The thing about carbon dioxide is that it flows freely across the surface of water—the water can both absorb the gas and give it off. In a typical river, organisms are consuming organic material and giving off CO₂, or respirating, just like humans do. Thus the river becomes a net carbon producer, because it’s saturated with so much CO₂ that the water just can’t dissolve any more CO₂ from the air. Same goes for ponds and lakes the world over—they’re greenhouse gas emitters.

Glacial meltwater, on the other hand, doesn’t have this organic respiration, so it can dissolve more CO₂ from the air. The sediments that the meltwater picks up along the way in turn consume the CO₂ that’s dissolved in the water. “You get the sediments mixing into the water and mixing with the carbon dioxide from the atmosphere that results in a change in the chemistry of the river as it moves downstream,” says St. Pierre. When the sediment reacts with the CO₂, some of material dissolves, so the river itself becomes a meandering carbon sink—to an impressive degree, in fact.

During the relatively low-melt season of 2016, the rivers in this Arctic watershed consumed half as much carbon daily per square meter as the Amazon rainforest. But the year before, which saw three times the glacial melt, on average the rivers consumed twice as much as the Amazon. At one point, they were capturing 40 times as much CO₂ as the Amazon per square meter. But again, this is not in total. The Amazon rainforest is 2 million square miles, a span that vastly eclipses the size of this glacial watershed.

Still, what emerges is a hitherto overlooked carbon sink. It’d be exceedingly hard to tell worldwide how much carbon glacial meltwater is capturing, even before climate change began sending Arctic systems into chaos. But the beauty of this work is that it brings some measure of understanding to complex phenomena. “The Arctic is changing much faster than even our best models predicted,” says Rose Cory, a biogeochemist at the University of Michigan, who wasn’t involved in this work. “And so to be able to model or project forward what's happening, we have to have this process information.”

Researchers have to better understand how rapidly melting glaciers impact freshwater systems. And also how much CO₂ these meltwaters might capture, so scientists can build more robust carbon budgets, or estimate how much carbon we can pump into the atmosphere if we want to meet the goals of the Paris Agreement. “I think this study is a really great example of the work that's needed,” Cory adds.