In March 2002, the Larsen B Ice Shelf collapsed catastrophically, destroying an area about one-sixth the size of Tasmania.
In a paper published today in Scientific Reports, we used nearly 1,000 film photographs of Antarctica from the 1960s to reconstruct exactly what the five glaciers looked like decades before the Larsen B ice shelf. This allowed us to precisely calculate their contribution to the rise of sea level.
Although Antarctica is far away and changing conditions there may seem remote, the changes can have a profound effect on all of us. The removal of an ice shelf can cause glaciers to melt rapidly in the ocean and raise global sea levels.
After consecutive years of unusually warm temperatures, the Larsen B ice shelf collapsed within a week. This caused a dramatic change in the glaciers flowing into it. Since then, the glaciers have been closely monitored – but there were few observations of them before 2002.
However, an archive of more than 300,000 historical images contains an invaluable record of this area from 1968 and helped us measure the difference between then and now.
The collapse of the Larsen B ice shelf from space, March 17, 2002.
NASA
Observation of glaciers
Ice shelves are thick bodies of floating ice attached to the Antarctic coastline. The melting of an ice shelf does not directly cause sea level rise.
However, ice shelves “restrain” the flow of glaciers. Once the shelves are removed, the glaciers melt rapidly into the ocean. This transfers ice from the land to the ocean and causes sea levels to rise.
To accurately predict how Antarctic glaciers will respond to future climate change, it is critical to understand how they have responded in the past. But some places in Antarctica are so remote that it’s almost too difficult and expensive to get there and collect data.
Scientists often look to satellites to collect data because it is relatively cheap and easy. However, persistent cloud cover over the Antarctic Peninsula can interfere with satellite observations for most of the year.
This means that for many areas in Antarctica, observations are rare and often short-lived.
Historical photographs are an invaluable record
Between 1946 and 2000, US Navy surveyors flew over nearly every corner of Antarctica, recording 330,000 high-quality, large-format film photographs in an effort to map the continent.
Photo scans are archived by the Polar Geospatial Center, University of Minnesota and are available for free download. These pictures are of high resolution that many modern satellites can capture.
We created accurate, real-world scaled 3D models of five glaciers in the Larsen B area using a technique called photogrammetry. Traditional photogrammetry uses two superimposed photos from different angles to create a 3D surface – like how our two eyes can visualize objects in three dimensions.
Advances in computing now allow hundreds of overlapping photos to be combined with relative ease. Matching points in the superimposed photos are automatically detected and their 3D position is calculated geometrically. An accurate glacier surface can then be made from a cloud of millions of matching points.
Identifiable features in images with known coordinates, such as nearby mountain peaks or uniquely shaped rocks, can then be assigned a GPS point to scale the model.
A virtual “flyover” of Crane Glacier in 1968, which was affected by the 2002 collapse.
Then and now
After comparing the five glaciers in 1968 and 2001 (the latter just a few months before the collapse), we found that they were relatively unchanged.
After the collapse, the glaciers lost 35 billion tons of ice to the ground. From one large glacier, 28 billion tons were lost, equivalent to about 0.1 mm of global sea level rise.
That doesn’t sound like much, but it’s the result of a glacier from one event. In other words, it is equivalent to every single person on Earth downing a liter of water every day for ten years.
These images were essential for observing the glaciers at high resolution decades before they were affected by the ice shelf collapse.
A new Antarctic record
As climate change accelerates, warming of the atmosphere and ocean threatens the remaining ice shelves on the Antarctic Peninsula. The historical image archive will become increasingly important in expanding change data and determining how significantly things are changing.
The same images can be used to investigate other ice shelves or glaciers, changes in coastlines, penguin colonies, expanding vegetation, or even direct human impacts.
The historical image archive is an invaluable resource waiting to be tapped.
This article was originally published on Conversation from Ryan North IN University of Wollongong and Tim Barrows at UNSW Sydney. Read on original article here.