Understanding the full history of glacial extent can help scientists better appreciate the significance of the glacial retreat measured today. This is evinced in a recent study, published in Science, that reveals a new technique to allow researchers to determine historical glacier extent and to assess just how extraordinarily low ice coverage in the Andes is today. Their findings indicate unprecedented glacial retreat.
To conduct this work, researchers climbed the mountains of four glaciers in Colombia, Peru and Bolivia, and chipped away samples of bedrock. These samples were then analyzed via a novel methodology known as “cosmogenic isotope analysis.”
When highly charged particles from space called cosmic rays enter our atmosphere, they create a chain reaction by interacting with other atomic particles in the air. As they bounce around, they produce secondary particles with less energy. This cascade of interactions eventually results in neutrons that slam into the Earth’s surface. When glaciers retreat, the bedrock beneath them becomes exposed to these neutrons.
“There’s this reaction that occurs and a byproduct is these new cosmogenic nuclides that form in the rock. The longer that rock is exposed to the atmosphere, the more and more of these nuclides will form,” said Nicolás Young, a paleoclimatologist at the Columbia Climate School. Nuclides are a distinct class of atoms. However, these nuclides can’t be formed without direct exposure to the atmosphere. Should the bedrock be covered with snow and ice, the cosmic radiation necessary to produce the nuclides is blocked. “Once that ice retreats and uncovers this new bedrock, we say that’s like the ‘cosmogenic clock’ has started. It’s now able to build up cosmogenic nuclides,” said Young.
From within each sample of chipped rock, researchers were able to date how recently the rock was exposed to the atmosphere. Specifically, they looked at the concentration of beryllium-10 and carbon-14 nuclides built up in the rock around the edges of the glaciers. Since this bedrock is the most recently exposed rock, high concentrations of nuclides would indicate that it must have been uncovered in the past, but a lack of nuclides would indicate a history of ice coverage.
“What makes this study unique is they basically measured nothing,” Young continued. With virtually no nuclides in the rock, the researchers were able to conclude that the bedrock must have been covered by ice for a period of thousands of years, long enough for any nuclides formed in the distant past to decay. “You can pretty confidently say that—at least in recent geologic time—this bedrock has probably never been exposed to the atmosphere,” Young said.
The study concluded that Andean glaciers are the smallest they have been in 11,700 years. This is roughly the length of time of our current geological epoch known as the Holocene, which began after the last Ice Age and is the epoch in which all of human history has occurred. In recent decades, the progression of anthropogenic climate change has brought warmer temperatures that melt ice and limit freezing. While it has been known that these effects shrink glaciers, the severity of such glacial retreat and its implications have not been so clearly determined until now.
“It’s pretty well established at this point that most mountain glaciers and ice sheets, to some extent, are retreating,” said Young. “What this study is getting at is the magnitude of the current retreat—how unprecedented is it?”
Future Implications
Throughout the Holocene, there have been periods of glacial growth and retreat, with most glaciers experiencing their glacial minimum long ago in the early- to mid-Holocene. Glaciers in the Northern Hemisphere, though currently shrinking, have not yet reached their Holocene minimuma as the tropical glaciers of this study have. The conclusions made about these Andean glaciers, though, may foreshadow what is to come for glaciers worldwide as anthropogenic warming continues.
Should glaciers around the world continue to shrink at the unprecedented levels these tropical glaciers already have, there will be effects both locally and globally. “These sorts of glaciers will contribute in the near term to some sea-level rise but what’s probably a bigger concern, at least on a human timescale, is water availability,” Young said.
Many communities rely on mountain glaciers for their water supply, as they have proven to be a secure source for hundreds of years. The regular cycle of snowfall to glacier melt that supports watersheds in mountain communities is becoming threatened by extreme glacial retreat. Paola Moschella Miloslavich, the director of glacier research at the Peruvian National Institute for Research on Glaciers and Mountain Ecosystems, told GlacierHub about some of the ways glacial retreat is already impacting mountain communities. “The most significant impact is reduced water availability, especially during the dry season, and decreased water quality due to acid drainage from recently exposed bedrock, which affects agriculture and livestock farming,” she said.
The extremely low ice coverage of glaciers identified by this study already infringes on the livelihoods of mountain communities who now risk drought and poor water quality. “It is fundamental to reduce global greenhouse gas emissions in order to slow the rate of glacier retreat and maintain their ecosystem services,” Moschella Miloslavich said. Reductions of land use changes that shrink the surface area of mountain ecosystems will also be necessary, and these must be coupled with the restoration of degraded ecosystems. “All these measures need a framework of inter-institutional collaboration in different scales and the inclusion of local communities’ knowledge,” she added.
Mountain communities are already experiencing the direct effects of climate change and glacial retreat. Yet, if this study is any indication of the progress of anthropogenic warming, they will not be the last. From within the rocks at the margins of Andean glaciers, this study paints an alarming picture of our planet’s fragile state.
