Glaciers’ secret cooling power won’t last much longer

Thomas Shaw remembers a particular summer day in August 2022 vividly. The postdoctoral researcher in Francesca Pellicciotti's lab at ISTA was high in the Swiss Alps under clear skies and a comfortable 17 degrees Celsius. Yet he stood atop the Glacier de Corbassière, more than 2,600 meters above sea level, taking measurements to assess the glacier's condition.

Although global air temperatures continue to climb, the surfaces of many glaciers appear to be warming more slowly. Some, such as the immense Himalayan glaciers, even send cold air streaming down their slopes, cooling the valleys below. This natural refrigeration may seem like a sign of resilience, but scientists say it's only temporary.

Shaw's new study indicates that glaciers' cooling response will reach its height in the 2030s. "The more the climate warms, the more it will trigger the glaciers to cool their own microclimate and local environments down-valley," says Shaw. "But this effect will not last long, and a trend shift will ensue before the middle of the century." After that point, melting and fragmentation caused by human-driven warming are expected to intensify, with glaciers heating faster near their surfaces and shrinking at an accelerating pace.

Large glaciers and cold winds

Studying these effects in some of the world's most isolated regions is no small task, especially since field data are scarce. This lack of information makes it difficult for scientists to fine-tune climate models. When Pellicciotti's team first examined records from a weather station 5,000 meters up Mount Everest, they were astonished. "Upon examining the data thoroughly, we understood that the glaciers were reacting to the warming air in summer by intensifying their temperature exchange at the surface," Pellicciotti says. The vast Himalayan glaciers cool enormous air masses that slide downslope under gravity, creating what scientists call katabatic winds. Similar patterns are seen at other major glaciers around the world.

Scientists going out of their way

To better understand this phenomenon globally, Shaw designed a new statistical model that could work even where data were limited. "We compiled data from past and recent projects across our research group, pooled them with all published data, and reached out to other researchers to request that they share with us their unpublished data," says Shaw. "Using this unprecedented dataset, we reassessed the physical processes to find generalizable aspects and developed a statistical framework that can give us a glimpse into the evolution of glacier cooling worldwide."

Peak cooling

Shaw and the team compiled an inventory of hourly data from 350 weather stations located on 62 glaciers worldwide, representing a total of 169 summer-long measurement campaigns. They specifically examined the ratio of near-surface temperature to ambient, non-glacier temperature right above each station and analyzed it over space and time. "We call the difference in temperature 'decoupling,' because it seems at odds with the warming of ambient temperatures," says Shaw. They showed that, on average, the near-surface temperature on mountain glaciers worldwide warmed 0.83 degrees Celsius for every degree rise in ambient temperature.

They also investigated the glacier properties most likely to limit the decoupling effect, such as the presence of a debris mantle on the lower part of a glacier, and refined their model with this information. By modeling future projections, they demonstrated that this cooling effect will peak between the 2020s and 2040s, before the glaciers' steady mass loss leads to their large-scale retreat, reversing the cooling trend. "By then, the worn-out and considerably degraded glaciers will 'recouple' to the steadily warming atmosphere, sealing their fate," says Shaw.

Accepting the loss and coordinating future actions

While the projection paints a bleak future for the world's majestic water towers, there are pragmatic consequences if the current trend continues. "Knowing that the glaciers' self-cooling will continue a little longer could buy us some extra time to optimize our water management plans over the next decades," says Shaw.

However, the team is fully aware that they can neither salvage nor recover the world's mountain glaciers. "We must accept the committed ice loss and put our full efforts into limiting further climatic warming rather than into ineffective geo-engineering strategies such as seeding clouds and covering glaciers. These are like putting an expensive Band-Aid on a bullet wound. The coming decades are a time for reflection, effective water management, and action to change public consciousness about human-caused climate change." The researchers further underline the need for coordinated global climate policies to drastically reduce emissions and safeguard human life on Earth from the unforeseeable effects of global warming. "Every bit of a degree counts," says Shaw, echoing the words that scientists have been stressing for decades.