Tough carbon budget could slow warming.

Tough carbon budget could slow warming

If the impending warming of Earth's climate--fostered largely by the release of "greenhouse" gases--grows too large or occurs too rapidly, many ecosystems may not adapt well enough to survive. To protect them, industrialized nations should halve their carbon dioxide releases from fossil-fuel combustion within 25 years and reduce those emissions to 75 percent below current levels by 2030, according to a new study commissioned by the Dutch government. However, the wealthiest and most fossil-fuel-intensive economies--including the United States--should reduce emissions even faster to allow for slower progress in eastern Europe, the study's authors say.

The Dutch government authorized public release of the report, entitled "Energy Policy in the Greenhouse," at the 70-nation Ministerial Conference on Atmospheric Pollution and Climate Change last month in Noordwijk, the Netherlands. In the United States, the report was formally unveiled last week at a briefing for members of Congress and their staffs.

Designed to identify benchmark limits on greenhouse-gas emissions, for use in drafting an international convention to limit climate change, the study began with an investigation of the warming rates most ecosystems might reasonably be expected to handle, says Florentin Krause of the Lawrence Berkeley (Calif.) Laboratory, who directed the effort. His team then calculated limits on greenhouse-gas releases that would likely hold warming within these limits.

The ecological risk assessment suggested warming should not exceed more than 0.1 [degrees] C (about 0.2 [degrees] F) per decade. Though this may not seem large, Krause notes that it's 10 to 20 times greater than the rate of change that typifies transitions from ice ages to warmer periods. He says it's also what "the best current data" suggest woodland ecosystems can withstand without risking a catastrophic dieback of mature trees.

While Krause acknowledges there's no way to predict with certainty how Earth's interdependent ecosystems will respond to a changing climate, he points out that the warmer the overall climate becomes, the less likely it is that many ecosystems will adapt.

So in this study, researchers set the limit on absolute warming at 2.5 [degrees] C above temperatures typical of the mid-19th century, just prior to the industrial revolution. Records indicate the average global surface temperature has already climbed about 0.5 [degrees] C since the 1850s. In their report, the authors point out that the last time Earth's climate averaged 2 [degrees] C warmer than preindustrial times was about 125,000 years ago--a period when humans, as hunter-gatherers, did not depend on cultivated agriculture. They add that Earth has not been 2.5 [degrees] C warmer in 2 million years--since before the emergence of humans or, indeed, about 70 percent of today's species.

On the basis of these constraints, the authors calculated an upper limit on allowable cumulative carbon dioxide releases between now and 2100. The best evidence suggests the world must live within a carbon budget of roughly 300 billion tons, or 50 years' worth of emissions at current rates, they report. The study's per-decade limit on warming also suggests humans must ultimately reduce their annual global carbon dioxide releases to about 25 percent of what's emitted today.

Krause cautions that the tough limits on carbon dioxide emissions -- which today account for almost 60 percent of the annual human contribution of greenhouse gases--do not reduce the need to limit other greenhouse gases. In calculating carbon budgets, the researchers assumed a total phaseout of chlorofluorocarbon emissions by 2000, a replanting of forest cover to maintain levels typical of the mid-1980s, a limit on methane releases and a slowing in the buildup of atmospheric nitrous oxide concentrations.

Allocating the carbon budget between industrialized and developing countries must rely on diplomatic negotiation, according to the report. But Krause's team expects to offer some guidance in a companion report due out next year. That study will detail technological and economic options already available for reducing carbon dioxide emissions--such as imposing a "carbon tax" on fossil-fuel use or installing "scrubbers" to remove the pollutant from powerplant stack gases. Though the necessary reductions won't be easy, Krause says his analyses also suggest that a dramatic phasedown in carbon dioxide releases need not require crash programs or sacrifices in economic competitiveness -- provided reduction measures begin immediately.

"I approve of what they're trying to do, and fundamentally, their approach is right," comments Stephen H. Schneider, a climatologist and global-change analyst with the National Center for Atmospheric Research in Boulder, Colo. But regardless of how well researchers try to ground such analyses in biology or atmospheric chemistry, there will remain sufficient scientific uncertainties to leave skeptics room for quibbling, he says.

Stewart Boyle of the Association for the Conservation of Energy in London, England, is more optimistic. The new report "is a blueprint for a future international agreement on carbon," he says, and "will help shape the [climate change] debate." Just last week, Boyle notes, Krause briefed high-ranking government officials in France and environmental leaders in the British parliament.