Melting glaciers are one of the iconic impacts of climate change, but the watersheds they feed sequester a surprising amount of carbon, according to a new paper published last week in the Proceedings of the National Academy of Sciences.
“On average during the 2015 melting season, per square meter (to be clear, not in total), these glacial rivers consumed twice as much CO2 as the Amazon rainforest,” Wired reports.
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“Using a whole watershed approach in the Canadian High Arctic, combined with additional dissolved CO2 measurements in glacial rivers in Greenland and the Canadian Rockies, we show that certain glacier-fed freshwater ecosystems are significant and previously unrecognized annual CO2 sinks due to chemical weathering,” writes the research team headed by University of British Columbia biogeochemist Kyra St. Pierre.
“As many of the world’s rivers originate from glacial headwaters, we highlight the potential importance of this process for contemporary regional carbon budgets in rapidly-changing, high-latitude and high-altitude watersheds.”
The unexpected benefit traces back to the basic reality that “glacial rivers are very different from rivers elsewhere in the world,” Wired explains. “One striking difference is they’re largely abiotic—algae and fish typically don’t colonize them because they’re just too cold. So instead of being chock-full of life, they’re chock-full of sediment.”
Which means that, “as these glaciers are retreating or advancing, which they do every year, they’re actually forming a lot of very fine sediments that are just wide open on the landscape,” St. Pierre told science writer Matt Simon. Those sediments gather in the meltwater and, eventually, in mineral-rich glacial lakes.
So while regular rivers, lakes, and ponds, populated by organisms that consume organic material and give off CO2, are net carbon producers, glacial melt can dissolve more CO2 from the air. “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,” St. Pierre said. That makes the river itself “a meandering carbon sink—to an impressive degree,” Simon writes.
It’s hard to tell how much carbon is being captured by glacial meltwater around the world. “If you were looking for a way out of our impending climate doom, however, this ain’t it,” Wired notes. “For one, the sequestering powers of glacial meltwater can’t keep up with our out-of-control emissions, or even other climate change-induced emissions from the Arctic like melting permafrost. And if we keep melting glaciers, we’ll run out of meltwater, too. Still, the findings are a key piece in understanding the monumentally complex carbon cycle on this planet.”