Recent models of future climate scenarios have assumed that this technique will be widely used in the future. Few have explored a world in which we can keep the planet’s warming within at least a 2-degree temperature threshold without the help of negative-emission technologies. But some scientists are arguing that this assumption may be a serious mistake.

In a new opinion paper, published Thursday in the journal Science, climate experts Kevin Anderson of the University of Manchester and Glen Peters of the Center for International Climate and Environmental Research have argued that relying on the uncertain concept of negative emissions as a fix could lock the world into a severe climate-change pathway.

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“[If] we behave today like we’ve got these get-out-of-jail cards in the future, and then in 20 years we discover we don’t have this technology, then you’re already locked into a higher temperature level,” Peters said.

Many possible negative-emission technologies have been proposed, from simply planting more forests (which act as carbon sinks) to designing chemical reactions that physically take the carbon dioxide out of the atmosphere. The technology most widely included in the models is known as bioenergy combined with carbon capture and storage, or BECCS.

In a BECCS scenario, plants capture and store carbon while they grow — removing it from the atmosphere, in other words — and then are harvested and used for fuel to produce energy. These bioenergy plants will be outfitted with a form of technology known as carbon capture, which traps carbon dioxide emissions before they make it into the atmosphere. The carbon dioxide can then be stored safely deep underground. Even more carbon is then captured when the plants grow back again.

The idea sounds like a win-win on paper, allowing for both the removal of carbon dioxide and the production of energy. But while more than a dozen pilot-scale BECCS projects exist around the world, only one large-scale facility currently operates. And scientists have serious reservations about the technology’s viability as a global-scale solution.

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First, the sheer amount of bioenergy fuel required to suit the models’ assumptions already poses a problem, Peters told The Washington Post. Most of the models assume a need for an area of land at least the size of India, he said, which prompts the question of whether this would reduce the area available for food crops or force additional deforestation, which would produce more carbon emissions.

When it comes to carbon capture and storage, the technology has been used already in at least 20 plants around the world, not all of them devoted to bioenergy. In fact, carbon capture and storage can be applied in all kinds of industrial facilities, including coal-burning power plants or oil and natural gas refineries. But the technology has so far failed to take off.

“Ten years ago, if you looked at the International Energy Agency, they were saying by now there would be hundreds of CCS plants around the world,” Peters said. “And each year the IEA has had to revise their estimates down. So CCS is one of those technologies that just never lives up to expectations.”

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This is largely a market problem, according to Howard Herzog, a senior research engineer and carbon capture expert at Massachusetts Institute of Technology.

“There’s no doubt you can do it,” he said. “We have coal plants that do CCS, you can have biomass that can do CCS — the technology’s not a big deal. The question is the economics.”

Because it’s more expensive to produce energy with carbon capture than without it, there’s little incentive for the private sector to invest in the technology without a more aggressive policy push toward curtailing emissions, he pointed out. A carbon price, for instance, would be one way of creating a market for the technology.

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It’s not that the modelers have no reason for incorporating BECCS so heavily, though. Over a long enough time period, and at the scale needed to make a dent in our global climate goals — especially assuming a high enough carbon price in the future — it may be the cheapest mitigation technology, Peters said. But this may not be enough for policymakers to invest in its advancement now.

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“Decision-makers today don’t optimize over the whole century,” he said. “They’re not asking: What technology can I put in place now to make a profit in 100 years? So the sort of strategic thinking in the model is different from strategic thinking in practice.”

Additionally, the models that are commonly relied on to project future climate and technological scenarios assume that the CCS technique works perfectly within the next few decades, when it’s only just emerging.

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“The models don’t have technical challenges; they don’t run into engineering problems; the models don’t have cost overruns,” Peters said. “Everything works as it should work in the model.”

The bottom line, he and Anderson note in their paper, is that all these assumptions make for a huge gamble. If policymakers decide we’re going to meet our climate goals only with the aid of negative-emission technologies, and then these technologies fail us in the future, we will already be locked into a high-temperature climate scenario.

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In this light, the authors write, “negative-emission technologies should not form the basis of the mitigation agenda.” Indeed, they conclude, nations should proceed as though these technologies will fail, focusing instead on aggressive emissions-reduction policies for the present, such as the continued expansion of renewable energy sources.

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Other scientists agree. Daniel Kammen, an energy professor at the University of California in Berkeley and director of the Renewable and Appropriate Energy Laboratory, has published several recent papers on BECCS technology, and agrees that it is “nowhere near ready to be considered a component of a viable carbon reduction strategy.”

“The paper is right,” he continued in an emailed comment to The Washington Post. “A run to endorse BECCS as a key component of the needed 80 percent or greater decarbonization we need by 2050 is unproven, premature and potentially costly. It is worth research, but has a ways to go before it can enter the realm of a solutions science for climate change.”

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Herzog also agreed that “the focus of today should be on mitigation as opposed to worrying about negative emissions sometime in the future.” In the future, he said, as we approach the end of our decarbonization schemes, negative emissions could still have a place when it comes to offsetting carbon from those last activities it’s most difficult or most expensive to decarbonize.

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But Herzog added that, in his opinion, we’ve likely already overshot a 2-degree temperature threshold, to say nothing of the more ambitious 1.5-degree target described in the Paris climate agreement. At the very least, he noted, a reliance on renewables alone would be unlikely to get us there, if it were still possible. Indeed, multiple recent analyses have suggested that the combined pledges of individual countries participating in the Paris Agreement — very few of which have even considered negative emissions — still fall short of our temperature goals.

“I think what you’re going to see in the long run is a mix of technologies coming in to help solve the problem,” he said. “You need a mix of renewables, efficiency, nuclear, CCS, lifestyle changes — just a whole litany.”

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