find out what ice sheet thinning and loss of area mean for the ability of ice sheets to resist being pushed in by new ice from upstream glaciers.

Our work shows that the decline in ice sheet area since 2007 has led to more ice flowing into the sea, as calving has weakened ice sheets and could accelerate some of the world’s largest glaciers.

We found the Pine Island Glacier and the so-called “Doomsday” Thwaites Glacier – which could destabilize the entire West Antarctic ice sheet if it melts – are highly susceptible to calving, increasing their contribution to sea level rise even as their protective ice sheets crumble.

Iceberg calving is a natural process. In any climate, we expect massive flat-topped icebergs to periodically break off and drift away. So while no single calf event should be taken for granted cause for alarm, the long-term trend is concerning. We found that a majority of the Antarctic ice sheets have lost mass since the late 1990s.

Why are Antarctic ice sheets shrinking? There is no single answer. Some ice sheets such as the Wilkins Ice Shelf have already seen catastrophic melting, while others are slowly retreating and some are even advancing. But generally these ice sheets are shrinking.

We know one cause of ice shelf retreat is the thinning of ice sheets, which is largely caused by relatively warm sea water eroding the base of these boards.

We also know that iceberg calving increases when Antarctica is protected ring of sea ice weakened. Antarctica saw the lowest sea ice extent this year ever recorded since measurements began in the 1970s. We have also seen entire ice shelves collapse when warmer air temperatures create surface meltwater that be able to cut through hundreds of meters of ice sheet.

Read more:
Warmer summers threaten Antarctica’s giant ice sheets through the lakes they create

Four giant sheets of ice are still in good shape

The four largest ice sheets of Antarctica are the Ross, Ronne, Filchner and Amery. These large floating sheets of ice tend to calve giant icebergs once every few decades.

All four are on track for major calving events in the next 10 to 15 years, and none would normally be cause for alarm. The problem is calving will come on top of steady ice shelf loss. As the great ice sheets melt huge icebergs, they will leave the Antarctic ice sheet smaller than we have ever seen it.

table iceberg

Huge icebergs like this are often formed by calving of ice sheets. This iceberg near the coast of West Antarctica is seen from a window of a NASA Operation IceBridge aircraft in 2016.

But while we still don’t see any abnormal behavior in these four large ice sheets, the overlooked losses from all the smaller ice sheets bounding the continent add up. Earlier this year another smaller ice sheet completely collapsed.

The most troubling changes of the past few decades are less photogenic than sudden ice sheet collapses. Bit by bit, the Thwaites Ice Sheet has retreated from West Antarctica.

Read more:
How an almost perfect rectangular iceberg was formed

Each calving event has weakened the ice sheet and allowed the Thwaites Glacier behind it – the size of the state of Victoria – to flow faster into the ocean. While the Thwaites Ice Sheet is relatively small, it is vital. Until now it acted as a plug. If it continues, it could potentially destabilize the entire West Antarctic Ice Sheet and unleash several meters of sea level rise.

Climate change is the big picture

Our warm atmosphere and ocean are the cause. Given the long lag between heat-trapping greenhouse gases and actual warming, it makes sense that what we’re currently seeing in Antarctica is at least in part a response to greenhouse gases being dumped into the atmosphere decades or even a century ago. That means we’re already locked into more ice sheet retreat as emissions continue to rise.

Antarctica has about 30 million cubic kilometers of ice, a really huge figure. That represents about 90% of the world’s fresh water. If it all melted, seas would rise almost 60 meters. Humanity’s decisions will shape what Antarctica will look like in the coming decades, and how much ice will remain.

Read more:
Ice world: Antarctica’s riskiest glacier is under attack from below and is losing its grip

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