Why are Patagonian glaciers rapidly losing mass?

Over the past two decades, satellite-based planetary observations have recorded rapid mass loss of Patagonian glaciers, contributing approximately 0.07 mm per year to global sea-level rise. A study published in Nature Communications links this mass loss to a poleward shift of subtropical high-pressure systems. This large-scale atmospheric circulation change brings more warm air to Patagonia, thereby accelerating glacier melt.

Located in the southern Andes between Chile and Argentina, Patagonia hosts the largest and wettest glaciated region in the Southern Hemisphere outside Antarctica. “The Southern Andes act as a natural barrier, blocking moisture-laden westerly winds from the Pacific Ocean,” explains Brice Noël, climatologist at the University of Liège. “As a result, glaciers locally receive over fifteen metres of snowfall annually, particularly on the western flank of the Andes.”

While snow accumulation at higher elevations contributes to glacier growth, rapid melting occurs at lower altitudes. “Glaciers can extend down to sea level, where warmer air triggers substantial summer melt. This meltwater eventually runs off into the ocean, leading to sea-level rise.” Scientists estimate that since the 1940s, Patagonian glaciers have lost over a quarter of their total ice volume, raising global sea level by 3.7 mm.

High-resolution climate model

The research team from Liège, Leuven, and Delft estimated the surface mass balance of Patagonian glaciers since 1940—that is, the difference between winter snowfall and meltwater runoff in summer. “We used MAR, our regional climate model developed at the University of Liège,” adds Xavier Fettweis, climatologist at ULiège. MAR is a polar climate model that simulates snow and ice processes on a five-kilometre spatial grid, which is too coarse to represent the small-scale Patagonian glaciers. “High spatial resolution is essential to study the glacier surface mass balance in Patagonia, so we spatially refined our model to a 500-metre grid,” notes Brice Noël. Lower-resolution models fail to accurately capture narrow glacier tongues* that melt rapidly or estimate realistic precipitation over the rugged Andes. “Our high-resolution model closely aligns with in situ and satellite mass loss observations,” confirms Bert Wouters from Delft University of Technology.

What drives glacier mass loss?

Sustained mass loss since 1940 is attributed to a long-term increase in meltwater runoff to the ocean, a consequence of atmospheric warming in Patagonia. “We identify increased surface runoff as the primary driver of glacier mass loss, as snowfall has remained steady since the 1940s,” explains Brice Noël. Surface runoff intensifies when firn - the porous, perennial snow layer covering the upper glacier zones – melts away, exposing the underlying bare ice. “Bare ice is darker than the surrounding firn, thus absorbing more solar energy in turn enhancing melt and runoff,” explains Stef Lhermitte of KU Leuven.

Poleward shift of subtropical highs

Besides the effect of global warming, researchers attribute the fast increase in Patagonian temperatures to a large-scale atmospheric shift, whereby subtropical high-pressure systems migrate poleward. This shift, observed over the past forty years, channels more warm air into Patagonia, thereby amplifying mass loss. Ocean-atmosphere interactions underpinning this circulation change are driven by global warming and are likely to persist in the future. “Complete melting of Patagonian glaciers could raise global sea-level by an extra centimetre,” warns Brice Noël. “Their disappearance would endanger South American communities reliant on summer meltwater supply.” At the current rate of mass loss, scientists project that Patagonian glaciers could vanish within the next 250 years.

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* A glacier tongue refers to the downstream extension of a glacier, resembling an icy “arm” descending into a valley or flatter terrain.