By Mark Golden and Mark Shwartz

President Obama has called for big investments in wind, solar and other renewable forms of energy to cut U.S. carbon dioxide (CO₂) emissions 80 percent by 2050. But a growing number of scientists warn that low-carbon technologies will not be enough to meet climate goals, which will require a new suite of technologies that remove CO₂ from the air.

This negative-emissions approach to reducing atmospheric CO₂ was the focus of a symposium at the annual meeting of the American Association for the Advancement of Science in San Jose.

One strategy for achieving negative greenhouse gas emissions may be to store massive amounts of atmospheric carbon dioxide in the soil of sustainably managed farms and ranches.
PHOTO: Terry Sohl, U.S. Geological Survey

"Renewables – such as solar, wind, hydro and bioenergy – and sequestration technologies, like carbon capture and storage (CCS), could help curb CO₂ emissions," said symposium organizer Jennifer Milne, an energy assessment analyst at the Global Climate and Energy Project (GCEP) at Stanford University. "To augment these, technologies exist that remove atmospheric CO₂ and potentially keep it out of the atmosphere. These negative-emissions technologies have benefits and downsides, and vary drastically in predicted cost."

The vast majority of climate models predict that limiting the Earth's temperature rise to 2 degrees Celsius (3.6 degrees Fahrenheit) will require significant deployment of negative-emissions technologies in the second half of this century, said symposium speaker Peter Smith of the University of Aberdeen. Smith outlined the categories of negative-emissions technologies, including directly capturing CO₂ from the air, crushing certain minerals that capture carbon naturally and dispersing them widely on land or at sea, and afforestation, which is creating carbon-absorbing forests where they previously did not exist.

However, the combination of bioenergy with carbon capture and storage (BECCS) may be getting the most attention of the negative-emissions technologies. It was one of the topics of a 2013 GCEP report co-authored by Milne that assessed energy-producing technologies that could reduce atmospheric carbon. The BECCS approach can be used in power plants that generate electricity or factories that make chemicals and fuels. BECCS takes advantage of the innate ability of plants to capture atmospheric CO₂ for photosynthesis. In nature, the CO₂ is eventually released back into the atmosphere as the plant decays. At a BECCS facility, grass and other vegetation is burnt along with coal or natural gas. The CO₂ emissions are captured and sequestered in the ground instead of going into the atmosphere, thus bypassing the decaying process. The result is a net-negative reduction in atmospheric CO₂.

A number of technical and policy issues have to be addressed before negative-emissions technologies can be implemented at scale. Some of the key challenges were discussed by symposium speakers Smith, Jennifer Wilcox of Stanford, Ken Caldeira of Stanford and the Carnegie Institution's Dept. of Global Ecology, and James Edmonds of the Joint Global Change Research Institute.

Wilcox, a professor of energy resources engineering, noted that thermodynamic laws dictate that the direct air capture of CO₂ would require so much energy that any system to do so would have to be run on renewable energy or it would produce more emissions than it would remove. In addition, the total costs of the crushed mineral approach – known as "accelerated weathering" when heat and processing are used – are currently prohibitive at $1,000 per ton of CO₂ removed, Wilcox said. Using byproducts of certain industrial processes, like cement kiln dust, to absorb carbon passively is economic, she said, but the total global potential for this approach represents a very small percentage of the gases that need to be removed.

Lisamarie Windham-Myers of the U.S. Geological Survey compared BECCS with land-management techniques – such as wetland restoration and sustainable agriculture – that could lead to the large-scale removal of atmospheric carbon as well as other environmental benefits. Peter Byck, a filmmaker at Arizona State University, screened and discussed his short documentary on a sustainable ranching project that restores grazing land while soaking up atmospheric CO₂.

While some of these negative-emissions technologies may be great to do, said Caldeira, they are not going to solve the climate problem. "For each CO₂ molecule you put in, you’d have to pull one out of the atmosphere, so the scale of the system that you would need is similar to the scale of our energy system," he said.

"If we have to do something on the scale of the current energy system," Caldeira argued, "why not build a new energy system that doesn’t put carbon into the atmosphere in the first place."

However, simply not emitting carbon will be insufficient to meet climate goals, which will require negative-emissions technologies, said symposium moderator Sally Benson, a professor of energy resources engineering at Stanford and director of the Precourt Institute for Energy.

"Negative-emissions technologies, such as BECCS, can be thought of as part of an insurance policy for climate-change mitigation," said Benson. "This approach still leaves unanswered questions, but to not consider it carefully would be too risky."

Media Contact

Mark Golden, Precourt Institute for Energy: (650) 724-1629, mark.golden@stanford.edu

February 18, 2015