This post originally appeared on Climate Central.
The manmade emissions fueling global warming are accumulating so quickly in the atmosphere that climate change could spiral out of control before humanity can take measures drastic enough to cool the earth’s fever, many climate scientists say.
The most important way the earth’s rising temperature can be tempered is to reduce the use of fossil fuels. But scientists say another critical solution is to physically remove greenhouse gases from the atmosphere — something called “negative emissions” — so that carbon dioxide and rising temperatures could peak, and then begin to decline over time.
Many of the assumptions underlying the landmark Paris Climate Agreement rely on the idea that humans will be actively removing carbon from the atmosphere late this century because reducing emissions won’t be enough to prevent global warming from exceeding levels considered dangerous.
But that assumption relies on technology that hasn’t been proven to work on a global scale. Removing carbon dioxide from the atmosphere on a scale large enough to slow global warming is untested, and the technology is in its infancy. The effect it could have on the earth is largely unknown, and some scientists warn that some of the consequences of using negative emissions technology could be catastrophic.
Because of all those unknowns, it’s critical that humanity doesn’t bet its future on negative emissions, Stanford University Woods Institute for the Environment scientists Katharine Mach and Christopher Field write in a paperpublished Thursday in the journal Science.
The paper argues that both negative emissions technology and a commitment to quickly cutting carbon dioxide emissions as much as possible are critical to solving the climate crisis.
Carbon concentrations in the atmosphere must not exceed 450 ppm if global warming is to be prevented from exceeding a level considered dangerous by most climate scientists — 2°C (3.6°F), the primary goal of the Paris Climate Agreement. The problem, though, is that humanity is quickly running out of time to limit more warming. The atmosphere blew past the 400 ppm mark last September and it’s on a trajectory to pass 450 ppm within 22 years.
Most of the Intergovernmental Panel on Climate Change models underlying the Paris Climate Agreement assume some level of large-scale carbon removal will be occurring in the coming decades, but nobody knows exactly how that will be accomplished.
Ben Sanderson, a climate scientist at the National Center for Atmospheric Research who is unaffiliated with the paper, said the study shows that carbon removal shouldn’t be treated as a cure-all for climate change because the future of humanity can’t rely on untested technology.
“The major risk is that the planned-for CO2 removal might never come to pass — and this is a very real concern,” Sanderson said.
The paper warns of dire consequences if the effects of negative emissions technology aren’t fully accounted for before they’re implemented.
For example, one of the negative emissions technologies carbon-removal proponents often cite as the most promising — bioenergy and carbon capture and storage, or BECCS — could create widespread food insecurity because it could take half of the world’s farmland out of production.
BECCS relies on converting agricultural areas and other land to vast new forests, which absorb atmospheric carbon in tree trunks and roots. The trees would be harvested for biomass energy and burned in power plants. The resulting carbon emissions would be captured and stored permanently — a method some scientists believe could be worse for global warming than burning fossil fuels.
“Converting land on this staggering scale would pit climate change responses against food security and biodiversity protection,” the paper says.
Chief among the many other negative emissions technologies being developed include expanding forests globally to store more carbon naturally, and building hundreds or thousands of facilities that directly remove carbon from the atmosphere and store it permanently. Those facilities, called “direct air capture” plants, have never been built on a large scale and scientists say they would require a large amount of energy to operate and many thousands of them would have to be constructed to make a dent in global warming.
The paper criticizes the idea of peak and decline — the theory that carbon removal could bring about a peak in global temperatures and then begin to cool the planet. That may be risky because the costs and consequences of global cooling following a temperature peak are not well understood. Some of the effects of climate change such as sea level rise and melting polar ice sheets can’t be reversed as the globe cools.
“These scenarios bet the future on CDR (carbon dioxide removal) technologies operating effectively at vast scales within only a few decades,” the paper says, referring to climate models assuming a peak and decline in atmospheric carbon concentrations. “Estimates of economic costs are crude for such scales and environmental tradeoffs are potentially stark.”
Ecosystems that will have begun to adapt to higher global temperatures as the world warms may struggle to adjust to the global cooling that peak and decline envisions. Scientists haven’t done much research on what the effects might be, Field said.
Sanderson said that peak and decline is unlikely within this century, but much more likely in the next century or beyond because changes in the climate system are hard to turn around even as atmospheric carbon concentrations decline.
The paper says that massive deployments of negative emissions technologies might work, but if they don’t, “future generations may be stuck with substantial climate change impacts, large mitigation costs, and unacceptable tradeoffs.”
Field said that when all the unknowns about negative emissions are considered, the best strategy to solve the climate crisis is to both develop carbon removal technology and work as quickly as possible to cut emissions today.
The paper generated a range of responses from negative emissions experts.
Klaus Lackner, director of the Center for Negative Carbon Emissions at Arizona State University and one of the world’s leading experts on carbon removal, said that he agrees with most of the paper’s conclusions, but it implies that climate change can be mitigated if emissions are drawn down quickly today.
“Yes, we should reduce emissions and move away from fossil fuels, but we will overshoot acceptable CO2 concentrations — indeed, we may already have overshot,” Lackner said, adding that no matter how much emissions are cut today, failure to develop negative emissions technologies will seriously damage the planet.
He said cutting carbon emissions gets more and more difficult the more they are reduced. Carbon removal is necessary to prevent atmospheric carbon dioxide concentrations from growing further.
Glen Peters, a scientist at the Norway-based climate research organization CICERO whose 2016 paper called focusing on negative emissions technology a “moral hazard,” said he agrees with the paper’s conclusion that countries need to both radically cut greenhouse gas emissions while also upscaling negative emissions technology.
Peters said he isn’t worried about the consequences of peak and decline because paleontological records suggest there are few catastrophic consequences of small declines in the earth’s average temperature over a period of decades.
Sanderson said the questions the paper raises about the degree to which negative emissions technologies should be developed may be best solved by a putting a price on carbon.
“A well designed carbon market would find the optimal combination of emissions reductions and carbon removal,” he said. “This rather breaks the deadlock for decision makers today — they don’t have to decide whether to reduce emissions or invest in reduction technologies, all they need to do is put a price on carbon.”
John DeCicco, a professor at the University of Michigan-Ann Arbor Energy Institute, called the paper “excellent and thoughtful,” but it does not sufficiently recognize that the global carbon cycle already includes carbon removal processes. He said the paper doesn’t sufficiently factor in ways forests and soils could be managed to store more carbon than they do naturally.
DeCicco said that he agrees with the paper’s conclusion that it’s unwise for countries to automatically assume that technology will be developed to bring atmospheric carbon concentrations and global temperatures down to tolerable levels eventually.
“Hoping that future generations might somehow figure out an atmospheric CO2 decline in a way that undoes climate catastrophe is just foolish,” he said, adding that it’s critical for humanity to pursue both emissions cuts and carbon removal technology as quickly as possible.
“We can’t wait for some sort of technological deus ex machina to save us from ourselves," DeCicco said.