Klaus Lackner has a picture of the future in his mind, and it looks something like this: 100 million semi-trailer-size boxes, each filled with a beige fabric configured into what looks like shag carpet to maximize surface area. Each box draws in air as though it were breathing. As it does, the fabric absorbs carbon dioxide, which it later releases in concentrated form to be made into concrete or plastic or piped far underground, effectively cancelling its ability to contribute to climate change.

Though the technology is not yet operational, it’s “at the verge of moving out of the laboratory, so we can show how it works on a small scale,” says Lackner, director of the Center for Negative Carbon Emissions at Arizona State University. Once he has all the kinks worked out, he figures that, combined, the network of boxes could capture perhaps 100 million metric tons (110 million tons) of CO 2 per day at a cost of US$30 per ton — making a discernible dent in the climate-disrupting overabundance of CO 2 that has built up in the air since humans began burning fossil fuels in earnest 150 years ago.

Lackner is one of hundreds, if not thousands, of scientists around the world who are working on ways to remove CO 2 from the atmosphere, capturing carbon from the atmosphere using plants, rocks or engineered chemical reactions and storing it in soil, products such as concrete and plastic, rocks, underground reservoirs or the deep blue sea.

“We can’t just decarbonize our economy, or we won’t meet our carbon goal.” – Noah Deich

Some of the strategies — known collectively as carbon dioxide removal or negative emissions technologies — are just twinkles in their envisioners’ eyes. Others — low-tech schemes like planting more forests or leaving crop residues in the field, or more high-tech “negative emissions” setups like the CO 2 -capturing biomass fuel plant that went online last spring in Decatur, Illinois — are already underway. Their common aim: To help us out of the climate change fix we’ve gotten ourselves into.

“We can’t just decarbonize our economy, or we won’t meet our carbon goal,” says Noah Deich, co-founder and executive director with the Center for Carbon Removal in Oakland, California. “We have to go beyond to clean up carbon from the atmosphere. … [And] we need to start urgently if we are to have real markets and real solutions available to us that are safe and cost effective by 2030.”

Many Approaches

Virtually all climate change experts agree that to avoid catastrophe we must first and foremost put everything we can into reducing CO 2 emissions. But an increasing number are saying that’s not enough. If we are to limit atmospheric warming to a level below which irreversible changes become inevitable, they argue, we’ll need to actively remove CO 2 from the air in fairly hefty quantities as well.

“It’s almost impossible that we would hit 2 °C, and even less so 1.5 [°C], without some sort of negative emissions technology,” says Pete Smith, chair in plant and soil science at the University of Aberdeen and one of the world’s leaders in climate change mitigation.

In fact, scientists from around the world who recently drew up a “road map” to a future that gives us good odds of keeping warming below the 2 ºC threshold lean heavily on reducing carbon emissions by completely phasing out fossil fuels — but also require that we actively remove CO 2 from the atmosphere. Their scheme calls for sequestering 0.61 metric gigatons (a gigaton, abbreviated Gt, is a billion metric tons or 0.67 billion tons) of CO 2 per year by 2030, 5.51 by 2050, and 17.72 by 2100. Human-generated CO 2 emissions were around 40 Gt in 2015, according to the National Oceanic and Atmospheric Administration.

“It’s almost impossible that we would hit 2 °C, and even less so 1.5, without some sort of negative emissions technology.” –Pete Smith

Reports periodically appear pointing out that one approach or another is not going to cut it: Trees can store carbon, but they compete with agriculture for land, soil can’t store enough, machines like the ones Lackner envisions take too much energy, we don’t have the engineering figured out for underground storage.

It’s likely true that no one solution is the fix, all have pros and cons, and many have bugs to work out before they’re ready for prime time. But in the right combination, and with some serious research and development, they could make a big difference. And, as an international team of climate scientists recently pointed out, the sooner the better, because the task of reducing greenhouse gases will only become larger and more daunting the longer we delay.

Smith suggests dividing the many approaches into two categories — relatively low-tech “no regrets” strategies that are ready to go, such as reforestation and improving agricultural practice, and advanced options that need substantial research and development to become viable. Then, he suggests, deploy the former and get working on the latter. He also advocates for minimizing the downsides and maximizing the benefits by carefully matching the right approach with the right location.

“There are probably good ways and bad ways of doing everything,” Smith says. “I think we need to find the good ways of doing these things.”

Deich, too, supports the simultaneous pursuit of multiple options. “We don’t want a technology, we want lots of complementary solutions in a broader portfolio that updates often as new information about the solutions emerges.”

With that in mind, here is a quick look at some of the main approaches being considered, including a ballpark projection based on current knowledge of CO 2 storage potential distilled from a variety of sources — including preliminary results from a University of Michigan study expected to be released later this year — as well as summaries of advantages, disadvantages, maturity, uncertainties and thoughts about the circumstances under which each might best be applied.

Afforestation and Reforestation

Pay your entrance fee, drive up a winding road through Sequoia National Park in California, hike half a mile through the woods, and you’ll find yourself at the feet of General Sherman, the world’s largest tree. With some 52,500 cubic feet (1,487 cubic meters) of wood in its trunk, the behemoth has more than 1,400 metric tons (1,500 tons) of CO 2 trapped in its trunk alone.

Though its size is clearly exceptional, the General gives an idea of trees’ potential to suck CO 2 from the air and store it in wood, bark, leaf and root. In fact, the Intergovernmental Panel on Climate Change estimates that a single hectare (2.5 acres) of forest can take up somewhere between 1.5 and 30 metric tons (1.6 and 33 tons) of CO 2 per year, depending on the kinds of trees, how old they are, the climate and so on.

Worldwide, forests currently sequester on the order of 2 Gt CO 2 per year. Concerted efforts to plant trees in new places (afforest) and replant deforested acreage (reforest) could increase this by a gigaton or more, depending on species, growth patterns, economics, politics and other variables. Forest management practices emphasizing carbon storage and genetic modification of trees and other forest plants to improve their ability to take up and store carbon could push these numbers higher.

Another way to help enhance trees’ ability to store carbon is to make long-lasting products from them — wood-frame buildings, books and so on. Using carbon-rich wood for construction, for example, could extend trees’ storage capacity beyond forests’ borders, with wood storage and afforestation combining for a potential 1.3–14 Gt CO 2 per year possible, according to The Climate Institute, an Australia-based research organization.

Carbon Farming

Most farming is intended to produce something that’s harvested from the land. Carbon farming is the opposite. It uses plants to trap CO 2 , then strategically uses practices such as reducing tilling, planting longer-rooted crops and incorporating organic materials into the soil to encourage the trapped carbon to move into — and stay in — the soil.

“Currently, many agricultural, horticultural, forestry and garden soils are a net carbon source. That is, these soils are losing more carbon than they are sequestering,” notes Christine Jones, founder of the Australia-based nonprofit Amazing Carbon. “The potential for reversing the net movement of CO 2 to the atmosphere through improved plant and soil management is immense. Indeed, managing vegetative cover in ways that enhance the capacity of soil to sequester and store large volumes of atmospheric carbon in a stable form offers a practical and almost immediate solution to some of the most challenging issues currently facing humankind.”

Soil’s carbon-storing capacity could go even higher if research initiatives by the Advanced Research Projects Agency–Energy, a U.S. government agency that provides research support for innovative energy technologies, and others aimed at improving crops’ capacity to transfer carbon to the soil are successful. And, points out Eric Toensmeier, author of The Carbon Farming Solution, the capacity of farmland to store carbon can be dramatically increased by including trees in the equation as well.

“Generally it is practices that incorporate trees that have the most carbon [storage] — often two to 10 times more carbon per hectare, which is a pretty big deal,” Toensmeier says.

Other Vegetation

Although forests and farmland have drawn the most attention, other kinds of vegetation — grasslands, coastal vegetation, peatlands — also take up and store CO 2 , and efforts to enhance their ability to do so could contribute to the carbon storage cause around the world.

Coastal plants, such as mangroves, seagrasses and vegetation inhabiting tidal salt marshes, excel at sequestering CO 2 — significantly more per area than terrestrial forests, according to Meredith Muth, international program manager with the National Oceanic and Atmospheric Administration.

“These are incredibly carbon-rich ecosystems,” says Emily Pidgeon, Conservation International senior director of strategic marine initiatives. That’s because the oxygen-poor soil in which they grow inhibits release of CO 2 back to the atmosphere, so rather than cycling back into the atmosphere, carbon simply builds up layer by layer over the centuries. With mangroves sequestering roughly 1,400 metric tons (1,500 tons) per hectare (2. 5 acres); salt marshes, 900 metric tons (1,000 tons); and seagrass, 400 metric tons (400 tons), restoring lost coastal vegetation and extending coastal habitats holds potential to sequester substantial carbon. And researchers are eyeing strategies such as reducing pollution and managing sediment disturbance to make these ecosystems absorb even more CO 2 .

And, Pidgeon adds, such vegetation provides a double climate benefit because it also helps protect coastlines from erosion as warming causes sea level to rise.

“It’s the perfect climate change ecosystem, especially in some of the more vulnerable places,” she says. “It provides storm protection, erosion control, maintains the local fishery. In terms of climate change, it’s immensely valuable, whether talking mitigation or adaptation.”

Bioenergy & Bury

In addition to tapping vegetation’s capacity to store CO 2 in plant parts and soil, humans can enhance sequestration by socking away the carbon plants absorb in other ways. A US$208 million power plant that started operation earlier this year in the heart of Illinois farm country is a tangible example of this approach and what is currently widely seen as the most promising technology-based strategy for removing large amounts of carbon from the air: bioenergy carbon capture and storage, or BECCS.

BECCS generally starts with converting biomass into a usable energy source such as liquid fuel or electricity. But then it takes the concept one key step further. Rather than sending the CO 2 released during the process into the air, as conventional facilities do, it captures and concentrates it, then traps it in material such as concrete or plastic or — as is the case for the Decatur plant — injects it into rock formations that trap the carbon far below Earth’s surface.

A related strategy proposes using ocean plants such as kelp instead of land plants. This would reduce the need to compete with food production and land habitat preservation for land. This option has not been explored as much as land-based BECCS, however, so the number of unknowns is even higher.

On the storage end of things, many of the technologies proposed are still in concept or early development stage. But if developed correctly, the approach has “potentially got quite a significant impact,” says the University of Aberdeen’s Smith.

Biochar

Another way to enhance plants’ ability to store carbon is to partly burn materials such as logging slash or crop waste to make a carbon-rich, slow-to-decompose substance known as biochar, which can then be buried or spread on farmland. Biochar has been used for centuries to enrich soil for farming, but of late has been drawing increased attention for its ability to sequester carbon — as evidenced by the fact that three of 10 finalists in a US$25 million Earth Challenge launched by Virgin in 2007 tap this approach.

Fertilizing the Ocean

Plants and plantlike organisms that live in the ocean absorb immeasurable amounts of CO 2 each year, their ability to do so limited only by the availability of iron, nitrogen and other nutrients they need to grow and multiply. So researchers are looking at strategies for fertilizing the ocean or bringing nutrients up from the depths to hyperdrive plants’ ability to trap and store carbon.

A decade or so ago companies began forming to do just that, with the plan of reaping rewards from the soon-to-be-established global carbon market. Such plans have largely remained on the drawing board, stymied by substantial uncertainties over how to put a price tag on carbon, concerns over disrupting fisheries and ocean ecosystems more generally, and the high energy requirements and costs that would likely be involved. In addition, we don’t have a clear picture of how much of the carbon trapped would actually stay in the ocean rather than reentering the atmosphere.

Rock Solutions

CO 2 is naturally removed from the atmosphere every day through reactions between rainwater and rocks. Some climate scientists propose enhancing this process — and so increasing CO 2 removal from the atmosphere — through artificial measures such as crushing rocks and exposing them to CO 2 in a reaction chamber or spreading them over large areas of land or ocean, increasing the surface area over which the reactions can occur.

As currently imagined, strategies to enhance carbon storage by reacting CO 2 with rocks are expensive and energy-intensive due to the need to transport and process large quantities of heavy material. Some also require extensive land use and so have potential to compete with other needs such as food production and biodiversity protection. Researchers are looking at ways to use mine waste and otherwise refine the strategy to reduce costs and increase efficiency.

Direct Air Capture and Storage

The carbon-sequestering containers from Arizona State University’s Lackner, along with other projects such as Climeworks’ just-opened carbon-trapping facility in Switzerland, represent one of the more widely discussed greenhouse gas capture and storage technologies being proposed today. Known as direct air capture and storage, this approach uses chemicals or solids to capture the gas from thin air, then, as in the case of BECCS, stores it for the long haul underground or in long-lasting materials.

Already used in submarines beneath the surface of the ocean and in space vehicles far above it, direct air capture theoretically can remove CO 2 from the air a thousand times more efficiently than plants, according to Lackner.

The technology, however, is embryonic. And because it requires plucking CO 2 molecules from everything else in the air it is a huge energy hog. On the flip side, this approach has the big advantage of being deployable anywhere on the planet.

Where to From Here?

If anything is clear from this summary, it’s these two things: First, there is a lot of potential to augment efforts to reduce CO 2 emissions with strategies to increase the removal of CO 2 from the atmosphere. Second, there’s a lot of work to be done before we’re able to do so at a meaningful scale and in a way that not only closes the carbon gap but also protects the environment and meets more immediate human needs.

“Based on current technology, there really is no combination of negative emissions technologies currently available that would be employable at sufficient scale to help meet the below-2 °C target without truly significant impacts,” says Peter Frumhoff, director of science and policy and a chief scientist with the Union of Concerned Scientists. “We can in principle deploy negative emissions technologies, but we do not have the understanding or the policies to do so on a sufficient scale.”

With the need to do something becoming ever more urgent, researchers are starting to take a closer look at the pros, cons and potential of the various opportunities and put together research agendas to advance the most promising in the right places at the right time. In May 2017, a National Academy of Sciences study panel began holding a series of strategy sessions to identify research priorities for moving forward.

“Our job on this committee is to recommend a research agenda to solve a lot of these problems, to bring the cost down, to bring the efficiency of the program up, to overcome the barriers for scale up and implementation and governance and especially verification and monitoring,” panel chair Stephen Pacala, professor of ecology and evolutionary biology with Princeton University, said in a video describing the initiative.

That said, it’s important to remember that technology may not be the limiting factor in the long run.

In the end carbon storage is not cheap, Smith admits — but, he points out, neither is climate change.

“I don’t think it’s a technical challenge,” says Deich. “I think it’s a willingness to pay and a willingness to get clear, consistent and fair regulations around these solutions.” In other words, getting carbon storage up and running ultimately is about creating markets and/or policies that reward it while also taking into consideration social and environmental dimensions. “It’s not necessarily, ‘Can these things get to scale?’ It’s, ‘Is there somebody who’s willing to pay for them to get to scale?”

The most obvious way to do this would be to affix a price to carbon, which would translate into financial benefit for socking it away.

In the end carbon storage is not cheap, Smith admits — but, he points out, neither is climate change.

The way Lackner puts it is this: We’re traveling at high speed down a mountain in a car coming up to a hairpin turn, and it’s not so much a question of whether we hit the guard rail as to whether we can slow down enough so that when we do we bounce off rather than catapult over it into oblivion.

“I cannot guarantee it will work,” he says of his CO 2 -trapping devices. “I’m an optimist, but I likely cannot guarantee it. The fact that it might not work, the possibility that it might not work, is not by itself an excuse not to try. If we don’t make it work I am very certain we will be in for very tough times.”

UPDATED 7.19.17: The description and disadvantages of coastal vegetation carbon storage were updated.