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While technologies are being developed that can remove carbon dioxide from the air, they aren't yet feasible on the scale needed to slow global warming, Europe's national science academies warn in a new report.

A wide array of technologies—from land management to ocean fertilization to capturing carbon dioxide from the air and storing it—are in various stages of testing and use, but according to the European Academies' Science Advisory Council, climate scientists and policymakers are being "seriously over-optimistic" about how much these approaches can help deal with the global warming crisis.

In recent years, climate experts have suggested that it's not enough to just decrease the amount of greenhouse gases emitted. To avoid more than 2 degrees Celsius of global warming this century, they say, net emissions will have to fall to zero within a few decades, and it's worth considering "negative emissions"—steps that subtract pollution from the atmosphere to offset what is being added.

But despite the appeal of that notion, which in theory allows the world to overshoot its emissions budget for a while and make up the difference later, the new report warns against banking on it.

"These technologies offer only limited realistic potential to remove carbon from the atmosphere, and not at the scale envisaged in some climate scenarios," wrote the report's authors, a group of experts representing the national science academies of the European Union member states, Norway and Switzerland.

The global efforts to slow warming typically rely on two methods: enacting policies to drastically reduce greenhouse gas emissions and developing technologies that can remove CO2 from the atmosphere.

While the policy side of mitigating the crisis made great strides with the Paris climate agreement of 2015, on the negative emissions side, there are still more questions than answers.

That's troubling, because most of the pathways laid out by the UN's Intergovernmental Panel on Climate Change (IPCC) rely on deploying negative emissions approaches by the middle of this century.

The inclination to think that technological breakthroughs will eventually save the day may be dangerous, warns Thierry Courvoisier, president of the European Academies' Science Advisory Council.

"It is no exaggeration to see responding to the real threats of climate change as a race against time: the longer action is delayed, the more acute and intractable the problem becomes," he wrote in the report's foreword. "If such technologies are seen as a potential fail-safe or backup measure, they could influence priorities on shorter-term mitigation strategies."

But others say it's also a mistake to rely wholly on emissions cuts, which are unlikely to come fast enough to avoid a crisis.

Klaus Lackner, the director of Center for Negative Carbon Emissions at Arizona State University, explains it with an analogy: The global emissions trajectory is like being in a car that's careening toward a curve. Just taking your foot off the gas (slowing emissions) isn't enough, Lackner says—you need to step on the brake, too, and remove some of what has already been emitted.

"I know that with that curve coming in front of us, we are going to hit the guardrail," Lackner said. "My way of looking at it is not 'can we avoid hitting it,' but 'can we avoid a rollover'."

What's Challenging These Technologies?

The EU scientists' conclusion that negative emissions technologies represent more of a wish than a promise followed an exhaustive review of academic studies on each technology. The report examined seven technologies and weighed how likely each was to make a difference on global climate:

Afforestation and reforestation : Simply put, more trees means less carbon in the atmosphere. But offsetting emissions from fossil fuels would require huge forests, competing with food production and posing other problems.



: Simply put, more trees means less carbon in the atmosphere. But offsetting emissions from fossil fuels would require huge forests, competing with food production and posing other problems. Land management to increase carbon in soil : Changing agricultural practices to increase the carbon stored in soil could make a significant contribution, but these practices can be easily reversed if farming returns to more intensive methods.



: Changing agricultural practices to increase the carbon stored in soil could make a significant contribution, but these practices can be easily reversed if farming returns to more intensive methods. Bioenergy with carbon capture and storage : Burning trees or other crops instead of fossil fuels in power plants, then capturing the CO2 from the smokestacks and storing it underground, would require huge tracts of land and risky changes to ecosystems.



: Burning trees or other crops instead of fossil fuels in power plants, then capturing the CO2 from the smokestacks and storing it underground, would require huge tracts of land and risky changes to ecosystems. Enhanced weathering : By adding minerals to oceans and soils, enhanced weathering is expected to be able to remove carbon, though on a smaller scale than the other technologies being explored. As of now, however, there are no projects to test the feasibility.



: By adding minerals to oceans and soils, enhanced weathering is expected to be able to remove carbon, though on a smaller scale than the other technologies being explored. As of now, however, there are no projects to test the feasibility. Direct air capture and storage : When air flows past a direct capture system, the carbon dioxide is selectively removed. It's then released as a concentrated stream for disposal or use. This technology is currently in operation on a small scale, but the size and cost of the equipment could get in the way of scaling it up.



: When air flows past a direct capture system, the carbon dioxide is selectively removed. It's then released as a concentrated stream for disposal or use. This technology is currently in operation on a small scale, but the size and cost of the equipment could get in the way of scaling it up. Ocean fertilization: Tiny plants in the ocean take up CO2, then die and sink to the ocean floor. Enhancing this process, such as by adding iron to stimulate phytoplankton growth, could have a substantial impact on atmospheric CO2 concentrations over several decades to centuries. But there are drawbacks, including risks from algal blooms and other ecological damage.

Carbon capture and storage: This basically is a way to continue burning fossil fuels by capturing their greenhouse gases and storing them, keeping them out of the atmosphere. Technology and policy experts have been hoping to make it work for years. But so far, CCS has not proven affordable, and governments have been unwilling to pay for it on a large scale. A few projects are up and running, but many others have been cancelled.

That Doesn't Mean Abandon the Work

The report's authors aren't suggesting that the technology should be abandoned—just that its limitations have to be fully understood. "In the event of mitigation failing to deliver a safe future operating space for humanity, failure of such technologies to deliver would then condemn humanity to a dangerously warming world," the authors wrote.

Lackner said the high-stakes nature of the climate change battle are precisely why both mitigation and technology need to be pursued simultaneously.

"We have a demonstrated record of having not succeeded with mitigation alone," he said. "We, at this point, have reached a point where even heroic efforts won't get you there."

He believes the most likely candidate is direct capture of carbon dioxide from air. "The reason the cost is high is because it's new," he said. "If you look at PV (photovoltaic solar energy), it's 100 times cheaper now than in 1960."

Peter Kelemen, a professor of earth and environmental science at Columbia University, said he favors an "all of the above" approach. "It is a mistake to wait for complete implementation of other mitigation approaches, since meanwhile huge damages will accrue, and we will be left focusing on the consequences, rather than attempting to avoid the damages in advance," he said.

Kelemen sees the most potential in technologies that aim to emulate natural processes.

"We should be ready to implement negative emissions at scale if, in 10 years, progress in the energy transition and/or greenhouse gas capture has not been sufficiently fast to avert huge damages due to climate change," he said. "The rest is guesswork. And politics."