The Little Ice Age was a period of significant cooling from the early 14th to mid-19th centuries, which saw mean temperatures across the northern hemisphere drop by up to 2°C and the advancement of glaciers.
Such icy conditions had a marked imprint on the landscape and continue to the present day. New research, published in Earth and Planetary Science Letters, has considered how the Mont-Blanc massif in France has responded to changing environmental conditions since the termination of the Little Ice Age in the 1850s.
To explore this, Dr. Léa Courtial-Manent of France’s Université Savoie Mont Blanc and colleagues investigated the long-term erosion trends of the mountain range through the history of rockfall events. Sharing a personal connection to the research, Dr. Courtial-Manent lives in a region surrounded by mountains and so became invested in understanding the degradation of the landscape as a result of climate change.
“Our research focuses on understanding how the climate crisis impacts mountain erosion by studying rockfall events in the Mont-Blanc massif. By analyzing the effects of permafrost degradation and freeze-thaw cycles on rock stability, we show that erosion rates are accelerating, which has significant implications for the safety of mountain enthusiasts and professionals, infrastructure and economies,” Dr. Courtial-Manent explains the significance of the work.
The scientists used cosmogenic nuclide data from material transported on the glacier surface (supraglacial load) to measure concentrations of the isotope beryllium-10 (10Be) which reflects the duration of exposure of rock fragments to cosmic rays, indicating the time the rock remained stationary before falling.
Focusing specifically on the Mer de Glace basin, the research team used terrestrial laser scanning to identify 123 smaller rockfalls (up to 566m3) and direct observations of 71 much larger rockfalls (up to 20,000m3), combined with the 10Be data, to calculate erosion rates through time. These direct observations come from a network of observers launched in 2007 that relies upon citizen science.
“Mountaineering guides, hut keepers and mountaineers report if they see a rockfall,” Dr. Courtial-Manent says. “Then they specify the date and location and provide an estimate of rock volume, trying to be as precise as possible. They can also share pictures of the rockfall and give information about the weather conditions to produce a better overview.”
Combining the three methods (terrestrial laser scanning, direct observations and 10Be data) allowed the team to “overcome the biases of each method individually” and means they “can go further back in time, cover a wider space, and, thanks to power laws, estimate the potential erosion rates due to larger rockfalls that weren’t monitored.”
They identified an erosion rate of >4.1mm/yr for 2006 to 2011, which has approximately doubled from the estimated maximum of 2.2mm/yr during the Little Ice Age. Some isolated rock faces have even seen a five-fold increase in local erosion rates.
Less than 6% of rockfalls are attributed to blocks <1m3 in size, while over 20% may be credited to volumes <100m3, and the remainder by large-scale mass movements. The material from these rockfall events suggests a continuous erosion period equivalent to approximately seven years, since the 1850s.
The scientists explain this is due to intense permafrost action on the Mont-Blanc rockwalls below 3,800 meters above sea level. Here, water entering cracks in the rockwall freezes when temperatures drop, with the expanding ice causing the crack to widen; when the ice eventually thaws, the water travels further down into the crack and when it freezes again the cycle keeps repeating until eventually the rock splits entirely, leading to rockfall events.
Given that degradation of rockwall permafrost is predicted to continue as global warming melts ice and encourages freeze-thaw processes, this research highlights the significance of understanding the patterns of rockfalls now and into the future, as their increasing frequency has implications for endangering local communities.
“At the current escalation rate, it’s almost certain that the frequency of rockfalls in the Mont-Blanc massif will continue to increase,” Dr. Courtial-Manent concludes.
“It’s difficult to say with any certainty whether this trend will double again in the future, or to give a precise date of when that may happen. However, what I can say is that the north faces and rockwalls, even higher in altitude than those studied, are suffering the consequences of the climate crisis, and that will have an important impact on rockfall frequency. Our current research in the Mont-Blanc massif suggests that we have reached a point of no return.”
More information: Léa Courtial-Manent et al, A significant doubling of rockfall rates since the Little Ice Age in the Mont-Blanc massif, inferred from 10Be concentrations and rockfall inventories, Earth and Planetary Science Letters (2024). DOI: 10.1016/j.epsl.2024.119142
Journal information: Earth and Planetary Science Letters
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