Nasa finds Antarctica ice losses are double previous estimates

One study maps how iceberg calving – the breaking off of ice from a glacier front – has changed the Antarctic coastline over the last 25 years.

The researchers found that the edge of the ice sheet has been shedding icebergs faster than the ice can be replaced. The finding doubles previous estimates of ice loss from Antarctic’s floating ice shelves since 1997, from 6 trillion to 12 trillion tonnes.

Ice loss from calving has weakened the ice shelves and allowed Antarctic glaciers to flow more rapidly to the ocean, accelerating the rate of global sea level rise.

The other study shows in unprecedented detail how the Antarctic ice is thinning as ocean water melts it. The thinning has spread from the continent’s outward edges into its interior, almost doubling in the western parts of the ice sheet over the past decade.

“Antarctica is crumbling at its edges,” said scientist Chad Greene, lead author of the calving study. “And when ice shelves dwindle and weaken, the continent’s massive glaciers tend to speed up and increase the rate of global sea level rise.”

Most Antarctic glaciers flow to the ocean, where they end in floating ice shelves up to three kilometres thick and 800 kilometres across. Ice shelves act like buttresses to glaciers, keeping the ice from simply sliding into the ocean. When ice shelves are stable, they have a natural cycle of calving and replenishment that keeps their size fairly constant over the long term.

But in recent decades, the warming ocean has been destabilising Antarctica’s ice shelves by melting them from below, making them thinner and weaker. Satellite altimeters measure the thinning process by recording the changing height of the ice, but until this study, there hasn’t been a comprehensive assessment of how climate change might be affecting calving around the continent.

That’s partly because satellite imagery has been challenging to interpret.

“You can imagine looking at a satellite image and trying to figure out the difference between a white iceberg, white ice shelf, white sea ice, and even a white cloud. That’s always been a difficult task,” Greene said.

“But we now have enough data from multiple satellite sensors to see a clear picture of how Antarctica’s coastline has evolved in recent years.”

The researchers synthesised satellite imagery of the continent in visible, thermal infrared (heat), and radar wavelengths since 1997. Combining these measurements with an understanding of ice flow gained from an ongoing Nasa glacier-mapping project, they charted the edges of ice shelves around 50,000 kilometres of Antarctic coastline.

Losses from calving have outpaced natural ice-shelf growth so greatly that the researchers think it’s unlikely Antarctica can grow back to its pre-2000 extent by the end of this century. In fact, the findings suggest that greater losses can be expected: Antarctica’s largest ice shelves all appear to be headed for major calving events in the next 10 to 20 years.

In the complementary study, scientists combined almost 3 billion data points from seven space-borne altimetry instruments to produce the longest continuous data set on the changing height of the ice sheet – an indicator of ice loss – from as early as 1985.

They used radar and laser measurements of ice elevation, accurate to within centimetres, to produce the highest-resolution monthly maps of change ever made of ice loss.

The new record reveals how long-term trends and annual weather patterns affect the ice. It even shows the rise and fall of the ice sheet as subglacial lakes regularly fill and empty miles below the surface. “Subtle changes like these, in combination with improved understanding of long-term trends from this data set, will help researchers understand the processes that influence ice loss, leading to improved future estimates of sea level rise,” said Johan Nilsson, lead author of the study.