Old aerial photos give scientists a new tool to predict sea level rise

On 28 November 1966, an American aeroplane flies over the Antarctic Peninsula just south of the southernmost tip of Chile.

On board is a photographer, probably from the US Navy, whose job is to map the Antarctic landscape. But it turns out that the photographer is also documenting a very special situation that is in progress. He shoots an aerial photo of the Wordie Ice Shelf, which, 30 years later, has almost vanished after a total collapse.

The consequence of this collapse was that the ‘plug’ that held large amounts of glacier ice broke off, leaving the ice sheet floating freely into the sea.

Fortunately, Wordie is a relatively small ice shelf and the sea level rise it has caused can be measured in millimetres. But there are much larger ice shelves in Antarctica, which, like Wordie, could collapse due to climate change. Just the two largest ice shelves, Ronne and Ross, are believed to hold enough ice to account for sea level rises of up to five metres.

If, in this context, we think that Antarctica is far away, we need to understand that the melting of ice in the Southern Hemisphere will cause sea levels to rise in places like Denmark in the Northern Hemisphere, due to the effects of gravity. A new research study provides insights that can help identify signs of incipient collapse in these ice shelves and assess the stage of collapse.

The photo of Wordie from 28 November 1966 - the first in a long series of images that continually document the collapse of the Wordie Ice Shelf through the 1960s - has become a valuable first data point in a study of the ice shelf collapse recently completed by researchers at the University of Copenhagen.

In their paper, now published in Nature Communications, they present a unique dataset based on the vast archives of old aerial images combined with modern satellite observations, which, for the first time, shows the collapse of an ice shelf as a constant evolution in a long time series. A critical insight that can be used to improve scientists' understanding of ice shelves and the mechanisms behind their collapse.

"We have identified several signs of incipient ice shelf collapse that we expect will be observed in other ice shelves, but perhaps more importantly, the dataset has given us a multitude of pinning points that can reveal how far advanced a collapse is. It's a completely new tool that we can use to do reality checks on ice shelves that are at risk of collapsing or already in the process of collapsing," says Postdoc Mads Dømgaard from the Department of Geosciences and Natural Resource Management, who is lead author of the study.

According to the researchers, this knowledge will enhance computer models of sea level rise and lead to more accurate predictions of when the water will rise, allowing for prioritisation of investments in climate change adaptation in the most effective way.

Warmer seawater and melting under the ice were most conclusive

The hundreds of historical aerial photos were analysed using a technique known as structure-from-motion photogrammetry. The method has enabled scientists to accurately reconstruct ice thickness, as well as its extent, surface structure and flow velocity, dating back to the 1960s.

In this way, the researchers analysed the collapse of Wordie and learned more about the factors involved. It was previously assumed that a warmer atmosphere was the primary factor leading to the collapse, and, similarly, that the formation of meltwater lakes on the ice surface had also played a part. The new study has disproved both assumptions.

Instead, the researchers highlight melting under the ice where the sea and ice meet as definitive factors.

"Our findings show that the primary driver of Wordie's collapse is rising sea temperatures, which have generated the melting beneath the floating ice shelf," Mads Dømgaard says.

Collapse is tougher than we thought – like it or not

Furthermore, the study's findings have already altered the foundation of scientists' knowledge about ice shelf collapse. According to co-author Anders Anker Bjørk, the new data advances our understanding of how and at what speed these collapses occur.

"The tentative conclusion from our findings is that ice shelf collapse may be slower than we thought. This means that the risk of a very rapid development of violent sea level rise from melting in Antarctica is slightly lower, based on knowledge from studies like this one," says Anders Anker Bjørk, Assistant Professor at the Department of Geosciences and Natural Resource Management.

But there is also a flip side to this, he explains:

"It was already a supertanker that needed to be turned to stall the melting of ice in Antarctica, but our data shows a collapse process that is even more protracted than previously assumed. And this longer process will make it harder to reverse the trend once it has started. This is an unambiguous signal to prioritise halting greenhouse gas emissions now rather than sometime in the future," Anders Anker Bjørk says.