Seeking the world’s largest glaciers

What exactly qualifies as a glacier? Which three glaciers are the largest on Earth? And which glaciers hold the top position in each part of the world? With the rapidly changing cryosphere receiving increasing attention through reports about shrinking Arctic sea ice, thawing permafrost and disappearing ice sheets, these questions might seem as though they should have simple and well-established answers. However, when NSIDC’s Ann Windnagel began looking into them, she discovered that the answers were considerably more complicated than expected.

“It’s surprising that we don’t already have a clear answer,” she explained. Different organizations and sources use different definitions for glaciers, ice sheets and ice caps. Determining exactly what should be classified as a glacier turned out to be the most difficult part of the entire investigation.

Strengthening a partnership

Windnagel, a project manager working with the National Oceanic and Atmospheric Administration (NOAA) at NSIDC program, was pursuing a professional certificate in Earth Science Data Analysis through the University of Colorado Boulder when she was invited to participate in a professional work exchange with the World Glacier Monitoring Service (WGMS) in Zurich, Switzerland.

WGMS has maintained a long-standing partnership with NOAA at NSIDC. Windnagel had spent years mentoring WGMS interns and postdoctoral researchers at NSIDC, so she welcomed the opportunity to travel to Switzerland and experience how her colleagues there conducted their work in their own environment.

During her time in Zurich, Windnagel was asked to develop a project that could provide value to both NSIDC and WGMS. She chose to undertake a systematic examination of the largest glaciers around the planet, an analysis that had not previously been carried out in this way.

“Glaciers are disappearing at worrying rates because of climate change,” Windnagel said. “The loss of glacial ice can have serious consequences for society, including sea level rise and reductions in freshwater availability. Examining glaciers can show us how climate change is influencing different parts of the world and can provide valuable information for preparing for the future.”

The project also supported the broader goals of NOAA, whose mission includes understanding and forecasting changes in climate, weather, oceans and coastal environments, sharing that knowledge with others, and protecting and managing coastal and marine ecosystems and resources.

Gathering data

For her worldwide glacier assessment, Windnagel aimed to determine which glaciers were the largest globally and which were the largest within each of the 19 glacier regions identified by the Global Terrestrial Network for Glaciers.

She saw several important reasons for establishing this information. Identifying the largest glaciers can help researchers organize existing data, provide a clearer sense of how much water is locked within glacial systems, and help scientists and policymakers recognize which glaciers could potentially have the greatest effects. This knowledge could also help direct monitoring resources toward areas where glacier loss may be especially significant.

To complete the analysis, Windnagel collected information from two major worldwide glacier databases: the Global Land Ice Measurements from Space (GLIMS) project and the Randolph Glacier Inventory (RGI).

She used shapefiles from both databases to identify the ten largest glaciers by surface area in each data set. She then compared the two lists and examined differences between them by consulting scientific literature.

As she worked deeper into the data, however, additional questions emerged. What separates a glacier from an ice sheet or an ice cap? When a glacier becomes divided into separate sections, should those sections still be considered part of the original glacier, or should they be classified as independent glaciers? And why do scientific organizations sometimes use different definitions for what constitutes a glacier?

“I’m not a glaciologist by profession,” Windnagel explained. “I didn’t realise that the definition of a glacier itself was still debated. Working on this project and spending so much time with glaciologists every day completely changed my perspective. I learned an enormous amount.”

For the purposes of her study, Windnagel ultimately adopted the GLIMS definition of a glacier. Under this definition, a glacier is a body of ice located on land that moves downhill under the influence of gravity while being controlled by internal stresses and friction along its base and sides. Generally, glaciers develop and persist through the accumulation of snow at higher elevations, with that accumulation balanced by melting at lower elevations or by ice calving into seas or lakes.

The takeaway

Windnagel spent five weeks developing her research project at the WGMS office in Zurich. Based on the GLIMS database, the three largest glaciers identified in the world are Vatnajokull Glacier in Iceland, Flade Isblink Ice Cap in Greenland and Seller Glacier in Antarctica.

The results from the RGI database were different. According to RGI, the three largest glaciers are Flade Isblink Ice Cap in Greenland, Carney Island Ice Cap in Antarctica and Seward Glacier in Alaska.

Interestingly, Pine Island Glacier in Antarctica, which is often described as the largest glacier on Earth, did not appear among the top three in either database. The reason is connected to the GLIMS definition. Under that classification system, Pine Island Glacier does not qualify as a conventional glacier. Instead, it is an ice stream that flows into the Amundsen Sea at a faster rate than the surrounding ice mass belonging to the West Antarctic Ice Sheet.

Although Windnagel successfully identified candidates for the world’s largest glaciers, the results still require additional verification. She returned to the United States with several unanswered questions and continued working on the project.

Determining the largest glaciers within each individual region has been particularly challenging because glacier databases such as GLIMS are not necessarily updated consistently across every part of the world. To complete the regional analysis, Windnagel will need assistance from regional glacier specialists who can review and verify the information.

Her research will also continue beyond simply identifying glacier size. She plans to further examine the distinctions between glaciers and ice caps and conduct an analysis of glacier retreat to determine which glaciers around the world are losing ice most rapidly.

Looking further ahead, Windnagel hopes the results of this work can contribute to the development of improved glacier databases as well as educational and public outreach resources that can help people better understand the world's changing glaciers.