A STAR attraction at the Science Museum in London right now is a tree. Not an elegant product of evolution, but something that looks rather like a steampunk collision of an industrial air-conditioning unit and an accordion. What researcher Klaus Lackner’s mechanical tree has in common with the natural variety, however, is that it is great at sucking carbon dioxide out of the air.

We are going to need a lot of that in the coming decades if we are to achieve net-zero carbon emissions by mid-century and so head off the worst of the climate crisis. The key word here is “net”. Even when we have wiped out all the emissions we can, intractable sources will remain, from the likes of food production, flying and heavy industry. Negative emissions technologies are intended to bridge the gap – by removing CO₂ already in the atmosphere.

This past year, individuals and companies from Elon Musk to Microsoft and US oil firm Occidental Petroleum have committed significant sums to various schemes to do just that. But they are controversial. Campaigner Greta Thunberg recently derided governments for pinning their climate plans on “fantasy-scaled” versions of “barely existing” technologies. Even if they can scale up, there are concerns over whether the cure would be worse than the disease, due to potential downsides of negative emissions technology for biodiversity, water consumption, food production and energy use. Time to ask: when it comes to carbon removal, do we really know what we are doing?

As last week’s report from the Intergovernmental Panel on Climate Change (IPCC) made plainer than ever before, we are running out of time to stave off the worst of global warming. And for all the warm words on climate action, our carbon emissions continue on the up. Last year, even with the pandemic, they amounted to 39 gigatonnes of CO₂. Back in 2014, AR5, the IPCC’s fifth climate science assessment report , reckoned that staying under 2°C of warming – the goal agreed at the Paris climate change summit in 2015, with a lower, desired target of 1.5°C – would mean removing around 730 gigatonnes of CO₂ from the air this century.

That scenario foresaw the heavy lifting – 480 gigatonnes – being done by “bioenergy with carbon capture and storage” (BECCS). This involves planting a crop that rapidly absorbs CO₂ because it grows quickly, then burning the vegetation for energy generation and capturing and storing the resulting carbon emissions. The remainder of the 730 gigatonnes would be removed by planting trees – the original, and for many still the best, negative emissions technology.

The IPCC’s modelling in the 2014 report was derided by some for requiring impossibly large areas of land for energy crops, as much as two and half Indias in some scenarios. Rob Bellamy at the University of Manchester, UK, says the 480 gigatonnes figure shouldn’t be taken literally – it is a result of models focusing on cost, rather than practicalities. “Nobody is ever going to do that amount of BECCS, it’s ridiculous,” he says.

Only one BECCS facility, in Illinois, is currently storing CO₂ , but this decade it could be joined by plants in Japan, Norway and the UK. “The key issue is we are running out of time to hit the 1.5°C target. The case for BECCS is it’s available today,” says Will Gardiner, chief executive of UK energy firm Drax.

Its power station in North Yorkshire once burned coal and was Europe’s biggest CO₂ emitter. It has since started burning biomass, mostly wood pellets shipped from forests in the southern US. That is technically a low‑carbon technology, as the CO₂ emitted on burning was only recently absorbed from the air by the trees. Now Gardiner wants to retrofit two of the power station’s units so
each captures 4 million tonnes of CO₂ per year.

That would be a significant step towards doing BECCS at scale – the UK’s CO₂ emissions were 326 million tonnes last year. Ultimately, Gardiner and other members of the Coalition for Negative Emissions think there is room for BECCS to eventually remove about 4 gigatonnes of CO₂ a year globally, roughly what aviation emits.

Missing policies

But there are lots of missing pieces for even this single project in the UK. They include financial incentives such as a minimum guaranteed price for the power and a negative emissions payment for storing the CO₂. The UK, like most countries, has no policies specifically to support CO₂ removal.

“The incentives landscape is a bit of a black hole when it comes to negative emissions,” says Bellamy. The fact that most of the biomass burnt by Drax comes from trees felled on the other side of the Atlantic, with only a small fraction from crops in the UK, may prove a PR stumbling block too.

Land use impacts are why Bob Watson at the University of East Anglia, UK, a former adviser to the UK government and chair of both the IPCC and the UN’s biodiversity science panel, has turned against BECCS. “I used to be a real big fan of this. But boy, I think we have to be careful,” he says. Among his fears are fast-growing monocultures replacing biodiverse areas and energy crops impinging on arable land and threatening food security. Recent research also indicates that the irrigation requirements of a mass BECCS roll-out could make water access harder for billions of people.

For such reasons, the next IPCC assessment report, AR6 – whose first part, on the basic science, came out last week – will advocate a portfolio of carbon-removal technologies. “AR5 largely focused on BECCS because that was pretty much all there was. AR6 will look at more technologies and be less simplistic,” says Wil Burns at American University i 0n Washington DC, who reviews IPCC reports.

The main high-tech rival to BECCS is direct air capture. Typically, this uses fans to draw air into a machine, where chemicals remove the air’s relatively dilute CO₂. Three main players – Climeworks in Switzerland, Carbon Engineering in Canada and Global Thermostat in the US – have been joined by two start-ups, Carbon Collect in Ireland and Heirloom in the US. This is the approach Microsoft and Occidental Petroleum are investing in, and the UK recently created a £100 million competition for direct air capture research.

Lackner, a professor at Arizona State University whose Science Museum mechanical tree was made in 2017, has been working on the tech since the 1990s. Its advantages, he says, are its small land footprint and easy scalability. But it is elaborate and expensive, for now at least: Climeworks cites a cost of $600 per tonne of CO₂ removed. “The world cannot afford that at 40 billion tonnes [emitted a year],” says Lackner. It also uses a lot of energy. One study projected the machines could account for a quarter of world energy demand by 2100.

Climeworks is now building its 15th plant, but is removing a tiny 6000 tonnes of CO₂ a year. Christoph Beuttler at the firm thinks it can eventually scale up to billions of tonnes and get the cost down to $100 to $250 a tonne through automated mass production. “We are a bit like Tesla when we have built the Roadster, but we are a long way from a Gigafactory for mass production of parts,” he says.

Vexed questions

Carbon Collect is about to deploy commercial versions of Lackner’s tree in Arizona that look a bit like giant Alexa speakers. The company aims to reduce energy costs by dispensing with the fans, creating a purely passive unit where wind blows the air in. The captured CO₂ will initially be sent to an algae farm at Arizona State University, and later sold as a “green CO₂”  alternative to hydrocarbon-produced CO₂ for horticulture and other sectors, perhaps for carbonated drinks or making synthetic fuel for planes. Storing the CO₂ will be the next step. “[The storage] market isn’t quite there yet, but it is emerging,” says Carbon Collect’s Reyad Fezzani.

Storage is one of the most vexing questions with both BECCS and direct air capture. Using the removed CO₂ as Carbon Collect proposes may help the climate fight a little by displacing higher-carbon ways of making the gas, but such uses mean it is eventually released back into the air. If we are to remove billions of tonnes of CO₂, the vast majority will need to be locked away. “There are only so many Diet Cokes that need the CO₂,” says Burns.

The Drax plan is to build a storage facility in bedrock 1 to 2 kilometres under the North Sea, in old oil and gas fields, with a pipe some 176 kilometres long to connect it to the plant. Others, including Climeworks, are looking to capture CO₂ in basalt rock, a process known as mineralisation. “That could be a game changer. That truly is long-term storage,” says Burns.

New economic incentives will still be needed for firms to sit on the CO₂. The US has a tax credit for storing the gas that was recently extended until 2026. Emily Cox at Cardiff University in the UK says that scheme needs to run for longer to incentivise projects, but could still be a model for other countries.

Storage of CO₂ may yet face other obstacles. Plans for an underground facility at Barendrecht in the Netherlands caused protests 12 years ago, and Burns expects to see the rise of the NUMBY – “Not Under My Backyard” – activist, particularly in the US. “If you’re going to have millions or billions of tonnes of CO₂ and are storing it, you may well see substantial resistance. Litigation could slow those projects down,” he says.

Cox has found people in the UK are generally supportive of research into CO₂ removal, but prefer BECCS over direct air capture. “People favour using plants in the process,” she says, because it makes it seem more “natural”. But she says it has been hard to communicate the scale of plantations needed for BECCS. “I don’t think global north populations have been exposed to the downsides of growing biomass,” she says. People in other parts of the world who have seen the ravages of monocultures on biodiverse environments such as rainforests might have different views.

Bellamy says his research shows people do support paying power stations for storing CO₂ , as long as it is perceived as part of an overall, coherent package of reducing emissions too. That is a point Watson can’t emphasise enough, either: the faster we cut emissions today, the less we will need to rely on sucking huge amounts of CO₂ out of the air. Yet there is no plausible road to limiting the global temperature rise to 2°C, let alone 1.5°C, without negative emissions technologies. Even the International Energy Agency, which is much more conservative on how much CO₂ needs to be removed than the IPCC, thinks 2.4 gigatonnes a year will have to be captured in 2050 by BECCS and direct air capture, with 1.9 gigatonnes of that stored.

Can we get to that scale by mid-century? And can we do that without environmental impacts that could rival those from the catastrophic temperature rises they are meant to avoid? It’s possible, but it is a Herculean task, says Burns. “If this really is going to come to fruition, there is going to have to be a tremendous acceleration of efforts.”

Take a leaf

One negative emissions technology already exists at scale and usually costs less than $100 per tonne of carbon removed to implement: planting trees. A recent estimate found that Earth has a potential 678 million hectares, twice the size of India, for forest regrowth. That would remove roughly 6 gigatonnes of CO₂ a year. “That’s a significant amount,” says Simon Lewis at the University of Leeds, UK.

The economic reality, however, is that only a third of that area can probably be affordably planted, he says. Even so, one UN-backed group estimates that nature-based carbon removal alone could be worth more than the world’s biggest oil and gas companies by 2040.

Lewis says government pledges of restoring 350 million hectares of forest would scoop up most of the economically attractive tree-planting land, leaving little for the corporate world to cheaply offset its emissions. The potential carbon capture via forest creation cited by oil company Shell, of 11 gigatonnes of CO₂ , is “colossal” and “absurd”, according to Lewis.

And there are concerns over new forests if done at scale and in the wrong places. Tree planting in inappropriate areas such as savannah could harm biodiversity and food production, says Lewis. He has found that almost half of global forest plans involve monoculture plantations of single species such as eucalyptus, which are poor for wildlife and store less carbon. Planting trees on peatland and carbon-rich soils can cancel out CO₂ savings or even lead to carbon emissions. Another potential downside of trees is that a warming world makes it harder for them to keep the carbon locked up, due to more fires and droughts, says Lewis.

 Other ways to remove carbon

Growing trees, burning biofuels and sucking CO₂ from the air (see main story) aren’t the only ways  we might remove our carbon emissions.

Rock dust  Grinding up rocks to increase their surface area and spreading them on cropland speeds up how rocks absorb CO₂ naturally. Known as enhanced weathering, this could take away between 0.5 and 2 gigatonnes of CO₂ a year by 2050, one team found, and might boost crop yields in the process. But the technique is still at the stage of small field experiments, and there are concerns over rock availability, the energy needed to grind it up and the risk of soil contamination.

Ecosystem restoration  Beyond woodlands, many habitats can help remove CO₂ emissions, including seagrass beds, salt marshes and other marine and coastal environments. On land, the biggest potential in the UK comes from restoring and protecting the country’s peatlands.

Biochar  This is a carbon-rich charcoal-like remnant made by heating plant waste in an oxygen-free environment, a technique known as pyrolysis. Advocates suggest burying the biochar in soil to lock it away. Like rock dust, there are some signs that it could boost crop yields. Some companies, such as UK-based Carbon Gold, are already producing biochar for gardeners to use, but it is far from clear the approach could scale to removing billions of tonnes of CO₂.

Iron filings  Iron fertilises the growth of marine phytoplankton, which absorb CO₂ from the air, eventually locking it away in the ocean. An international team plans to seed three locations across the globe with extra iron to test the effects, although such geoengineering schemes are controversial.