Clearing the air

From space, the Earth's atmosphere appears paper thin, as if it doesn't weigh much at all. In fact, it weighs 5 and a half quadrillion tons. How much does Earth’s atmosphere weigh? — Britannica

It's a lot of air, and somehow we need to scrub it clean — to remove the excess CO2, and to do it soon before climate change can do its worst.

We already have the technology to do this. In operation now, are two companies — Climeworks in Switzerland Climeworks and Carbon Engineering in Canada, Carbon Engineering that are removing CO2 directly from the atmosphere.

Cleaning solutions

From their location near Zürich, Switzerland, Climeworks is currently removing 900 tons of CO2 a year. On the outside of the Climeworks building, large fans draw in air which is filtered to remove the CO2. Climeworks then sells the CO2 to greenhouses to make plants grow faster or to drinks manufacturers to make the drinks fizzier. News and press — Climeworks

The second company, Carbon Engineering, situated in scenic B.C., Canada, removes a ton of CO2 a day from the surrounding air using a chemical process to extract the CO2. Carbon Engineering They turn the CO2 into fuel, or sell it to oil companies. Uses — Carbon Engineering

Both Climeworks and Carbon Engineering have a working solution, in operation now and removing CO2 from the atmosphere. With 30,000 Carbon Engineering facilities, or another 30 million Climeworks units, the world could remove 30 gigatonnes of CO2 per year, which is as much as is produced each year. An inter-model assessment of the role of direct air capture in deep mitigation pathways — science article

It's not an impossible number either. There are 30,000 power plants in the world, A global database of power plants — World Resources Institute or 70 million cars produced annually. Worldometer An equivalent number of Carbon Engineering plants or Climeworks units is within reach.

What is impossible is the amount of electricity they would need. To build enough plants to capture 30 gigatonnes of CO2 per year, one estimate suggests that altogether, they would require half of the world's current electricity production. An inter-model assessment of the role of direct air capture in deep mitigation pathways — science article

The new CO2 scrubbers

There are solutions in the pipeline which require much less electricity. Professor Klaus Lackner of Arizona State University has spent the last 20 years working on what he calls a "Mechanical Tree".

Actually it looks more like a spring.

Artist's rendition of a mechanical tree. Arizona State University, CC BY 4.0

The magic lies in a special resin that absorbs CO2 blown by the wind, and which can be released in a container of water. Powerful 'mechanical trees' can remove CO2 from air to combat global warming at scale — Arizona State University

None of this requires much electricity at all, and seems to solve the massive energy requirements of Carbon Engineering or Climeworks.

Prof. Lackner's hard work may have paid off. Irish investor Silicon Kingdom has agreed to build and install its first direct air capture machine by November 2020. Start-ups test ideas to suck CO2 from atmosphere — Financial Times In the longer term, they intend to build 250 Mechanical Tree farms around the world, which they say, is enough to remove 1 gigatonne of CO2 per year. Impact - Mechanical Trees

If those figures are correct, Mechanical Trees could potentially remove thousands of gigatonnes of CO2. Instead of building 250 farms and removing 1 gigatonne of CO2 per year, they could build 12,500 farms and remove 50 gigatonnes.

In 20 years Mechanical Trees might be able to remove 1000 gigatonnes of CO2 from the atmosphere.

Removing this much CO2 could be what's needed to slow down global warming. Special report on global warming of 1.5 °C — IPCC With no fans, heating or cooling requirements, Mechanical Trees wouldn't need half the world's electricity either.

Going underground

Regardless of whether it's Climeworks, Carbon Engineering, Mechanical Trees or an even newer technology still in the works, e.g. Verdox, Verdox all these companies come up against the same issue — what to do with all that CO2 they've captured.

With so much work required to get CO2 out of the atmosphere, it would be nice to think they are getting rid of the CO2, permanently.

Carbon Engineering is working with oil company Occidental Petroleum to get rid of the CO2 it's captured. In a project planned for 2022, in the Permian Basin, Texas, the intention is to inject CO2 into old, disused oil wells to force out the remaining oil. The process is called "enhanced oil recovery". The oil rises to the surface, and the CO2 stays deep underground, hopefully permanently.

Enhanced oil recovery at Lost Hills oilfield, California. Richard Masoner, CC BY 2.0

This odd alliance between environmentalists and oil companies could mean profits for all. Carbon Engineering can sell the CO2 it has extracted from the atmosphere, which, they say, will be "permanently, safely and securely stored deep underground". Occidental Petroleum gets a reliable source of CO2 which it can use to keep older oilfields in operation. News and updates — Carbon Engineering

But just how environmentally friendly is it? The CO2 may stay permanently underground, but the oil it forces out of the ground still ends up burning and releasing CO2 into the atmosphere. One estimate is that, for every ton of CO2 stored underground, a similar quantity of CO2 (about 75‑90%) will end up in the atmosphere from the extracted oil. Is EOR a dead end for carbon capture and storage? — Power Magazine This means, however much CO2 Carbon Engineering removes from the atmosphere, three quarters or more will be added again by their partner Occidental Petroleum.

Walking away from CO2

Climeworks has an entirely different plan for getting rid of CO2. Together with an Icelandic company — CarbFix, Carbfix they have been pumping CO2 down holes drilled deep in basaltic rock. Within 2 years the CO2 reacts with the rock and becomes a mineral. It's a permanent solution for CO2, and there is the potential to store thousands of gigatonnes of CO2 in this kind of rock around the world. CO2 storage potential of basaltic rocks offshore Iceland — science article

"We are permanently getting rid of the CO2. We can walk away from it", says Sandra Snæbjörnsdóttir, head of CO2 Mineral Storage at CarbFix. Basalts turn carbon into stone for permanent storage — Eos In other words, the CO2 is out of the equation. Converted into rock and locked underground, the CO2 will no longer cause climate change.

So far CarbFix has permanently removed 50,000 tonnes of CO2, and is currently removing 12,000 more each year. There's room to grow. The process relies on basalt rock which is the most common rock on the planet. In Iceland alone there's capacity to store 400 gigatonnes of CO2. FAQ - Carbfix Globally, there is enough basalt to store 60 million gigatonnes of CO2, easily absorbing the CO2 resulting from burning all fossil fuels on Earth, if we ever went that far. An overview of the status and challenges of CO2 storage in minerals and geological formations — research article

One concern might be the amount of water that CarbFix uses. To pump one ton of CO2 underground, CarbFix uses 25-30 tons of water. FAQ — Carbfix If CarbFix ever expanded enough to remove gigatonnes of CO2, which is what is needed, then this would require many cubic kilometres of water. Whats a gigatonne? — science blog Surrounding lakes would be quickly emptied.

Permanently storing CO2 underground at Hellisheiði Geothermal Power Plant, Iceland. Árni Sæberg, CC BY-SA 4.0

Fortunately, CarbFix isn't limited to freshwater, and can use seawater or even work as a way of purifying wastewater. Or they could reuse the water they used. But if none of that works, the Blue Sky Carbon Sequestration Project, based in Montana, has developed a method that uses 100 times less water. Burying the sky: Turning carbon dioxide into rock — Earth It's just not quite as effective, storing only 60% of CO2 underground. Quantification of CO2 mineralization at the Wallula basalt pilot project — research article On the other hand, 95% of the CO2 that CarbFix pumps underground, stays underground. FAQ — Carbfix

Trillions of dollars

With all this technology, removing hundreds or even thousands of gigatonnes of CO2 from the atmosphere could soon become possible. It just might not be affordable. Mechanical Trees estimates that they can remove CO2 from the atmosphere at $100 per ton. Carbon tax should fund CO2 removal, says CEO of 'mechanical trees' firm — The Independent At this price removing 1000 gigatonnes would cost a hundred trillion dollars, which is more than the global economy. GDP (current US$) — World Bank

Fortunately technology is rapidly reducing the zeros. Climework's latest partner, Svante, Svante has developed a CO2 filter based on nanotechnology, which could lower costs to $30 per ton. Our carbon problem is a multibillion-dollar opportunity - Forbes If this downward trend continued and costs came down to, for example, a dollar a ton, the bill to remove a 1000 gigatons becomes a mere trillion dollars, which is how much governments might spend in a global financial crisis or pandemic. Global coronavirus spending overtakes financial crisis response — Yahoo

CO2 scrubbers may be the way forward, back to a more stable and safer climate, similar to how it was a few decades ago. The technology is rapidly improving, costs are coming down. But with a price tag in trillions of dollars, who is going to pay for it?

Trillions of dollars to clean up trillions of tons of CO2 from the atmosphere. NASA, CC BY 2.0