For years, emissions of greenhouse gases in developed countries—and throughout the world—have been going down while economic activity increased. Even as the economies of the U.S. and European Union continued to grow, the amount of carbon dioxide (CO2) per car built, burger served or widget sold was on the decline. No more. "It appears that the carbon intensity of economic activity has stopped improving," says Chris Field, director of the Carnegie Institution of Washington's Department of Global Ecology in Stanford, Calif. "Each dollar of economic activity is requiring more rather than less carbon, which reverses a long-term trend."

In fact, the growth of CO2 emissions tripled between 2000 and 2004—growing by more than 3 percent per year—according to a new study published in Proceedings of the National Academy of Sciences USA. From 1990 to 1999, emissions growth had averaged a little over 1 percent per year. (Researchers based their findings on data from the U.S. Department of Energy, the United Nations Statistics Division and the International Monetary Fund.)

Carbon dioxide is responsible for trapping roughly 63 percent of the extra heat blamed for global warming. By 2005, emissions from man-made fossil fuel combustion had reached 7.9 billion metric tons per year (or 1.7 x 1013 pounds), according to the Global Carbon Project (GCP)—an Australia-based research consortium devoted to analyzing the problem.

Developing countries such as China and India, which have experienced economic booms, are leading the charge in increasing CO2 emissions. Although the U.S., Europe and other developed countries have contributed 77 percent of the cumulative emissions since the dawn of the Industrial Revolution in the 19th century, developing nations were responsible for 73 percent of the total growth in 2004 alone. "Basically, the increase reflects a surge in economic activity," Field says. "There is a tight link between economic activity and energy use." In other words, the more widgets produced, the more energy consumed—and therefore the more CO2 emitted.

Carbon intensity is going up because countries like China are relying on the cheapest and dirtiest of fossil fuels to power their growth. "Basically, their economy is growing on coal," Field notes. But according to the U.S. Department of Energy, pollution is on the rise in the U.S. and world energy use is expected to grow 57 percent by 2030, with coal being the fastest growing energy source.

Study lead author Michael Raupach, GCP co-chair and atmospheric physicist at Australia's Commonwealth Scientific and Industrial Research Organization, says it will take economic, policy and social changes to reverse the trend, such as capturing the CO2 emitted by coal-fired power plants and increased international cooperation.

This is particularly true as national governments continue to strive to enhance the economic well-being of their populations. "In an era of rapidly increasing economic growth and increasing carbon emissions, you can't assume that we're going to continue to see improvement in carbon intensity," Field says. "We have to figure out some way to get the carbon intensity of the energy system to go down."

Fields argues the burden rests with countries like the U.S. that have the resources and technological know-how to undertake solutions, such as carbon capture and storage, which will be needed quickly. "We have to try harder to control global warming," Raupach adds. "The final judge of our efforts is the global atmosphere and its judgment at present is harsh."