IN MAY some 250 scientists and policy types from around the world convened in Gothenburg, Sweden, to discuss a dirty secret of the three-year-old Paris climate agreement. Virtually all simulations which chart paths toward meeting that compact’s goal—to keep temperature rise “well below” 2°C relative to pre-industrial levels—assume not just a sharp reduction in actual emissions but also the removal of carbon dioxide from the atmosphere on a massive scale. One reason such “negative emissions” have been absent from climate discussions—the Swedish shindig being the first of its kind—is that no one has a good idea of how exactly to bring them about. The obvious solution is to plant lots of trees, to convert CO 2 into wood. But this would mean foresting an area with a size somewhere between that of India and Canada. Alternative, engineered fixes have been dogged by potentially stratospheric costs, uncertain efficacy or both.

No longer, reckons David Keith. Besides his day job as a climate expert at Harvard university, Dr Keith is a co-founder of Carbon Engineering, a nine-year-old firm that counts Bill Gates among its backers. Dr Keith and his colleagues argue in a paper they have just published in Joule that the CO 2 removal technique they have been perfecting is no pipe dream—even if it does contain pipes aplenty.

Their process has four steps. First, air is channelled by fans onto a honeycombed plastic slab called a contactor, where CO 2 , which is acidic, reacts with aqueous potassium hydroxide, which is alkaline. The resulting solution of potassium carbonate is filtered and exposed to a slurry of calcium hydroxide. This produces potassium hydroxide, which is recycled back to the contactor, and pellets of calcium carbonate. These are whisked to the third receptacle, called a calciner. There the calcium carbonate is heated to 900°C to release pure carbon-dioxide gas ready for capture, and calcium oxide. Finally, the calcium oxide is piped to a “slaker”, where it is dissolved in water to form calcium hydroxide, which is reused in the second step.

If that all sounds complicated, chemically speaking it is not. Nor is the idea new. A researcher called Klaus Lackner came up with the principles 20 years ago and Dr Keith patented his version in 2015. A pilot plant with a contactor three by five metres across and three metres deep has been running for three years. It extracts a tonne of carbon dioxide from the air per day.

What sets Dr Keith’s latest paper apart from his earlier publications—and, indeed, those of other putative carbon-hoovers—is that it offers a hard-nosed estimate of the system’s cost and scalability. The results look encouraging.

That is principally because each step in Dr Keith’s scheme is adapted from known industrial processes. The contactor was pinched from factory cooling towers. The pellet reactor came from water-treatment plants. The calciner was developed from metal-ore purification apparatus. And the slaker was adapted from pulp mills. The required tweaks were small enough to permit Carbon Engineering to procure the paraphernalia for the prototype plant from existing suppliers. Crucially, this also enabled the suppliers—and an independent engineering consultancy hired by Carbon Engineering—to estimate how much it would cost to build a fully fledged facility (envisaged in the picture above) capable of extracting between 100,000 and 1m tonnes of carbon dioxide a year.

Factoring in operating costs and the cost of capital, the study concludes that Carbon Engineering’s system could capture a tonne of the greenhouse gas for between $94 and $232. That is well below the $600 per tonne suggested by authors of an influential American Physical Society report from 2011 that reviewed proposed carbon-dioxide-removal schemes. Admittedly, it is still much pricier than the $10 or so that a tonne of the gas is worth in emissions-trading schemes such as the European Union’s. But it is of the order of the $100 or so that most climate economists think would eventually be needed to prompt the transition to the low-carbon economy implicit in the Paris agreement. And Dr Keith thinks the cost can be brought down further with a bit of tinkering.

Carbon Engineering and its investors believe they can make money even before this happens. To start with, revenue would be generated by turning captured CO 2 back into fuel (technology to do this already exists). Though that sounds thermodynamically bonkers, such fuel would, from a legal point of view, count as “zero carbon” because making and then using it involves no net release of CO 2 into the atmosphere.

Demand for zero-carbon liquid fuels looks poised to rise as climate-friendly places adopt low-carbon fuel standards. California did this in 2007 and the European Union followed in 2009. Such standards force distributors to keep the average carbon intensity of petrol below a certain threshold, and therefore to offset dirty fuels with clean ones—and none is cleaner than Carbon Engineering’s. California’s requirement, which is in effect a cap-and-trade scheme, translates to a price of $165 per tonne of carbon dioxide, making the company’s product competitive. Steve Oldham, the firm’s boss, therefore hopes to license know-how to fuel producers doing business in such jurisdictions. Mr Oldham says that ground should be broken on the first industrial-scale plant, which is to be built at an undisclosed location in America, before the end of the year.