A STAR attraction at the is a tree. Not an elegant product of evolution, but something that looks rather like a steampunk collision of an industrial air-conditioning unit听and an accordion. What researcher Klaus Lackner鈥檚 mechanical tree has in common with听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 鈥渘et鈥�. 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听鈥� by removing CO鈧� already in the atmosphere.
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This past year, individuals and companies from to and have committed significant sums to various schemes to do听just听that. But they are controversial. Campaigner Greta Thunberg recently derided听governments for pinning their climate听plans on 鈥渇antasy-scaled鈥� versions of听鈥渂arely existing鈥� technologies. Even if they can scale up, there are concerns over whether the cure would be worse than the disease, due听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鈥檚 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 . Back in 2014, AR5, the IPCC鈥檚 , reckoned that staying under 2掳C of warming听鈥� the goal agreed at the Paris climate change summit in 2015, with a lower, desired target of听1.5掳C听鈥� would mean removing around 730听gigatonnes of CO鈧� from the air this century.
That scenario foresaw the heavy lifting 鈥� 480 gigatonnes 鈥� being done by 鈥渂ioenergy with carbon capture and storage鈥� (BECCS). This involves planting a crop that rapidly absorbs CO鈧� 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 鈥� the original, and for many still the best, negative emissions technology (see 鈥淭ake a leaf鈥︹��).
The IPCC鈥檚 modelling in the 2014 report was听derided by some for requiring impossibly large areas of land for energy crops, as much as听. at the University of Manchester, UK, says the 480 gigatonnes figure shouldn鈥檛 be taken literally听鈥� it is a result of models focusing on cost, rather than practicalities. 鈥淣obody is听ever going to do that amount of BECCS, it鈥檚听ridiculous,鈥� he says.
Only one BECCS facility, in Illinois, is , but this decade it could be joined by plants in Japan, Norway and the UK. 鈥淭he key issue is we are running out of time to hit the 1.5掳C target. The case for BECCS is听it鈥檚 available today,鈥� says Will Gardiner, chief听executive of UK energy firm Drax.
Its power station in North Yorkshire once burned coal and was Europe鈥檚 biggest CO鈧� emitter. It has since started burning biomass, mostly wood pellets shipped from forests in听the southern US. That is technically a low鈥慶arbon technology, as the CO鈧� emitted on听burning was only recently absorbed from the air by the trees. Now Gardiner wants to retrofit two of the power station鈥檚 units so
each captures 4 million tonnes of CO鈧� per year.
That would be a significant step towards doing BECCS at scale听鈥� the UK鈥檚 CO鈧� emissions were . Ultimately, Gardiner and other members of the think there is room for听BECCS to eventually remove about 4听gigatonnes of CO鈧� 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 CO鈧�. The听UK, like most countries, has no policies specifically to support CO鈧� removal.
鈥淭he incentives landscape is a bit of a black hole when it comes to negative emissions,鈥� 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鈥檚听biodiversity science panel, has turned against BECCS. 鈥淚 used to be a real big fan of this. But boy, I think we have to be careful,鈥� he听says. Among his fears are fast-growing monocultures replacing biodiverse areas and听energy crops impinging on arable land and听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听鈥� whose first part, on the basic science, came out last week听鈥� will advocate a听portfolio of carbon-removal technologies. 鈥淎R5 largely focused on BECCS because that was pretty much all there was. AR6 will look at听more technologies and be less simplistic,鈥� says at American University, Washington DC, who reviews IPCC reports.
The main high-tech rival to BECCS is direct air capture. Typically, this uses fans to draw air听into a machine, where chemicals remove the air鈥檚 relatively dilute CO鈧�. Three main players听鈥� in Switzerland, in Canada and in the US 鈥� have been joined by two start-ups, in Ireland and in听the听US. This is the approach Microsoft and听Occidental Petroleum are investing in, and听the听UK recently created a .
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 CO鈧� removed. 鈥淭he world cannot afford that at 40 billion tonnes [emitted a year],鈥� says Lackner. It also uses a lot of energy. One study projected the machines could account for a听quarter of world energy demand by 2100.
Climeworks is now building its 15th plant, but is removing a tiny 6000 tonnes of CO鈧� a听year. Christoph Beuttler at the firm thinks it听can eventually scale up to billions of tonnes and get the cost down to $100 to $250 a tonne through automated mass production. 鈥淲e 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,鈥� he says.
Vexed questions
Carbon Collect is about to deploy commercial versions of Lackner鈥檚 tree in Arizona that look a听bit like giant Alexa speakers. The company aims to reduce energy costs by dispensing with听the fans, creating a听purely passive unit where wind blows the air听in. The captured CO鈧偺齱ill initially be sent to听an algae farm at听Arizona State University, and later sold as听a听鈥済reen CO鈧傗��听 alternative to hydrocarbon-produced CO鈧� for horticulture and other sectors, perhaps for carbonated drinks or making synthetic fuel for planes. Storing the听CO鈧� will be the next step. 鈥淸The storage] market isn鈥檛 quite there yet, but it is emerging,鈥� says Carbon Collect鈥檚 Reyad Fezzani.
Storage is one of the most vexing questions with both BECCS and direct air capture. Using the removed CO鈧� 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 CO鈧�, the vast majority will need to听be听locked away. 鈥淭here are only so many Diet听Cokes that need the CO鈧�,鈥� says Burns.
The Drax plan is to build a storage facility in听bedrock 1 to 2 kilometres under the North Sea, in old oil and gas fields, with a pipe some 176听kilometres long to connect it to the plant. Others, including Climeworks, are looking to capture CO鈧� in basalt rock, a process known as mineralisation. 鈥淭hat could be a game changer. That truly is long-term storage,鈥� says Burns.
New economic incentives will still be needed for firms to sit on the CO鈧�. 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听run for longer to incentivise projects, but could still be a model for other countries.
Storage of CO鈧� may yet face other obstacles.听Plans for an underground facility at听Barendrecht in the Netherlands caused protests 12 years ago, and Burns expects to see听the rise of the NUMBY 鈥� 鈥淣ot Under My Backyard鈥� 鈥� activist, particularly in the US. 鈥淚f听you鈥檙e going to have millions or billions of听tonnes of CO鈧� and are storing it, you may well see substantial resistance. Litigation could听slow those projects down,鈥� he says.
Cox has found people in the UK are generally supportive of research into CO鈧� removal, but prefer BECCS over direct air capture. 鈥淧eople favour using plants in the process,鈥� she says, because it makes it seem more 鈥渘atural鈥�. But听she says it has been hard to communicate the scale of plantations needed for BECCS. 鈥淚听don鈥檛 think global north populations have been exposed to the downsides of growing biomass,鈥� she says. People in other parts of听the听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听support paying power stations for storing CO鈧傗��, as long as it is perceived as part of an听overall, coherent package of reducing emissions too. That is a point Watson can鈥檛 emphasise enough, either: the faster we cut emissions today, the less we will need to rely on听sucking huge amounts of CO鈧� out of the air.听Yet there is no plausible road to limiting the听global temperature rise to 2掳C, let alone 1.5掳C, without negative emissions technologies. Even the International Energy Agency, which is听much more conservative on how much CO鈧� needs to be removed than the IPCC, thinks will have to be captured in听2050 by BECCS and direct air capture, with 1.9听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鈥檚 possible, but it is a Herculean task, says Burns. 鈥淚f this really is going to come to fruition, there is going to have to be a tremendous acceleration of efforts.鈥�
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 found that Earth has a potential 678听million hectares, twice the size of听India, for forest regrowth. That would remove roughly 6 gigatonnes of听CO鈧� a year. 鈥淭hat鈥檚 a听significant amount,鈥� says 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 could be worth more than the world鈥檚 biggest oil and gas companies by 2040.
Lewis says government pledges of would scoop up most of the economically attractive tree-planting land, leaving little for the听corporate world to听cheaply offset its emissions. The potential carbon capture via forest creation , of听11听gigatonnes of CO鈧傗��, is 鈥渃olossal鈥� and 鈥渁bsurd鈥�, 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 such as eucalyptus, which are poor for wildlife and store less carbon. Planting trees on and can cancel out听CO鈧� savings or even lead to carbon听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 CO鈧� from the air (see main story) aren鈥檛 the only ways听 we might remove our carbon emissions.
Rock dust听 Grinding up rocks to increase their surface area and spreading them on cropland speeds up how rocks absorb CO鈧� naturally. Known as enhanced weathering, this could take away between 0.5 and 2 gigatonnes of CO鈧� a year by 2050, one team found, and might boost crop yields in the process. But the technique is still at听the stage of small听field experiments, and there are concerns over听rock availability, the听energy needed to grind听it up and听the risk of听soil contamination.
Ecosystem听restoration听 Beyond woodlands, many habitats can help remove CO鈧� emissions, including seagrass beds, salt marshes and other marine听and coastal environments. On听land, the听biggest potential in the听UK听comes from restoring and protecting the听country鈥檚 peatlands.
Biochar听 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听rock dust, there are听some signs that it could听boost crop yields. Some companies, such as听UK-based , are already producing biochar for gardeners to use, but it is far from clear the approach could scale to听removing billions of tonnes of CO鈧�.
Iron filings听 Iron fertilises the growth of听marine phytoplankton, which absorb CO鈧� from the听air, eventually locking it听away in the ocean. An international team plans to seed three听locations across the globe with听extra iron to test the effects, although such geoengineering schemes are controversial.
