The retreat of Andean tropical glaciers

The current retreat of Andean tropical glaciers exceeds that recorded during the warm periods of the last 11,000 years.

Today, mountain glaciers are retreating in all regions of the world, and 80% of this retreat is linked to human activities, the remaining 20% being of natural origin. Since the Earth has experienced warm periods of natural origin in the past, is the current situation exceptional? To answer this question, an international team of researchers from various laboratories studied the evolution of tropical Andean glaciers during the Holocene, i.e. over the last 11,700 years, a period during which temperatures were sometimes similar to those of today. However, the authors show that the current retreat of Andean tropical glaciers is unprecedented compared with the Holocene. Their study was published on 02 August 2024 in the journal Science.

Researchers from various American universities, the CNRS and the Universities of Aix-Marseille and Grenoble have analysed the multi-millennia evolution of Andean tropical glaciers located in Colombia, Peru and Bolivia over the Holocene (the last 11,700 years).

The particularity of the Holocene period is that it is characterised by a long warm phase between the last 10000 and 4000 years, known in the northern hemisphere as the Holocene Thermal Maximum (HTM).

Researchers have discovered that the current size of Andean tropical glaciers is smaller than it was during this long warm phase of the Holocene. To document the evolution of the glaciers over time, the researchers focused on the bedrock recently deglaciated by the retreat of the glaciers. They then measured the concentration of cosmogenic isotopes, in particular beryllium-10 and carbon-14. in situ, contained in rock samples taken in the immediate vicinity of the current glacier front. These two isotopes, produced by nuclear reactions caused by the impact of cosmic ray particles on rock minerals, accumulate once the glacier retreats. This triggers a “geological stopwatch”. The ice itself acts as a shield, protecting the rock from this cosmic bombardment. In short, the concentration of cosmogenic isotopes in the rock depends on the time of exposure to cosmic radiation and on glacial erosion, which strips away the cosmogenic isotopes accumulated on the surface.

In the Alps and in other regions of the world, similar analyses carried out in the past have shown high isotopic values measured in rocks. These high levels are due to the fact that between ~10000 and 4000 years ago, the Alpine glaciers and their cousins in the northern hemisphere retreated considerably. Their fronts were positioned at altitudes slightly higher than those observed today. The rock was therefore bombarded for around 6000 years by cosmic radiation, allowing the accumulation of ¹⁰Be and ¹⁴C in it.

Over the last 4,000 years, glaciers have undergone flooding phases that have halted the production of isotopes in the bedrock. By using a glacial flow model combined with an erosion model and comparing the isotope values obtained in the Andean rocks with those already known from samples taken in the Alps and other regions of the northern hemisphere, the researchers have revealed an extremely low, almost zero, isotope content accumulated in the Andean rocks.

This particularly low rate can be explained by the fact that the fronts of the Andean tropical glaciers have never been at such high altitudes over the last 11700 years. In other words, the Andean tropical glaciers have never been as small as they are today.