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Thousands of lakes dot Greenland鈥檚 coast, their waters held back by walls of ice. But sometimes, the meltwater can surge through and spill into the sea. The first survey of its kind has found a 1200 per cent increase in the number of such leaky lakes compared with how many had previously been documented to burst, which could help better model sea level rise.

The biggest factor in this spike is underreporting, according to the researchers, but it is possible that a rapidly warming Arctic has played a role. Greenland鈥檚 glaciers are retreating twice as fast as they did in the 20th century.

Mads D酶mgaard at the University of Copenhagen in Denmark and his colleagues used water level data from satellites and aircraft measurements to determine that 326 glacial lakes produced 541 glacial lake outburst floods (GLOFs) between 2008 and 2022.

鈥淭his is an important first step in assessing risks,鈥� says D酶mgaard.

GLOFs can harm wildlife already made vulnerable by climate change. In 2009, for example, local hunters reported dead fish in Nuup Kangerlua, a fjord close to Nuuk, the capital of Greenland. They also reported upwelling of water and large amounts of sediment. This, along with satellite observations showing a nearby glacial lake had recently emptied, indicated that water had flowed beneath the glacier and plunged into the ocean. The impact shifted water upwards and brought fish to the surface. This caused them to experience the bends, or decompression sickness.

D酶mgaard says it is possible for GLOFs to cause tsunamis, depending on the path and the amount of water released. In some of the lakes identified in the study, water levels dropped by more than 140 metres.

Celeste Labedz at the University of Calgary in Canada says that 鈥渉aving a bigger dataset of examples can help us understand how to better mitigate the hazards鈥�.

These findings will also help researchers assess how GLOFs could evolve. 鈥淭here are indications that as the climate gets warmer, we will see more GLOFs,鈥� says D酶mgaard, referring to how thinning glaciers may not be able to hold back increased meltwater.

The team found that most of the GLOFs happened in 2019 鈥� a year marked by extreme levels of meltwater and record-low albedo, or the amount of sunlight reflected to space by ice. With a larger sample in the future, scientists could determine whether a link between GLOFs and warming exists.

The team鈥檚 new data is also a trove for researchers trying to understand how Greenland鈥檚 ice responds to warmer temperatures. For example, GLOFs can change the speed and direction of a glacier鈥檚 flow, which can accelerate sea level rise.

鈥淕reenland is a big, complicated place, and knowing where all the water goes between the different parts of the system matters,鈥� says Labedz. 鈥淵ou can鈥檛 really do that without including the lakes.鈥�