Author: Mary Hoff | Published: July 19, 2017 

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 CO₂ per day at a cost of US$30 per ton — making a discernible dent in the climate-disrupting overabundance of CO₂that 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 CO₂ 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 CO₂-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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ per year by 2030, 5.51 by 2050, and 17.72 by 2100. Human-generated CO₂ 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 CO₂ 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 CO₂ trapped in its trunk alone.

Though its size is clearly exceptional, the General gives an idea of trees’ potential to suck CO₂ 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 CO₂ 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 CO₂per 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 CO₂ 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 CO₂, 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 CO₂ 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.

Although forests and farmland have drawn the most attention, other kinds of vegetation — grasslands, coastal vegetation, peatlands — also take up and store CO₂, 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 CO₂ — 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 CO₂ 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 CO₂.

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.”

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