EDITORIAL

Negative-emissions insurance

Science  27 Jun 2014:
Vol. 344, Issue 6191, pp. 1431
DOI: 10.1126/science.1257423
PHOTO: STANFORD UNIVERSITY

In its April 2014 report, the Intergovernmental Panel on Climate Change (IPCC) recognized that reducing greenhouse gas (GHG) emissions by 40 to 70% by mid-century will require more than just implementing emission-free solutions. Many scenarios for stabilizing GHG concentrations that were evaluated by the panel included removing carbon dioxide (CO2) from the atmosphere: so-called “negative emissions” or carbon dioxide removal (CDR). Among the most promising CDR methods are reforestation, afforestation (planting new forests), and bioenergy with carbon capture and storage (BECCS). However, for BECCS strategies to succeed, major hurdles must be overcome.

Ethanol refinery

“This approach still leaves unanswered questions, but to not consider it carefully would be too risky.”

PHOTO: TADPHOTO/ISTOCKPHOTO.COM

All BECCS approaches rely on removal of CO2 from the atmosphere by plants during photosynthesis. In nature, as a plant decays, it releases CO2 back into the atmosphere. But with BECCS, CO2 is captured and permanently stored underground, resulting in a net negative reduction in atmospheric carbon. At least three BECCS technologies are being investigated today. CO2 released during the microbial fermentation of plant sugars to ethanol can be captured. CO2 can also be captured during the gasification of biomass to synthetic gas for conversion to transportation fuels, chemicals, or electricity generation. And CO2 from the combustion of biomass, either with or without coal, can be captured.

Since 2009, a consortium supported by the U.S. Department of Energy has successfully operated a BECCS test facility in Illinois. CO2 emitted during the fermentation of corn is captured and stored in a sandstone formation about 7000 feet underground. The project removes 300,000 metric tons of CO2 per year from the atmosphere—the equivalent of removing about 70,000 cars from the road annually. But that's just a fraction of the amount of CO2 that will have to be removed to curb global warming, and of the estimated 2 to 10 gigatons (Gt) of CO2 per year that could be removed from the atmosphere with BECCS by 2050.*

For either conventional CCS or BECCS, the cost must come down. Right now, CCS costs range from about $30 to $140 per ton of CO2, depending on the source from which it is captured, the capture technology, and the form of storage. Improving energy-conversion efficiency would address the cost hurdles, but this requires further research and development. Increased confidence in long-term geological storage security is also needed to better understand the risks of BECCS strategies. Capturing and storing 1 Gt of CO2 from the atmosphere using BECCS would require about 0.5 to 1 Gt of biomass (equivalent to 10 to 20 exajoules of primary energy). Concerns about whether this much biomass could be practically and sustainably harvested, dried, and collected at this scale without interfering with food production or negatively affecting other ecosystem services must be examined.

Combined with sustainably managed reforestation and afforestation, the potential co-benefits of habitat creation, carbon mitigation, and renewable energy make BECCS an attractive choice. Rigorous research and development are needed so that the potential of BECCS is clear to scientists, policy-makers, and the public.

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

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