BioEd Online

The United States could slash its greenhouse gas emissions by switching to today's fuel-efficient cars, US scientists have calculated, challenging the idea that reducing emissions is too expensive or will require new technologies to be developed.

Many methods have been put forward to curb emissions of heat-trapping carbon dioxide, such as growing plantations of trees to soak up the gas. Another idea is to stop ploughing farm fields, which would slow the release of carbon dioxide through the normal bacterial breakdown of plant waste.

Robert Jackson and William Schlesinger of Duke University in Durham, North Carolina, wanted to know how realistic these proposals really are. Using figures from previously published studies, they calculated how widespread such measures would have to be to slice US carbon emissions by 10%.

If farmers stopped tilling all the cropland in the country, and either switched to alternative farming techniques or left land fallow, carbon emissions would drop by less than 4%, the researchers report today in Proceedings of the National Academy of Sciences¹.

In order to hit the 10% target, at least one third of that land would have to be planted with forests to sop up carbon dioxide, which would reduce the food supply and have unknown additional consequences for the environment. The study shows that many people expect too much from these methods, says Jackson: "It points out the limits."

The United States could, however, achieve a 10% emissions cut by doubling the fuel efficiency of all cars and sports utility vehicles (SUVs), Jackson and Schlesinger's estimates show. This could be done by switching to existing hybrid electric vehicles, which run partly on electricity.

Although there is a price to pay for cleaner cars - hybrid vehicles currently cost a few thousand dollars more than conventional gas-guzzlers - Jackson says the exercise highlights how small changes in existing technology could make considerable dents in US emissions.

"We're trying to say we can do it now, and relatively cheaply," he says. Additional price subsidies for consumers, or a hike in gasoline prices, could encourage drivers to switch to hybrid vehicles, he suggests.

Showing that cars could make such a big difference is "really, really important", agrees Chris Field, a specialist in global climate change at the Carnegie Institution of Washington in Stanford, California. "It's a very useful set of calculations," he says.

Immediate action

The United States pumps out more greenhouse gases than any other country in the world. Environmentalists have criticized President Bush for not taking more immediate action and for pulling out of the Kyoto protocol, an international pact to curb emissions.

The Bush administration focuses instead on voluntary emission cuts and longer-term improvements in fuel efficiency, such as moves towards hydrogen-powered vehicles and nuclear energy. Jackson says that his study shows that current technology might have the ability to lower pollution now.

Even if the United States did adopt drastic steps to clean up, Field says the country would still struggle to achieve cuts equivalent to those required of industrialized countries by the Kyoto agreement: a reduction of at least 5% relative to 1990 levels by 2012. This equates to a 30% cutback from today's levels, he says.

No single policy or technology switch can make such a big dent in US pollution, Field says - but combined, they might make a real difference. "The sum of all these is compatible with finding a solution," he says.