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The Larsen Ice Shelf in Antarctica has been breaking for decades, but 2002’s Larsen B collapse was especially dramatic. After being stable for at least 10,000 years, a large portion of the shelf broke apart, with the consequences felt around the entire planet. The widespread changes in Antarctica have been widely studied and publicized, but contextualizing and analyzing how the changing conditions in Antarctica impact the rest of the world has been challenging. To combat this, scientists have used film photographs from the 1960s to help understand how collapsing Antarctic glaciers have impacted global sea levels. In a research paper published in Nature, Ryan North and Timothy T. Barrows investigate historic film photos of Antarctica dating back to the 1940s and apply a sophisticated modern analytical technique called structure-from-motion (SfM) photogrammetry. Researchers say, “The technique creates digital elevation models (DEMs) by constructing 3D point clouds of matching features in overlapping photos without the need for the original camera positions or orientations.” The Larsen B ice shelf collapse as seen from space on March 17, 2022. | NASA As North and Barrows explain in an article on The Conversation, accurately understanding the past is essential to predicting the future. “To accurately predict how Antarctica’s glaciers will respond to future climate change, it is critical to understand how they have responded in the past,” the researchers write.
Figure 1. This shows an example of overlapping historical aerial imagery from December 1968, the resulting three-dimension models derived from the historical aerial photographs, and corresponding glacial outlines. Ice shelf outlines are from the SCAR Antarctic Digital Database. A significant challenge is that Antarctica is remote, and capturing good data there is prohibitively expensive. While satellites are excellent for collecting data about much of Earth’s surface, the Antarctic Peninsula is shrouded by clouds for much of the year. The result is that observations of the area are irregular and short-term. However, U.S. Navy cartographers captured more than 300,000 aerial photos of Antarctica, all of which are freely available at the Polar Geospatial Center at the University of Minnesota, as part of a large-scale mapping effort of the continent from 1946 to 2000. The large-format film photos are extremely high resolution, so North and Barrows applied SfM photogrammetry to 871 specific photos from 1968 to construct historical data for the Larsen B region. The selected photos were captured on large format 9×9 grayscale film on December 21, 23, and 27 in 1968. The film was then scanned at 1,000 DPI by the United States Geological Survey (USGS). 503 of the 867 photos were used to construct data for the Jorum, Crane, Mapple, and Melville glaciers, while more than 360 were used to determine the Flask Glacier’s elevation. The images were also fine-tuned by hand to reduce errors in the photogrammetric process, including changes to cropping, exposure, contrast, and clarity. “We use the historical DEMs to precisely measure the net change in surface elevations of Larsen B tributary glaciers (Jorum, Crane, Mapple, Melville, and Flask) between 1968 and 2021. For the same glaciers, we also calculate the surface elevation differences between 1968 and 2001… Using precise elevation differences, we provide new mass balance and sea-level contribution estimates spanning 53 years and discuss these measurements in the context of existing pre-collapse and post-collapse literature,” they explain.
Figure 2. ‘Details of high spatial resolution (1.6-meter pixel size) historical orthophoto mosaic covering the Larsen B region in December 1968. (A) Full extent of the orthophoto mosaic, (B) enlarged region on Crane Glacier, (C) a tributary of Crane Glacier enlarged further, and (D) meter-scale surface debris and meltwater channels visible in the same tributary of Crane Glacier.’ As a result of their research, the duo determined precise elevation changes across discrete regions of Larsen B and detailed the exceptionally minor changes over decades that ultimately led to the ice shelf’s collapse. They found that after the 2002 collapse of Larsen B, the glaciers lost a staggering 35 billion tons of land-based ice. Just the largest glacier alone lost 28 billion tons, which resulted in the Earth’s sea level rising about 0.1 millimeters. “This doesn’t sound like much,” the researchers concede. “But is the result of one glacier from one event. Put another way, it is equal to every single person on Earth tipping out a one-liter water bottle every day for 10 years.” Figure 5. This shows the elevation change for Larsen B and its tributary glaciers from 1968 to 2001. The net mass balance is labeled as the mouth of each tributary. North and Barrow call the historical archive of aerial film photographs a “priceless resource waiting to be tapped,” and say that the same process they used for this investigation can be used to analyze other ice shelves or glaciers, penguin colonies, vegetation expansion, or even direct human activity. Antarctic ice shelves and glaciers will continue to evolve as climate change accelerates, impacting the rest of the planet. Of course, one of the most critical steps toward dealing with the issue requires that the problem itself be understood. Image credits: Unless otherwise noted, the photos in this article are courtesy of the Polar Geospatial Center and the United States Geological Survey. The research referenced is ‘High-resolution elevation models of Larsen B glaciers extracted from 1960s imagery,’ written by Ryan North and Timothy T. Barrows.
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