Why the world needs negative emissions

Orignally published on 2021-10-30 00:00:00 by www.economist.com

THIRTY KILOMETRES down the road from Reykjavik, the Hellisheiði geothermal power plant sits amid black boulders draped in phosphorescent green moss. Behind its plumes of rising steam, steep mountains sweep up from the rocky plain. Boulders and mountains alike are made of basalt, as is some 90% of the rest of Iceland. It is a 300trn tonne sawn-off tree stump of basalt sitting on the floor of the Atlantic—which is itself just more basalt. There is no commoner rock in Earth’s crust.

The cylinder of basalt that Kári Helgason is holding out, though, is different. Young basalt is riddled with tiny holes, but in this case most cavities are filled with flecks of white crystal. “This,” he says, pointing to the white flecks, “is mostly calcite.” Calcite, a form of calcium carbonate, is not a rare mineral on the island—it is commonly known as Iceland spar—or elsewhere. But this basalt-bound calcite is exceptional. It is the physical manifestation of CO2 emissions being turned to stone.

Small amounts of CO2 are part-and-parcel of the hot fluids piped up from the underlying crust at geothermal power stations like Hellisheiði. Since the early 2000s, Carbfix, the Icelandic company where Mr Helgason works, has been capturing CO2 and pumping it back into the porous bedrock in the form of carbonated water. Once there it reacts with calcium in the basalt. Tracer studies have shown that 95% of the gas injected by Carbfix is mineralised within two years.

Carbfix says Iceland’s basalt could store a century of CO2 emissions, even at today’s rate. By 2030 it hopes to have a “mineral storage terminal” west of Reykjavik that can petrify 3m tonnes of CO2 a year, most or all of it captured at industrial facilities in Europe. In early 2021 it signed a deal with Dan-Unity CO2, a Danish shipping company, for custom-made low-emission tankers to bring it CO2 earmarked for disposal.

Carbon capture and storage (CCS) along these lines has been a disappointment. There are ways to take CO2 out of the exhaust gases of power plants and steelworks, and injection into basalt is just one of various possible places to store it—others include gasfields and saline aquifers. The idea of putting these technologies together has been around for decades. The UNFCCC asked the IPCC to produce a report on the technology 20 years ago. Yet there is still not a single large gas- or coal-fired power station that is capturing and storing its emissions.

One problem is that fossil-fuel industries and governments that value them have an interest in saying they are pursuing CCS, because it seems to provide a future for some fossil fuels, but no pressing reason to make it an implemented reality. The technology makes plants more expensive and less efficient, and in the absence of a high carbon price that is a penalty nobody wants to pay. What is more, many people—sometimes termed “numbies”, for “not under my backyard”—dislike the idea of industrial waste being squirted into aquifers and gasfields anywhere near their homes. Hence the attraction of shipping it to Iceland where it can be turned into solid rock.

Given all this, it is a problem that the technologies on which CCS relies are now central to climate action, thanks to the spread of net-zero pledges. It is not just that these require decarbonising cement-making (for which CCS looks crucial), or that they have a role for hydrogen (which, if made from fossil fuels, needs CCS to be clean). It is that at some point they need CO2 to be drawn down from the thin but thickening air and stored away.

Take back what hurt you

Next to Carbfix’s operation at Hellisheiði is Orca, a facility built in partnership with Climeworks, a Swiss company. Fed with air from a bank of 96 industrial fans, Climeworks’s technology filters out atmospheric CO2 so it can be fed into the geothermal plant’s wastewater for disposal at depth. Orca, which opened in September, is the world’s largest “direct-air capture” (DAC) facility. Its 11 tonnes of carbon captured each day are the forerunner of an enterprise which, if models are right and pledges are adhered to, will grow a millionfold in the next half-century.

The negative emissions DAC is held to offer play two roles in climate stabilisation. One might be seen as balancing the current carbon account. Although most emissions can theoretically be eliminated using technologies that exist now, aviation, shipping and some industrial processes remain hard to decarbonise.Some agricultural greenhouse-gas emissions look as if they will prove recalcitrant. As long as emissions of long-lived greenhouse gases persist, stabilisation will require negative emissions.

The other role for DAC is getting rid of historical excess. As we have seen, the cumulative CO2-emissions budget consistent with a 50-50 chance of meeting the 2°C goal is 3.7trn tonnes. The budget for 1.5°C is just 2.9trn tonnes. With 2.4trn tonnes already emitted, that leaves a decade of emissions at today’s rates for 1.5°C, maybe 25 years for 2°C.

Those constraints could be eased if some of what has already been “spent” were repaid—that is, if CO2 were pulled out of the atmosphere faster than it were being put in, producing net-negative emissions. Removing a billion tonnes of carbon dioxide in 2050 is not quite the same as not having emitted it in 1950, but it is close. And this remains true even if the removal comes after the budget has been broken. Carbon budgets can be overshot, at least for a while.

This offers rich countries that benefited disproportionately from 20th-century emissions a way to create room in the budget for poor ones which were left out. But to do this on an appreciable scale they need to draw down huge amounts of carbon. Some scenarios have negative emissions of well over 10Gt a year—a global fossil-fuel industry running in reverse. Done through DAC that would require huge capital investment and use up a great deal of clean, renewable but still not free energy in the process.

This would not have to be done entirely through DAC. Nature takes half the carbon dioxide that humans put into the atmosphere back out, through either photosynthesis or geochemistry. Both processes could be ramped up.

For photosynthesis, more trees are the obvious option. They can be grown in plantations, including commercial ones where new trees replace each year’s harvest; or they can be encouraged in regenerated forests. The second option is much better. A study in 2019 found that over 80 years restoring natural forests stores an average of 40 times more carbon per hectare than new plantations. Restoration also scores better in preserving biodiversity. But plantations make money in an easily understood way. The same study found that 45% of commitments made under the Bonn Challenge, a voluntary NGO-led initiative to boost forests, involved planting poor-quality commercial plantations.

Another option is to raise the amount of carbon stored in agricultural land and forests that are already commercially exploited. So-called “nature-based solutions” along these lines are staples of the market for voluntary carbon offsets, where vendors promise to do things like growing trees, or stopping them being cut down, to absolve clients’ sins of emission.

Offsetting schemes seem able to deliver negative emissions at a reasonable price. When Microsoft and Stripe, a fintech company, sought carbon-removal projects to meet their net-zero pledges, they found that nature-based projects were priced at $5-50 per tonne. But there are three problems. One is that not all offsetting schemes are well run or well supervised. In Chile government subsidies helped establish 1.3m hectares of tree plantations since 1986—but a rule requiring that this expansion should not happen at the expense of native forests was not enforced. As a result the programme actually reduced the amount of stored carbon by some 50,000 tonnes.

A second is that offsetting is unlikely to solve deeper issues. The airline industry has strong economic incentives to use fuel efficiently, but they have not made the wrenching technological shifts required to stop buying kerosene. If the industry buys offsets to make its planes “carbon-neutral”, as it plans through a scheme called CORSIA, the efficiency incentives will increase further. But that will not of itself make a post-kerosene world more likely. That is one reason why the Science Based Targets Initiative, a standard-setting coalition, does not accept offsetting as a path to emission reduction.

The third drawback is limited capacity. This feels surprising, since in principle the scope for nature-based solutions could be very large. A team of researchers led by Cécile Girardin of Oxford University estimates that a radical commitment to the idea could see the amount of CO2 from human emissions absorbed by the biosphere more or less doubled by 2025, from 10Gt a year to 20Gt a year, making a real difference. Even more would be possible. Add the measures described by the researchers to a trajectory which would otherwise lead to 2°C of warming and you get 1.8°C; a 3°C trajectory comes down to 2.7°C.

Such massive effects cannot be achieved through expansion of current offsetting schemes. The Oxford plan would entail agricultural transformation around the world, an end to deforestation and the restoration of natural ecosystems across roughly 7m km{+2}of Earth’s surface—twice the area of India. It also requires forests and other ecosystems to stay healthy. Unfortunately climate change is making this much harder by increasing the risk of fire and other nasties, such as insect infestations. If nature-based solutions were to go ahead without a simultaneous effort to curb emissions, leaving the world to go on warming regardless, carbon stored in wood and soil could find its way back to the atmosphere.

Another process that could be co-opted is mineral weathering—reactions with rocks that use up CO2 dissolved in water. What is going on in the basalt under Hellisheiði is a form of weathering which produces calcium and carbonate ions that go on to precipitate out as calcite. Rock weathering already soaks up a billion tonnes of CO2 a year. Increase the area available for it by grinding suitable rocks into dust and you will get more. David Beerling, a researcher at the University of Sheffield, and his colleagues have calculated that spreading 3.5bn tonnes of finely ground basalt over 700,000 km{+2} of farmland every year—that’s 50 tonnes per hectare over an area roughly as big as Texas—could double the weathering rate, drawing down another billion tonnes.

Find a way

For weathering to work on a really large scale, though, turn to the oceans, where geochemical manipulation could in principle store trillions of tonnes of CO2. Increase the alkalinity of seawater—eg, by adding lime, an oxide of calcium used in cement—and the amount of carbon dissolved in it as carbonate atoms increases. That shift provides room for more CO2 to be absorbed. Unfortunately it takes some 700m tonnes of lime for 1Gt of CO2—and to make the lime requires putting a lot of energy into heating limestone in kilns which have to be fitted with CCS, since the process itself gives off CO2. The sheer scale of the carbon drawdown that ocean alkalinisation offers makes such schemes worth considering. But they are at best a long-term possibility. Their costs would be enormous, their effects on ocean ecosystems would need careful monitoring and they would require international laws governing marine pollution to be renegotiated from the bottom up.

A more plausible near-term approach to sucking down CO2 industrially is to fit CCS to power stations which burn recently cropped plants. Because the carbon in those photosynthesising plants was recently CO2 in the air, putting what is released by burning them underground is in effect a transfer from atmosphere to crust. And because biomass energy with CCS (BECCS) provides electricity as well as negative emissions it can be used to displace fossil fuels, further reducing emissions.

When the climate-and-economics models used to analyse emission pathways first began to be applied to negative emissions, it was by adding BECCS to them. This gave the technology a first-mover advantage in subsequent discussion in the IPCC, at the Paris summit and elsewhere. That it has been more discussed than other approaches, though, does not make BECCS better. Its large-scale deployment requires vast amounts of land be turned over to growing energy crops; in some estimates an area equivalent to up to 80% of that now used for food crops would be needed.

This is what gives purely technological DAC schemes like Orca their appeal. The installation is designed to tuck away 48,000 tonnes of CO2 over its 12-year life. A tree plantation in a temperate climate capable of soaking up that much would have to cover about 400 hectares. Orca is just a small hangar and four pairs of shipping-container-sized collector units on stilts; a plant the size of a small school doing the work of a forest as big as a fair-sized town. And not only does DAC require less land than BECCS. It can also use land that agriculture can’t: witness Hellisheiði’s barren boulders and moss. Sunny deserts far from anywhere where the process can be powered by cheap solar panels would do fine.

The problem is cost. Climeworks says it costs between $600 and $800 to separate a tonne of CO2 from the Icelandic air and store it away, though it may do better in larger plants. It sells customers the assurance that a tonne of CO2 has been turned to stone at their behest for over $1,100. Because Orca is exciting and its capacity small, these offsets have more or less sold out. But when non-novelty offsets sell for a hundredth of the price it doesn’t look like a very scalable business. One serious rival, a Canadian firm called Carbon Engineering, says it can offer offsets at $300 a tonne when it gets its 1m-tonne-a-year plant operating in Texas by 2025. That fits with an analysis in an academic journal by the company’s founder, David Keith, that puts the costs of the technology it is using in the $90-240/tonne range.

Neither company focuses on offsetting as a core business. Carbon Engineering, which is partnered with Occidental Petroleum, an oil firm, plans to pump the CO2 it isolates in Texas into oilfields to squeeze out oil that is otherwise reluctant to flow. Because the CO2 stays underground, the oil will count as a low-carbon fuel which can be sold at a premium, thanks to regulations in California. It is also looking at combining the CO2 it captures with hydrogen to make synthetic fuels—a business Climeworks is keen on. A startup called Prometheus Fuels claims to be able to do this profitably with a cheaper form of DAC, but has yet to provide details.

Such fuels may help with decarbonisation in some of the places electricity cannot reach, such as aircraft flying over oceans. But the greatest potential for DAC lies in changing the overall carbon budget. If applied on a scale close to that of today’s natural-gas industry it could in principle create space in the atmosphere for hundreds of billions of tonnes of further emissions as the world weans itself off fossil fuels and in the decades after it does so.

Such an idea seems utterly fantastical. So do a huge alkalinisation of the oceans and nature-based solutions or BECCS plantations on scales approaching those of a small continent. But if they remain so, in all likelihood so will a world where the temperature rise stays “well below 2°C”, in the words of the Paris agreement.

And, unfortunately, fantasies that do not become realities can still have real effects. The “net” in net zero functions as a notional safety net: it lets the world imagine that, if somewhere along the tightrope of emissions reduction it trips or tumbles, negative emissions will break its fall. But this is only true if the capacity for stonking great negative emissions is realised. If it remains a fantasy, such a fall could hurt a lot.

Full contents of this special report
The agenda for the COP 26 summit: Stabilising the climate
What the Paris agreement of 2015 meant: The state of play
How Asia is crucial in the battle against climate change: The Asian century’s emissions
The economics of the climate: Economics and energy
Why the world needs negative emissions: Negative emissions*
The case for geo-engineering: Veils and ignorance

This article appeared in the Special report section of the print edition under the headline “If I could turn back time”

Orignally published on 2021-10-30 00:00:00 by www.economist.com

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