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Can We Really Scrub Carbon Dioxide From the Atmosphere?

Your burning climate question: Can carbon dioxide be removed?

By Henry Fountain

Do you have questions about climate change? We’re answering them in this newsletter. Send us yours via the form at the bottom of our climate Q. and A. This one was sent to us from Walter Beinecke in Manchester-by-the-Sea, Mass.:

What are the leading/viable technologies to remove carbon dioxide from the atmosphere?

Removing carbon dioxide that is already in the air is seen as a potential way to combat global warming. There are various approaches, lumped together as “negative emissions technologies” to distinguish them from technologies that reduce or eliminate emissions from power plants and other sources.

In theory, reducing the concentration of heat-trapping carbon dioxide in the atmosphere might be one way to keep the world under the two-degree Celsius target for warming established by the 2015 Paris climate agreement. But in practice, removing carbon dioxide is far from simple. There are major questions about scale, cost, speed and energy requirements. In most cases, the carbon dioxide that was removed would have to be buried underground indefinitely — and carbon storage technologies have only been deployed on a small scale so far.

Some removal technologies are more fanciful than others, but as for which are most viable, it could be argued that none are, at least not yet. In a report last month, the European Academies Science Advisory Council offered a pessimistic outlook for carbon removal, saying that it offered only “limited realistic potential” to have a climate impact. The authors argued that the world should not count on removal technologies to make up for a failure to sharply reduce or eliminate emissions in the first place.

There are five major approaches to carbon dioxide removal:

Plant more forests. Trees remove carbon dioxide naturally, incorporating it into their tissues as they grow. Worldwide, forests store about one billion to two billion tons of carbon annually, offsetting a chunk of the roughly 10 billion tons emitted by human activity. Reforestation and afforestation, properly managed, could remove a lot more and keep it out of the atmosphere. But planting forests is slow work — as Icelanders know well — and requires a lot of land. The world is currently much better at cutting down forests than planting new ones.

Crush a lot of rock. This technique is called enhanced weathering, and is based on the fact that some types of rock weather by naturally combining with carbon dioxide in the air or water. One suggested approach would use the mineral olivine, which is plentiful, crushing it into fine sand and spreading it on land, perhaps along coastlines. But mining, crushing and transporting the billions of tons needed would be expensive and energy intensive. And the carbon removal would still be exceedingly slow.

Burn plants for energy and capture the carbon dioxide. In this high-tech approach, called bioenergy with carbon capture and storage, or BECCS, vegetation would be used to naturally remove carbon dioxide. The vegetation would then be burned in a power plant and the carbon dioxide in the exhaust gases would be captured and stored. So far there are only a handful of working BECCS projects; others have been canceled. Among the many questions about the technology is whether emissions are really negative if the carbon cost of growing and harvesting the vegetation is taken into account.

Sprinkle iron in the ocean. Like enhanced weathering, fertilizing the ocean by putting iron particles or other nutrients in the water is among the more far-fetched approaches. The idea is that the nutrients would stimulate the growth of tiny marine plants called phytoplankton, which would incorporate carbon dioxide as they grew and then sink to the bottom of the ocean when they died, taking the carbon with them. Generally, however, putting large amounts of metal or chemicals into seawater is considered ocean dumping. There have been only a few tests of the idea, one of which was conducted without scientific oversight off Western Canada in 2012 under the pretense of helping a native Canadian community improve its salmon catch.

Suck carbon dioxide out of the air. There has been a significant amount of research into “direct air capture.” Much of the technology is similar to what is used in carbon capture projects at power plants: chemicals bind with carbon dioxide molecules and then are heated or otherwise treated to release them for capture. Several companies, including Carbon Engineering and Climeworks, have developed machines to do this. But carbon capture at a fossil-fuel plant, where carbon dioxide can make up perhaps 5 to 10 percent of the exhaust gases, is one thing. Doing it from the air is another. For all the rightful concern about rising carbon dioxide levels, the gas still makes up only about 0.04 percent of the atmosphere. Removing a significant amount of it would involve moving huge volumes of air through thousands upon thousands of capture machines, and powering the machines for decades.