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April 30, 2007
Volume 85, Number 18
Web Exclusive

Greenhouse Gases

Running the numbers on CO2 emissions

Stephen K. Ritter

The global chemical industry uses up approximately 115 million metric tons of carbon dioxide each year as a feedstock in a variety of synthetic processes, according to the Intergovernmental Panel on Climate Change (IPCC) report "Carbon Dioxide Capture and Storage," which was issued in 2005. That number pales in comparison to the estimated 23.7 billion metric tons of annual global CO2 emissions caused by human activities, primarily the burning of fossil fuels.

Chemists David C. Grills and Etsuko Fujita of Brookhaven National Laboratory, who are developing catalysts that could convert some of the overabundant greenhouse gas into useful chemicals, have broken down the numbers from the IPCC report and other sources and put the scale of the CO2 issue into perspective for C&EN.

Of total anthropogenic CO2 emissions, about 45% remains in the atmosphere, which is resulting in a gradual buildup of atmospheric CO2, Grills and Fujita explained. Although that seems like a large fraction, total annual CO2 emissions are still less than 1% of Earth's total atmospheric CO2 content, which is close to 3 trillion metric tons.

There currently is no feasible way to extract large amounts of CO2 from the air because it's so dilute, a bare 0.038% by volume at present, the Brookhaven researchers noted. That equates to 380 ppm, which is up from about 280 ppm 150 years ago. Thus, for any practical mitigation effort to stave off expected global warming, CO2 must be captured before it's released into the atmosphere.

Researchers are developing technologies for capturing CO2 from coal-fired power plants, which could be the source of 80% of global CO2 emissions by 2100 (Science 2007, 315, 812). But for now, only three large-scale sequestration programs are operating to put about 3 million metric tons of the unwanted CO2 per year into deep underground saline formations or into oil wells where the gas helps boost oil production (C&EN, April 2, page 48).

The 115 million metric tons of CO2 used by the chemical industry takes out less than 0.5% of the total annual anthropogenic CO2 emissions, Grills and Fujita noted. Chemists working in the area of CO2 utilization believe this amount easily could be tripled using current technologies, but that still wouldn't put a dent in solving the CO2 problem.

"It's clear we can't rely solely on CO2 utilization by the chemical industry to significantly mitigate CO2 emissions from fossil-fuel burning," Grills said. "Because of the vast scale of the problem, several strategies will need to be employed simultaneously, including increasing the efficiency of current fossil-fuel processes, capturing and using or storing the CO2 generated by fossil-fuel burning, and switching to renewable fuels and renewable energy sources."

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