To combat climate change, companies bury plant waste at sea
Dror Angel, a marine ecologist at the University of Haifa, had for years heard his archaeologist colleagues talk about ancient shipwrecks on the bottom of the Black Sea that were perfectly preserved by the low-oxygen environment. “You can see ropes,” Angel says. “It’s something which is quite spectacular.”
Now, Angel wants to combat climate change by purposefully adding to the wreckage, sinking waste wood to the sea floor, where carbon that the trees stored up while living can remain locked away for centuries.
Angel is a science lead for an Israeli company called Rewind, one of many companies riding a wave of investment in technologies that could help limit global warming by drawing carbon out of the atmosphere and locking it up. Whereas some carbon capture schemes require expensive machines and complex chemistry, burying terrestrial biomass at sea is exceedingly simple: It requires tugboats, barges, and woody waste from forestry and agriculture.
The approach has advantages over another popular ocean-based carbon capture strategy: growing, and sinking, massive amounts of seaweed or phytoplankton. Because the plant material is grown on land rather than in the ocean, it is less likely to rob nutrients from the surrounding water and upset the ecology. Industrial agriculture and forestry have an extensive infrastructure for growing, processing, and transporting plants, in contrast to marine farming, which has never been attempted at scale. And because woody plants are tough and unlikely to degrade, they are good at hanging on to their carbon. “Decomposers don’t like to eat them — they don’t get much out of it,” says Ning Zeng, a climate scientist at the University of Maryland.
At the same time, the approach may fall short of what’s needed to fight climate change. To keep warming below 2°C, the world needs to capture and store about 10 billion tons of carbon dioxide per year by midcentury, according to the International Energy Agency. But terrestrial biomass can be sunk only where supplies of waste are located near suitable bodies of water. By one recent estimate, the approach could sequester a few tens of billions of tons of carbon dioxide in total — just a few years of the need.
“The terrestrial biomass thing is not going to solve the full problem,” says ocean engineer Kate Moran from Ocean Networks Canada, a group that is assessing the efficacy of carbon capture strategies. “It’s going to be a small piece of the pie if it is deemed to be more beneficial than risky.” But, she adds, “We need all the tools in the toolbox.”
In the Black Sea, Rewind has one of the world’s great carbon burial sites. The sea is much saltier at the bottom than at the top, so the two layers don’t mix much at all — one reason why very little oxygen makes it to the sea floor. Without oxygen, microbes can’t convert the carbon in biomass back into greenhouse gases, such as methane, and even if some methane is produced, chemical reactions in the sulfate-rich waters will break it down. And because the layers don’t mix, any trace greenhouse gases that are produced will be locked in the depths for hundreds or thousands of years. “There’s all these additional processes that add more layers of security,” Angel says.
The advantages are enough to lure investors hoping to sell credits for the carbon removed from the atmosphere. Carbon credit marketplace Supercritical recently became Rewind’s first customer, and next summer the company plans to start sinking biomass in burlap sacks — possibly including forestry residue, river driftwood, and agricultural waste. Bulgaria, Romania, Turkey, and Georgia have all shown interest in the project, Angel says.
Frontier Climate — a group that makes commitments to buy future credits from carbon sequestration startups — recently awarded $250,000 R&D grants to Rewind and another firm, Houston, Texas – based Carboniferous, which hopes to sink sugarcane waste in an oxygen-starved region of the Gulf of Mexico known as Orca Basin. The waste is abundant on Gulf Coast farms, says Morgan Raven, a biogeochemist at the University of California, Santa Barbara and the company’s chief science officer. “It’s already sitting in piles,” she says. “The alternative for this material is essentially that it degrades, releases methane, and requires tending so it doesn’t light on fire.” Carboniferous is now applying for permission to test its strategy from the Environmental Protection Agency.
Portland, Maine – based Running Tide is combining terrestrial and marine biomass in one carbon capture strategy. The company takes waste wood from a forestry operation in Nova Scotia that would otherwise be burned or left to decay and presses it together to create floating “buoys” the size of baseballs and basketballs that are seeded with seaweed. The buoys are released off the coast of Iceland, where ocean currents carry them over a deep region with little oxygen. Eventually they become waterlogged and sink, along with any seaweed that has grown en route. Last summer, Running Tide sold its first carbon credits to Shopify, and the company says it has sunk tens of thousands of tons of material into the North Atlantic Ocean.
Marine scientist David Koweek of the nonprofit Ocean Visions, which has previously supported Running Tide’s research, lauds the simplicity of sinking terrestrial biomass, because technology exists for almost every step in the process. That’s a strong reason why “you might think about doing this,” he says.
Beyond that, the benefits are murkier. Even though boats are a climate-friendly form of transportation (trucks emit at least 100 times more carbon per kilometer), Angel says it wouldn’t make sense to ship biomass around the world to get it to favorable sites. And although sunk terrestrial biomass doesn’t steal nutrients from marine life, removing it from land could deplete soil of nutrients. “Over time we’re going to also be losing some of the fertility that crops and forests need,” says Charlotte Levy, a biogeochemist at Carbon180, which advocates for scaling up carbon removal projects. Levy also worries that as innovators find new uses for scrap biomass — for example, as sustainable building materials or biochar, a charcoal-like soil additive — sinking the biomass might not be the most environmentally friendly use.
Zeng agrees that terrestrial biomass burial will be limited to a few areas of the ocean for the foreseeable future. But he believes the urgency of carbon removal demands that every possible scheme be explored thoroughly. “I think every idea deserves $1 billion of support to test it out,” he says with a smile.