In March 2002, the Larsen B Ice Shelf collapsed catastrophically, fragmenting 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 five glaciers looked like decades before the collapse of the Larsen B Ice Shelf. This allowed us to accurately calculate their contribution to sea level rise.
Although Antarctica is far away and changing conditions there may seem distant, the changes can have a profound effect on all of us. The removal of an ice shelf can cause glaciers to rapidly melt into the ocean and global sea levels to rise.
After consecutive years of unusually warm temperatures, the Larsen B Ice Shelf collapsed over the course of a week, causing dramatic changes to the glaciers that once fed into it. The glaciers have been closely monitored since then, but there were few observations prior to 2002.
However, there is an archive of over 300,000 historical images that provide an invaluable record of this area in 1968, allowing us to chart the difference between then and now.
NASA
Observing glaciers
Ice shelves are thick floating masses of ice that are attached to the coastline of Antarctica. The melting of an ice shelf does not directly cause sea level rise.
However, ice shelves hold back the flow of glaciers. Once the shelves are removed, glaciers melt rapidly into the ocean. This moves ice from the land to the ocean and causes sea level rise.
To accurately predict how Antarctica’s glaciers will respond to future climate change, it’s crucial to understand how they have responded in the past. But some places in Antarctica are so remote that it’s almost prohibitively 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 disrupt satellite observations for most of the year.
This means that observations for many areas in Antarctica are rare and often short-lived.
Historical photographs are an invaluable evidence
Between 1946 and 2000, U.S. Navy cartographers flew over virtually every corner of Antarctica, taking 330,000 high-quality, large-format photographs in an attempt to map the continent.
Scans of the images have been archived by the Polar Geospatial Center, University of Minnesota and are freely available for download. These images have a resolution as high as that of many modern satellites.
PGC, UMN
We created accurate and realistic 3D models of five glaciers in the Larsen B region using a technique called photogrammetry. Traditional photogrammetry uses two overlapping photographs from different angles to create a 3D surface – much like our two eyes can visualize objects in three dimensions.
Advances in computing now make it possible to combine hundreds of overlapping photographs with relative ease. Matching points in overlapping photographs are automatically detected and their 3D position is calculated geometrically. An accurate glacier surface can then be created from a cloud of millions of matching points.
Recognizable features in the images with known coordinates, such as nearby mountain peaks or uniquely shaped boulders, can then be assigned a GPS point to scale the model.
Then and now
After comparing five glaciers from 1968 and 2001 (the latter just months before the collapse), we found that they had remained virtually unchanged.
After the collapse, the glaciers lost 35 billion tons of land ice. One large glacier lost 28 billion tons, equivalent to about 0.1 mm of global sea level rise.
That may not sound like much, but it is the result of one glacier from one event. In other words, it is the equivalent of every person on Earth dumping a one-liter bottle of water every day for ten years.
These images were essential to observe the glaciers in high resolution, decades before they were affected by the collapse of the ice shelf.
A new record from Antarctica
As climate change accelerates, warming of the atmosphere and ocean threaten the remaining ice shelves on the Antarctic Peninsula. The historical image archive will become increasingly important to extend the record of change and determine how significantly things are changing.
The same images can be used to investigate other ice shelves or glaciers, changes in coastlines, penguin colonies, the expansion of vegetation, or even direct human impacts.
The historical image archive is an invaluable resource waiting to be tapped.