Whether by land, by sea or through human innovation, carbon sequestration is likely coming to (or already happening in) a destination near you. As our planet, overdosed on greenhouse gases, battles climate disasters, a logical solution is to simply stop pumping carbon dioxide into the air. Legislation worldwide is aimed at that target, but reducing output alone may not be enough. There are still billions of tons of extra CO2 already in the atmosphere—this crossroads is where sequestration comes into play.
Carbon sequestration is exactly what it sounds like—the storage of CO2. Once carbon is sucked out of the air, or in some cases pulled directly from industrial smokestacks, sequestration can be undertaken in a lot of different ways. Carbon storage happens naturally, when forests and oceans absorb and convert CO2 into organic matter, but carbon dioxide can also be artificially injected into deep underground rock formations (or wells), or in some cases technological approaches repurpose carbon into a resource like concrete, or as a catalyst in a closed-loop industrial system. However it’s accomplished, the point of sequestration is to stabilize carbon and ensure it doesn’t creep back into our atmosphere. Researchers, like those at the United Nations’ Intergovernmental Panel on Climate Change, now say that CO2 removal is vital to keeping global warming to 1.5 degrees Celsius (past that threshold, climate change could reach catastrophic levels). A 2023 University of Oxford study estimated that, currently, about two billion metric tons of carbon dioxide are being removed each year, primarily through land management (i.e., planting trees), and suggested that we need to double that amount to avoid dangerous global warming levels.
Along with recent Environmental Protection Agency (EPA) approvals for class VI injection wells in several states, carbon sequestration is also backed by a $444 million Biden Administration package for large-scale carbon management and infrastructure. Fresh legislation will certainly follow, to hold carbon innovators accountable for yet untested environmental impacts, and some detractors argue that sequestration is unproven and fails to disincentivize off gassing. Nonetheless, major players around the world, from coastal ecologists in Massachusetts and California startups, to big energy executives and government leaders, are pursuing better carbon storage and view it as a non-negotiable for zero-carbon goals. Ahead, we look at techniques, some familiar and some surprising, for carbon sequestration.
Five If by Land
Forests are well known for their carbon-absorbing capacity, while large-scale farming is more likely to be villainized in today’s environmental discourse. Agriculture, though, is a hotbed for research on carbon sequestration.
- More Salt? Some scientists are proposing the idea of growing biomass crops to capture CO2 from the atmosphere; they would bury the harvested vegetation in engineered dry bio-landfills. With the agro-sequestration process, researchers say that stabilizing the captured carbon is as simple as putting salt on top of the buried debris to halt decomposition, allowing the plants to hang onto captured CO2 for thousands of years.
- The BECCS We Can Do. The U.C. Berkeley team behind the biomass-based bioenergy with carbon capture and storage (BECCS) project touts it as a comparatively thrifty option. With BECCS, experts devised a system that captures carbon dioxide produced during the conversion of biomass crops into fuel or energy, permanently sequestering the CO2. The technique is considered one of the few that can potentially be scaled up to the massive volumes needed to make a dent in carbon reduction—and as a bonus it also produces green energy. However, BECCS would likely require tight regulations and thorough oversight to ensure biomasses are sustainably sourced.
- Powering Up the Soil. In addition to biomass crops, some startups want to maximize soil’s natural affinity for hanging onto carbon. Andes, a company based in California, developed a microbe-based fertilizer of sorts that causes farmland to absorb higher levels of CO2, and in 2023, it treated 50,000 acres of U.S. land with the product. Similarly, major food industry companies like General Mills and Land O’Lakes have begun paying farmers to embrace regenerative techniques that optimize soil sequestration, such as planting cover crops and moving previous harvest seasons’ “trash” aside rather than tilling it under the soil. Soil-based carbon credits are becoming big business, with some estimates indicating the industry could be worth $50 billion by 2030. Deborah Bossio, the lead soil scientist for the Nature Conservancy, told Science.org that “I and many other scientists have a lot of confidence that we can build carbon in soil.”
- Getting to the Roots of a Solution. In another study, farm industry leaders found that grazing dairy farms sequester a substantial amount of carbon, due in part to the grass and their root systems. Canadian researchers are even looking into incentivizing the preservation of the country’s swaths of rural grasslands, because they are such an effective tool for carbon sequestration—as long as they stay intact.
- The Forest, for the Trees. While farms could be an unexpected success story for carbon sequestration, forests remain an irreplaceable resource, with woodlands in the U.S. Northwest offsetting some 13% of U.S. greenhouse gas emissions. Forestry scientists are examining how to enhance our forests’ carbon sink by carefully timing harvests (which could look like thinning out trees, clear cutting or even trimming the understory, at the optimal time). Plus, trees can continue holding carbon for up to 80 years after their death, as they slowly decompose. One 2019 study indicated that if the right species of trees are planted, emerging forests could capture 205 gigatons of carbon dioxide in the next 40 to 100 years.
Straddling the line between land and sea, coastal habitats (think mangroves and wetlands) also remove a lot of carbon out of the atmosphere, at a rate 10 times greater than mature tropical forests. Researchers at the Smithsonian Environmental Research Center aim to boost the natural abilities of these wetlands by designing methodologies that would award carbon credits to tidal wetland projects.
Looking for a Sea Change
Oceans absorb at least a quarter of the world’s carbon emissions, with some sources putting that number in the 30% to 40% range. Investors are pouring millions into new projects that could further increase that amount.
- At the Startup Line. Despite questions about the impact of carbon sequestration on marine biodiversity—and how much of the carbon trapped in the ocean would actually stay there—startups are experimenting with creative approaches. Captura wants to remove dissolved CO2 gas from the ocean and pump it underground or reuse it for industrial needs. The company theorizes that shrinking carbon levels in the ocean will allow the bodies of water to absorb more greenhouse gases from the air.
In a major deal for ocean-based carbon sequestration, California startup Equatic agreed to remove 62,000 tons of carbon on behalf of Boeing. Equatic’s technology uses an electrolytic process, in which ocean water is split into hydrogen and oxygen; then atmospheric air is run through the processed seawater, in effect stabilizing carbon dioxide permanently in the form of dissolved bicarbonate (in seawater) and solid mineral carbonates. The process also extracts hydrogen as a green energy.
If you’ve ever been terrorized by sargassum, the stinky seaweed that has recently washed ashore in the Caribbean and Florida, scientists have a proposal for that aggravation. The National Renewable Energy Laboratory (NREL) conducted a study of several marine carbon management methods, including seaweed farming, microalgae farming and artificial upwelling (which forces algae bloom by pumping nutrients to the surface from deep in the ocean). They found that though seaweed captures greenhouse gases, it may not store them for long. One potential answer is to sink the seaweed—like that pesky sargassum—deep into the ocean, where carbon could stay sequestered for hundreds of years. Sinking huge clumps of seaweed is not risk-free, though. Sea creatures might make a snack of the marine plants during their downward descent, and researchers don’t know how long the carbon would then remain stored safely away in the belly of a fish. Additionally, swarms of wildlife might gravitate to the seaweed, leading to low-oxygen zones that would kill off more wildlife. There are kinks to be worked out, but one Maine startup, RunningTide, recently signed an agreement with Microsoft to remove 12,000 tons of carbon dioxide via a method that relies on floating kelp; the CO2 is captured in kelp and then sequestered into attached wooden buoys that are sunk into the ocean.
- A Whale of a Solution? Scientists have even proposed that recovering whale populations might aid in reaching carbon goals. The giant creatures—particularly great whales—can sequester 33 tons of carbon each year. Over their often 100-year lifespan, the impact could be noteworthy.
- Turning Over New Reefs. Studies suggest that reef sponges are, quite literally, a sponge for greenhouse gases. Off the Texas coast in October 2022, an Enbridge-funded project began exploring carbon sequestration on an artificial reef made up mostly of intentionally sunken vessels, concrete rail ties and cinder blocks. Researchers from the University of Texas Rio Grande Valley say that not only has the reef brought back growing numbers of fish and other sea life, but that its structure, sponges, soft coral, sediment, biomass—and even marine life—are capturing carbon in significant proportions. Natural reefs also help sequester carbon, but their manmade counterparts could be a beneficial and affordable tool for any country with a coastline.
Putting Our Heads Together
Novel technology targeting carbon sequestration is being developed on multiple fronts.
- Electrochemical Solutions. Both MIT and the University of Colorado Boulder have explored the role of electrochemistry in carbon storage. In a 2022 University of Colorado Boulder study, scientists found that a certain type of electrically charged compound—quinones—can bind with and capture carbon in a controlled fashion and may be especially effective in grabbing it from highly concentrated sources like power plants. Power plants produce more than 30% of all CO2 emissions in the United States. Some have installed equipment that catches greenhouse gas before it’s released into the atmosphere, but those systems are costly and energy intensive, and these efforts may only capture 0.1% of global carbon emissions annually. “What we want are technologies that are more modular and flexible and can be adapted to more diverse sources of carbon dioxide. Electrochemical systems can help to address that,” said one of the MIT study’s authors, Betar Gallant.
- Casting Call. Bill Gates has also jumped into the carbon sequestration business. His venture capital firm is backing Graphyte, a startup that pioneered what it calls carbon casting. The technique relies on gathering waste products like discarded wood, rice husks and more, that have already captured significant CO2. The waste is then dried and condensed into large, dense carbon blocks that are wrapped in a protective polymer, stacked and buried underground. Graphyte says the carbon can stay safely stored in this way for 1,000 years or more. The company also announced that it can do all of this work for under $100 per ton of sequestered CO2—an industry standard largely viewed as the threshold to hit for real-life affordability. (Many competing techniques cost closer to $600 per ton.) Graphyte is building its first pilot plant and aims to expand processing capacity to a rate of 50,000 tons per year by the end of 2024.
- The Repurpose of It All. Along with these tech-savvy approaches, other innovators are finding ways to not just store, but also repurpose, excess carbon. Novomer is one such startup; the company’s system will turn greenhouse gas into biodegradable plastics. Another, CarbonCure, mixes captured CO2—which is direct captured from the air by Heirloom Carbon Technologies—with concrete ingredients, in turn strengthening concrete and reducing the need for cement, which is the component of concrete with the largest carbon footprint. Once incorporated into concrete, the carbon should stay there for centuries. Authors of another study say that carbon pulled directly from the air could be converted to baking soda and then stored in the ocean (though international dumping treaties might be triggered, so as with many of these new technologies, legalities will be important considerations for moving forward). In a more niche approach, Finnish startup Aircohol is devising a process that captures carbon from fermented alcohol and reuses it for production of more alcohol, essentially creating a closed-loop system.
Making It Work
While scientists and climate experts agree that carbon sequestration is an urgent requirement in our efforts to reduce greenhouse gases, the costs, methodologies and scalability are factors that, much like carbon dioxide, remain up in the air. Microsoft’s senior director of energy and carbon, Brian Marrs, says his company is investing in a range of carbon removal technologies because it’s impossible to know which approaches will be most effective. Getting a firm grasp of exactly how much carbon is captured also remains scientifically difficult. Perhaps the combination of many carbon sequestration techniques (used alongside carbon-cutting measures) will be the answer, from land to sea to high-tech innovations. Legislation and initiatives around the world will continue to evolve as a means to boost carbon capturing goals, but laws will also need to address the risks involved in some tactics. Michelle You, cofounder of a carbon-accounting business, Supercritical, told Wired magazine, “I know how fast technology can scale. I know how quickly things can change. I believe that that kind of capability will help us in this crisis.”
