Scientific Method —

Measuring Greenland’s increasing weight loss using aerial photos

Current rate of ice melt is double the 20th century average.

Enlarge / Kangiata Nunata Sermia, Greenland.
Nicolaj Krog Larsen, Aarhus University, Denmark

Past performance may not always predict future results in the stock market, but in the Earth sciences, it can tell us a hell of a lot. Since we only have the one planet, examples of some processes can only be found in the past. That’s why so much effort goes into studying the past behavior of the Greenland and Antarctic ice sheets. We need context for what we’re currently seeing and some ideas about what’s likely to happen next.

While many studies look tens of thousands or even millions of years into the past, much more recent histories can also be of interest. We’ve only had satellites measuring changes in the Greenland ice sheet since the early 1990s, so what happened over the preceding century is much less clear. That makes it difficult to answer questions about Greenland’s contribution to the full century's sea level rise or the ice sheet’s natural short-term variability.

But in a new study, a team led by Kristian Kjeldsen and Niels Korsgaard of the University of Copenhagen has managed to fill in this gap through some clever, if tedious, research. They took advantage of a trove of stereo aerial photos taken in the late 1970s and 1980s as part of a survey of Greenland.

Luckily, there’s also an older reference point visible in these images. During the “Little Ice Age," the outlet glaciers that ring the great ice sheet reached their greatest size in recent time. They began to shrink back by around 1900 but, like the ring of foam that marks how full your cup of hot chocolate used to be, their mark on the landscape remains. That includes jumbles of rocks deposited at the edges of glaciers, as well as clean-scraped valley walls.

By overlaying these photos on a digital model of Greenland’s coastal topography, the researchers could carefully calculate how much ice had disappeared between about 1900 and the time the aerial photos were taken. From there, they could rely on various satellite datasets to bring us up to the current day.

The results showed that Greenland lost a fair amount of ice over the 20th century. Between 1900 and 1983, an average of 75 billion tons (±29 billion) of ice disappeared each year. That’s similar to what happened from 1983 to 2003, which saw a loss of 74 ±41 billion tons per year. Recent years stand out, however, as Greenland doubled that pace, losing 186 ±19 billion tons per year between 2003 and 2010.

Add it all up, and the researchers calculate that Greenland lost about nine trillion tons of ice between 1900 and 2010, which is enough to raise global sea level by about 2.5 centimeters. That’s interesting because most efforts to tidy up the ledger of contributions to 20th century sea level rise have penciled Greenland in for considerably less—not surprisingly, these estimates came up short of the observed sea level rise.

A recent study using a different technique came up with a slightly smaller estimate of sea level rise in the earlier part of the century, balancing the sea level budget without additional ice losses. However, the authors of this study argue that Greenland really did contribute more, and the sea level ledger still needs work.

In order to go a little deeper, the researchers also compared their calculations to the latest reconstruction of Greenland weather and snowfall over the 20th century. That allowed them to split apart losses caused by surface melting and reduced snowfall from losses caused by changes in the seaward flow of the ice. The estimated ice flow was actually pretty steady, with increasing surface melting mostly responsible for Greenland’s weight loss.

In an article accompanying the study in the journal Nature, University at Buffalo researcher Beata Csatho notes that there are other photos that could similarly be used to extract more detail—a US Air Force survey from the 1930s and some spy satellite imagery from the 1960s, for example. And the data already in hand provides better tests for the computer models of the ice sheet that help us understand its future outlook.

Nature, 2015. DOI: 10.1038/nature16183, 10.1038/528341a  (About DOIs).

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