The longer we wait to take serious action on climate change, the more necessary it becomes to remove some of the CO 2 we've already put in the air. In fact, the scenarios in the most recent Intergovernmental Panel on Climate Change report in which warming was limited to 2°C relied heavily on CO 2 removal.

Adoption of the necessary technology still seems too far off, given the current lack of market incentives and supporting policies. The technique seen as most likely to scale up involves growing biofuel crops and burning them in power plants that capture the CO 2 so it can be injected into underground storage. Like any scheme, this has drawbacks—it could compete with food crops for farmland, for example.

A new study led by the University of California, Santa Cruz's Greg Rau highlights another tool for our CO 2 removal toolbox: splitting seawater to produce hydrogen gas for fuel while capturing CO 2 with ocean chemistry.

Seawater chemistry

In electrolysis, a device powered by electricity is used to split H 2 O, producing hydrogen gas. Several chemical modifications to this process have been proposed that can also grab CO 2 from the atmosphere. Like the idea of using biofuels, this represents a "win-win" by producing an energy resource while capturing CO 2 , bringing the cost down.

For example, one method uses special membrane filters to separate the hydrogen and hydroxide ions produced during electrolysis. Adding the hydroxide to water allows it to take up CO 2 from the air, turning it into bicarbonate. If the hydrogen ions weren't separated, they'd push the chemical equilibrium away from bicarbonate and toward dissolved CO 2 . But when powdered carbonate rock is added, it can react with the dissolved (atmospheric) CO 2 to produce a bunch of happy, stable bicarbonate. Combined, these reactions allow people to tune the hydrogen production and carbonate formation.

That may be more chemistry than you wanted to hear, but the gist is that atmospheric CO 2 goes into the ocean as bicarbonate—which won't acidify the water or harm ecosystems. So if you power the electrolysis process with renewable energy, you can turn solar/wind/hydroelectric energy into hydrogen fuel while also removing CO 2 from the air.

The new study focuses on a basic estimate of the cost and maximum potential of this technique. First, the researchers worked out its efficiency of CO 2 capture—about 0.3 tons captured per gigajoule of electricity input, including the losses from quarrying and crushing rock. That's around 10 times greater than biofuel schemes, but it depends on the assumption that there is demand for all the hydrogen fuel you make. The hydrogen can be used by vehicles, and there's the possibility of using hydrogen as a type of storage for the electric grid—using excess power to make hydrogen that can run a power plant when needed. So it's not too farfetched that demand could rise to meet supply.

Relatively cheap and scalable

The researchers' back-of-the-envelope estimate puts the cost of this system at between $3 and $161 per ton of captured CO 2 , depending on which type of renewable energy powers it. That's equal to or cheaper than estimates for biofuels, which were thought to be the most practical method.

The researchers used existing estimates of the upper limits of potential renewable energy production to get a sense of how much CO 2 this system could possibly remove from the atmosphere. If all the world's potential hydroelectric, wind, geothermal, and biomass generation was used for this process (not that this would ever happen), you could capture twice as much CO 2 per year as we currently emit. For a slightly more meaningful comparison, this comes in at about eight times the maximum potential for capture using biofuels. Essentially, it's a lot of CO 2 if you go crazy.

Obviously, this scheme has its drawbacks. Quarrying rock has its own localized environmental impact, as could pumping all that extra bicarbonate into the ocean. But the researches argue that the idea is worth studying much more closely. The more options for removing atmospheric CO 2 we work up, the more likely it is that one catches on when serious incentives finally arrive.

Nature Climate Change, 2018. DOI: 10.1038/s41558-018-0203-0 (About DOIs).