Thanks to old aerial photographs, researchers have gotten a good picture of the evolution of so-called peripheral glaciers. And it will probably not surprise you that the speed at which these glaciers are retreating has doubled in the past twenty years.

Glaciers act as a sensitive barometer of climate change – and one that can be easily observed. Worryingly, however, we have known for some time that glaciers have been shrinking rapidly since the mid-20th century. Things are going particularly fast in Greenland. At least, with the glaciers we know about. However, Greenland also has a lot of ‘forgotten’ glaciers, of which we actually had no clear picture. Until now.

Peripheral glaciers
Although extensive research has been done on the impact of climate change on Greenland, the focus of most scientists is primarily on the Greenland Ice Shelf, which covers approximately 80 percent of the land. However, it appears that little is known about so-called ‘peripheral glaciers’. These are glaciers that are located on the edges or outside of an ice sheet or ice shelf. In the case of Greenland, the peripheral glaciers are the smaller ice masses scattered along the island’s coastline. These glaciers are separate from the central ice shelf and often have direct contact with the surrounding ocean. They contribute to sea level rise when they melt and release ice into the ocean. And we don’t actually have a detailed picture of these specific glaciers yet. This deficiency is partly caused by a shortage of observational data.

Change
In a recent study a team of scientists has examined how thousands of these peripheral glaciers have changed between 1890 and 2022. They used satellite images and a special archive of old aerial photographs (more on that later) to create the most detailed overview yet. With this unique data, the researchers have documented changes in the length of more than a thousand glaciers over the past 130 years. And the findings are probably already obvious. The team discovered that the retreat of the glaciers has accelerated dramatically in the 21st century. In fact, in the past twenty years, the retreat rate of these glaciers in Greenland has doubled. Glaciers in southern Greenland have lost an average of eighteen percent of their length, while glaciers in other areas have shrunk between five and ten percent over the past twenty years.

This map shows the change in position of a glacier in southern Greenland. Image: Laura Larocca

Withdrawal
In short, although glaciers in Greenland have already lost a lot of ice over the past century, the pace of this retreat has accelerated significantly in the past two decades. Remarkably, these findings are consistent across Greenland’s diverse climates and geographic features, even among the world’s northernmost glaciers. The results therefore highlight how vulnerable the region is to higher temperatures due to human-induced climate change. “Our research puts the recent retreat of peripheral glaciers in different climatic zones of Greenland into historical perspective,” says first author of the study Laura Larocca. “It suggests that the rate of retreat of these glaciers in the 21st century is largely unprecedented compared to previous centuries. The only exception is glaciers in northeastern Greenland, where recent increases in snowfall may be slowing the retreat.”

Old pictures
The fact that we now have a better picture of these ‘forgotten’ glaciers is, as mentioned, thanks to the analysis of satellite images and thousands of historical aerial photographs. “Some of these photographs were taken during the early cartographic expeditions of Greenland, conducted from open aircraft,” says researcher Yarrow Axford. “These old photos take the dataset back to an era before satellite observations, when detailed observations of the cryosphere were rare. It is special that we now have long-term data for hundreds of glaciers. This allows us to finally document more than a century of changes in the response of Greenland’s glaciers to climate change.”

Over Greenland in an open plane. Image: The Danish Agency for Data Supply and Infrastructure

Lack of data
Before the launch of Earth-observing satellites in the 1970s, researchers lacked a complete understanding of how temperature changes affected Greenland’s glaciers. There simply weren’t widespread and detailed observational data available, or so the researchers thought. A breakthrough came about fifteen years ago when long-forgotten aerial photographs of Greenland’s coastline were rediscovered in a castle outside Copenhagen. These images, now stored in the Danish National Archives, made it possible for researchers to reconstruct the history of the glaciers. “Starting in the 1930s, Danish pilots dressed in polar bear skins conducted aerial mapping campaigns in Greenland, collecting more than 200,000 photographs of the island’s coastline,” says Larocca. “In doing so, they also unintentionally recorded the condition of peripheral glaciers.”

Climate change
The research concludes that climate change explains the accelerated retreat of the glaciers. It means that glaciers across Greenland apparently respond quickly to changing temperatures. This highlights the critical importance of slowing global warming. “What we do in the coming decades will have a significant impact on glaciers,” Larocca underlines. “Every increase in temperature really matters.”

As global temperatures rise, it is more important than ever to better understand how the melting of peripheral glaciers will affect rising sea levels and reliable freshwater sources. “Although peripheral glaciers represent only about four percent of Greenland’s total ice-covered area, they contribute as much as fourteen percent of the island’s current ice loss, which is a disproportionate share,” Larocca explains. “If we look globally at all glaciers other than the Greenland and Antarctic ice shelves, they have contributed about 21 percent of the observed sea level rise over the past two decades. Therefore, these smaller ice masses are an important part of the sea level rise problem. Millions of people worldwide also depend on glaciers for fresh water, agriculture and hydropower, so it is deeply worrying that we allow this to continue. The choices we make in the coming years will have a major influence on how much ice we ultimately lose.”




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