Cars, not crops, should be chief targets in reducing greenhouse gases

Retiring croplands and switching to no-till agriculture can contribute in a modest way to reducing the increase of carbon dioxide in the atmosphere, but doubling fuel efficiencies of cars and light trucks would achieve much greater results, according to two Duke University ecologists.

In an analysis to be published the week of Oct. 25, 2004, in the online edition of the Proceedings of the National Academy of Sciences, Robert Jackson and William Schlesinger of Duke’s Nicholas School of the Environment and Earth Sciences and Department of Biology examined how far “carbon sequestration” versus increased fuel efficiency would go toward a goal of reducing net U.S. carbon dioxide emissions by 10 percent.

Carbon sequestration steps include adopting no-till agriculture to retain crop wastes in the soil rather than letting them decay after plowing, and retiring croplands by paying farmers to revert them to grassland or forests. Carbon dioxide has been implicated in global warming because it can trap heat in the atmosphere much like a greenhouse. “We emit a quarter of the world’s carbon dioxide emissions into the atmosphere, even though we constitute only 5 percent of the world’s population,” said Jackson, a professor of environmental science and biology who is also director of Duke’s Center on Global Change.

“Our goal was to compare two different scenarios – biological sequestration and reduced auto emissions – to give a sense of what we could do to lessen carbon dioxide emissions.” Said Schlesinger, who is a professor of biogeochemistry and dean of the Nicholas School, “We chose a 10 percent reduction as the basis of our analysis to place it in the realm of possibility.” Their calculations showed that converting all U.S. croplands to no-till agriculture, or even taking the unthinkable step of retiring all croplands, would yield only a reduction of less than 4 percent in carbon dioxide emissions. Achieving a 10 percent reduction would require converting one third of all croplands to carbon-storing forest plantations, they calculated.

In contrast, they noted that cars and light trucks emit roughly 20 percent of carbon dioxide emissions. Thus, a doubling of fuel efficiency in such vehicles – for example through hybrid gas-electric vehicles – could achieve the goal of a 10 percent carbon dioxide reduction. Reducing carbon dioxide emissions through more efficient vehicles makes more sense strategically, said Schlesinger. “It’s as if you have marbles leaking out of a bag,” he said. “It’s easier to figure out a way to stitch up the bag than to attempt to gather up the marbles after they have escaped.

“I think our analysis reinforces the view that it is very difficult, short of taking very large areas of land out of production to soak up carbon dioxide that’s escaped, compared to cutting down on the emissions as you might with simple technologies like driving hybrid cars that get twice the mileage. “Taking a third of our crop land out of production and using it to sequester carbon would mean very high prices for food in this country and a lot of hungry people around the world,” Schlesinger said.

He and Jackson concluded, however, that all possible strategies should be considered in plans to reduce carbon dioxide emissions. “Reducing emissions is the step that’s simplest and most directly under our control,” Jackson said. “But we need to examine all carbon sequestration technologies wherever they’re cost-effective, and to evaluate the suite of consequences that would arise if they were implemented.” Other solutions include the use of renewable energy sources, decarbonization and geological sequestration, said the researchers.

The researchers concluded in their article that: “a doubling in fuel efficiency through hybrid technology, advanced diesel engines and light-weight materials could precede a transition to hydrogen vehicles, which now require fossil fuels or other sources of energy to generate the hydrogen. Coupled with changes in the way that agricultural lands are managed, doubling the fuel efficiency of our nation’s vehicles seems a logical first step in balancing the carbon budget.”

Schlesinger emphasized that national leadership will be needed to address the problem of carbon dioxide emissions. Regardless of which candidate wins the presidential election, he said, “the climate change issue is not going to go away. The international community is going to make sure that it doesn’t go away. This country is going to have to come up with solutions, and our paper aims to begin to identify some of the changes that can be made without vastly changing the lifestyle of the citizens of the country.”

Both Jackson and Schlesinger also said that one implication of their paper is that the budgets for environmental research and monitoring, and development of alternative energy sources, will have to increase drastically for the country to meet the challenges of reducing its impact on global climate and achieving a more energy-efficient economy.