This is how we might pull CO2 out of the atmosphere

Source: Chelsea Harvey, E&E News reporter • Posted: Tuesday, June 5, 2018

Researchers convened in Sweden last month for a first-of-its-kind international conference on “negative emissions.” They were looking for strategies to combat climate change by pulling carbon dioxide out of the atmosphere.

At the same time, 20 scientists from around the world published a hefty three-part literature review in the journal Environmental Research Letters on the same topic: negative emissions.

Both events reflect rising interest in negative emissions technology. It’s fueled by growing concern about the world’s ability to meet its climate targets under the Paris Agreement. Nations are working to keep global temperatures within at least 2 degrees Celsius of preindustrial levels. But scientists increasingly warn that world leaders won’t hit those targets without stronger action.

Negative emissions technology offers the hope that climate goals can still be met, even if temperatures first rise above 2 degrees. Actually removing carbon dioxide from the atmosphere would theoretically cool the planet back down.

It’s a controversial subject. Some experts say putting too much stock in negative emissions could undermine global efforts to curb greenhouse gas emissions now. Others argue that world leaders should be prepared for the worst-case scenario and get the technology ready for such an outcome.

In the meantime, researchers have proposed an array of negative emissions strategies, involving everything from futuristic machines to massive carbon-sucking tree plantations. For now, aside from a few small-scale demonstration projects, they’re almost entirely theoretical. But more and more research is looking into the potential benefits — and challenges — of creative proposals.

Here are some of the ideas.

Greenhouse-gas guzzlers

Many scientists have touted the idea of planting or restoring vegetated landscapes to naturally suck up and store away large quantities of carbon.

These ideas usually involve planting terrestrial forests, although some researchers suggest the same strategy could be applied to carbon-rich coastal ecosystems, such as salt marshes or mangrove forests. The potential payoff for land-based forests is fairly large: Studies indicate that afforestation and reforestation could save up to 3.6 billion tons of carbon dioxide annually by midcentury.

The upside is that plants do the work. The downside is that they require a lot of space to make a dent in global greenhouse gas emissions: potentially millions of square miles of land to achieve the greatest storage potential.

Scientists have also proposed using the ground itself. Like trees, soil does a great job of storing carbon when it’s healthy and managed properly. Some researchers suggest that more sustainable agricultural practices — such as more efficient crop rotations, or the use of certain fertilizers — could increase the carbon storage ability of soils around the world, potentially putting away as much as 5 billion tons of carbon dioxide a year.

A boost from science

With a little creativity, landscapes could soak up even more carbon than they normally might, some experts say. One idea involves the use of biochar: wood or agricultural waste that’s been specially heated until it turns into a charcoal-like substance. Biochar can boost the productivity of cropland, helping soil retain more water and nutrients and soak up more carbon.

In another process, known as enhanced weathering, crushing up certain types of rocks or minerals can speed up a natural carbon-storing process. When rain forms, it dissolves a little bit of carbon dioxide from the atmosphere. The slightly acidic rain wears away at rocks when it falls to the earth, producing a chemical byproduct called bicarbonate, which mixes into the soil. Eventually, the bicarbonate may wash into nearby waterways and flow out to the ocean, where it can remain trapped for thousands of years.

Crushing rocks into powder can speed up the weathering process. Farmland is one obvious target for the pulverized rock dust, scientists say — partly because the landscape is already cleared and partly because the crushed rocks can potentially increase cropland’s fertility and productivity. Other researchers have suggested pouring the crushed rocks into the sea.

Some research indicates that enhanced weathering could put away several billion tons of carbon dioxide each year if applied at a global scale — but scientists are still investigating potential side effects of adding the powder to farmland or allowing it to wash into water sources.

Two birds with one stone

One of the most widely discussed negative emissions proposals involves not just planting forests, but also harvesting them for energy. It’s known as bioenergy with carbon capture and storage, or BECCS.

The idea calls for large tree plantations that would draw carbon out of the atmosphere before being cut down and used in biofuels or burned for energy. Special carbon capture technology would collect and store the resulting emissions before they could escape back into the atmosphere, rendering the entire process carbon neutral.

The strategy is attractive in that it promises both carbon removal and clean energy production. But scientists have some major concerns about its environmental impact at a large scale, mainly related to the sheer amount of water, land and other natural resources that would be needed for massive tree plantations (Climatewire, March 13).

Studies vary widely in their estimates of how much carbon BECCS could store globally. The literature review released last month shows that the range may run anywhere from 0.5 billion to 5 billion tons of carbon dioxide annually by midcentury.

By land or by sea

Land isn’t alone in its potential to store carbon. A technique known as ocean fertilization aims to boost the carbon-storing capacity of marine environments. The idea suggests iron or other nutrients could be poured into the ocean to enhance the growth of carbon-capturing algae.

But ocean fertilization carries a particularly high amount of uncertainty and potential risks, according to some scientists. For one thing, it would be difficult to control the types of organisms that respond to fertilization, meaning toxic algae blooms are a potential side effect. The ecological impacts of boosting certain types of algae, which form the baseline of the entire marine food web, are also unclear.

Moreover, algae aren’t actually that efficient at sequestering carbon — research suggests a lot of the carbon dioxide they take in gets cycled through the marine ecosystem and goes back into the atmosphere. As a result, the amount of extra algae growth necessary to make a dent in emissions is formidable. Based on the research so far, the authors of the literature review say ocean fertilization is probably “not a viable negative emissions strategy” compared with other proposed technologies.

High-tech machines

Natural carbon sinks aside, some experts propose using machines to draw carbon dioxide back out of the atmosphere. It may sound like the plot of a sci-fi movie, but some small-scale projects have already demonstrated it’s possible.

Last year, the Swiss company Climeworks became the first to open a commercial carbon capture plant, currently operating outside Zurich. It uses special filters to capture carbon with a chemical bond, making it easy to collect.

The plant captures about 1,000 tons of carbon dioxide each year, which it pipes into greenhouses to boost plant growth. A second plant, currently being tested in Iceland, would capture about the same amount of carbon dioxide each year but would mix it with water and pump it deep underground. Eventually, the company aims to capture 1 percent of all global greenhouse gas emissions. At current levels, that would be nearly half a billion tons of carbon dioxide each year.

While direct air capture is now proven in small projects, its potential at a large scale remains to be seen. The technology’s upfront costs are among the greatest barriers to its advancement. But there are other concerns, as well — namely, the large amount of energy required to power the machines. Unless the electricity comes from carbon-neutral sources, there’s the risk of offsetting or even negating the carbon dioxide being pulled out of the air.

Outside the box

For now, the above are among the most commonly discussed — and widely researched — negative emissions ideas. But others are emerging as interest in carbon dioxide removal grows.

Some researchers are investigating ways to funnel captured carbon dioxide into other industrial applications instead of just storing it away: for instance, using it to make synthetic chemicals or fuels. Finding profitable uses for carbon dioxide could provide greater incentives for companies to invest in carbon-capturing machines.

In the meantime, other scientists are searching for ways to remove other types of greenhouse gases from the atmosphere, such as methane — a shorter-lived but far more potent warming agent.

For now, even most research on carbon dioxide removal is highly preliminary, and it remains to be seen whether the proposed technologies can be scaled up to make a dent in global emissions.

Studies on individual negative emissions strategies, such as BECCS or direct air capture, suggest that even when applied at a global scale — accounting for factors like the amount of land, power or other resources required to maintain them — they might each store a few billion tons of carbon dioxide each year, on the high end. If multiple technologies were deployed together, though, their combined carbon-capturing potential could be far more substantial.

“There is no silver bullet: no single technology is likely to provide all the CO2 removal that is required,” two authors of the recent science review wrote in The Washington Post. “Only diversified portfolios can help us to hedge the risks associated with CO2 removal.”