Fjords Hoard More Carbon Than We Thought

Norway is famous for its fjords, but they're also found in many other places around the world. (Photo: Shutterstock)

Fjords aren't just gorgeous gorges. They're also a big part of the global carbon cycle, a new study finds, soaking up more than their share of the excess carbon that could otherwise fuel climate change.

A fjord is a deep, narrow and elongated ocean inlet formed by a glacier. Fjords cover less than 1 percent of Earth's total surface area, but they sequester 18 million metric tons of carbon every year, according to a new study published in the journal Nature Geoscience.

That's 11 percent of the total carbon absorbed by ocean sediments worldwide, which means fjords' rates of carbon burial are roughly double the ocean's average rate. It also suggests these picturesque valleys play a larger role than we realized in buffering the planet from man-made climate change.

The process begins with plants, which soak up carbon dioxide from the air to help them grow. Some of this carbon may return to the air when a plant dies, but some is also buried in soil or washed into rivers. Fjords excel at carbon storage because they funnel lots of carbon-rich river water into deep, calm reservoirs with low oxygen levels, which inhibit bacteria from freeing the carbon into the air.

For some reason, fjords in Alaska account for about half of all fjord-stored carbon on Earth. (Photo: getdirectlydown/Flickr)

Between ice ages, fjords prevent carbon from flowing out to the continental shelf, thus blocking the release of airborne CO2 that contributes to climate change. But when glaciers start to advance, this carbon would likely be pushed outward and CO2 production would rebound.

"In essence, fjords appear to act as a major temporary storage site for organic carbon in between glacial periods," says study co-author Candida Savage, a marine scientist at New Zealand's University of Otago, in a statement about the research. "This finding has important implications for improving our understanding of global carbon cycling and climate change."

The researchers tested how much carbon is stored in sediments at the bottom of four fjords in New Zealand, them combined those data with 573 surface-sediment samples and 124 sediment cores from fjords around the world. Their results suggest fjords rank "as one of the ocean's major hotspots for organic carbon burial, based on mass of carbon buried per unit area," the study's authors write.

"The total amount of annual organic carbon burial in fjords is only surpassed in continental margin sediments," writes University of Washington geochemist Richard Keil, who wasn't involved with the study, in a commentary for Nature Geoscience. "Despite being small, fjords are mighty."

This study sheds important light on the role of fjords in climate change, but we still have a lot to learn. Alaskan fjords seem to absorb more carbon than fjords in other parts of the world, for example, and scientists still aren't sure why. More research might reveal what aspects of fjords make them better at hoarding carbon, and thus help us understand the role they play in regulating Earth's carbon cycle.

As Keil points out in an interview with the journal Nature, however, "it is nowhere near enough to make up for what humans are doing to change the cycle."