Two new papers in Nature have explored carbon removal from the atmosphere and upper ocean. They demonstrate that taking carbon dioxide out of the air will be a hard slog, and there is a lot more we need to know before we can do it safely.

In the first paper, Dr. Phil Williamson of the University of East Anglia examined proposed methods for drawing carbon from the air. “Unless a lot more effort is made to cut carbon emissions... we will have to work out how to safely remove very large amounts of CO₂ from the atmosphere,” Williamson said in a statement. "The aim is to have a balanced global carbon budget. For that to work, from now on we have to think of matching the addition of greenhouse gases to the atmosphere with their subsequent removal.”

"Climate modelers estimate that as much as 600,000 million tonnes [661 billion tons] of CO₂ may need to be extracted from the atmosphere by 2100 to deliver the main goal of the Paris agreement,” Williamson added. "But removal will be expensive, and is currently unproven at the scale needed – so it would be much better to reduce emissions as rapidly as possible."

Methods Williamson considered include tree plantations, burning crops for bioenergy and storing the carbon produced underground or as biochar, mangrove restoration, chemical treatment of clouds, and ocean fertilization.

"Crucially, large-scale CO₂ removal, by whichever means, will have knock-on effects for ecosystems and biodiversity. There could be benefits, but damage seems more likely,” Williamson said. "For example, the amount of bioenergy crops we would need to grow could use up to 580 million hectares of land [1.4 billion acres] – or half of the land area of the U.S. This would in turn accelerate the loss of forests and natural grassland with impacts for wildlife, whilst also having implications for food security.”

Moreover, carbon storage is not the only issue. “Counter-intuitively, afforestation at mid-latitudes and in northern, boreal forests may have a net warming effect, despite increasing the storage of carbon,” the paper noted.

^{A privately funded research expedition used the ship Tara to study carbon transportation in the oceans. ©S. Bollet - Tara Expeditions}

The second paper draws on the three-year Tara Oceans expedition to expand our knowledge of how carbon leaves the upper ocean. "We're trying to understand, 'Does carbon in the surface ocean sink to the deep ocean and, if so, how?'" Ohio State University's Dr. Matthew Sullivan said in a statement. "The reason that's important is the oceans help mitigate our carbon footprint."

The Tara team genetically sequenced plankton and microbes from the ocean's surface and depths to see whether certain species are more likely to sink to the depths where their carbon can be safely stored for generations. "It's the first community-wide look at what organisms are good predictors of how carbon moves in the ocean," Sullivan said. 

"What was really surprising was that only a handful – less than 10 out of more than 5,000 – viruses seem to be specifically linked to carbon export,” Sullivan said.

It may be possible to alter conditions to promote the species associated with sinking carbon, but Williamson's work indicates we will have to understand processes much better before we can do this safely.