Good News in the Greenhouse

Good News in the Greenhouse

The reporter's enthusiasm for Kyoto notwithstanding, more evidence that the science underlying the global warming issue is changing.

Copyright SCIENTIFIC AMERICAN, June 1998
"Climate Change: Good News for the Greenhouse"
Subhead: "Growth of atmospheric methane might soon abate"
by David Schneider

Last December's climate talks in Kyoto, Japan, helped to keep the world's attention firmly focused on the threat of greenhouse warming posed by emissions of carbon dioxide. And to good effect: if industrial nations can meet the targets they established, the heat-trapping veil of carbon dioxide should thicken less swiftly.

But another, more subtle, event occurred last December that might also bode well for future climate. Researchers from the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado reported that methane, another potent greenhouse gas, appears to be accumulating in the atmosphere more slowly than anticipated. If this trend continues, the concentration of methane might soon stabilize, miraculously stemming some 20 percent of the burgeoning greenhouse problem.

This news from NOAA's Edward J. Dlugokencky and his colleagues (delivered at the last meeting of the American Geophysical Union) represents a departure from previous conceptions. Researchers had known that the growth of atmospheric methane had been ebbing since comprehensive measurements began in the early 1980s. And they were aware of an abrupt decrease in the rising tide of methane and several other trace gases that occurred in 1992.

But many, including members of the influential Intergovernmental Panel on Climate Change (IPCC), regarded that sudden downturn as a short-term "anomaly." After all, the main sources of methane--wetlands, rice paddies and livestock--had not gone away. According to their best guesses, the 1992 decline was caused, perhaps, by the drop in natural gas production (and, presumably, methane leakage from pipes) in the former Soviet Union. Or it came from the eruption of Mount Pinatubo the previous year, which reduced stratospheric ozone and allowed more ultraviolet light to reach the lower atmosphere, where it breaks up methane.

So Dlugokencky's observation that the growth of methane has remained comparatively low for years after the atmospheric effects of Pinatubo subsided has sparked some rethinking. Inez Y. Fung, an expert on atmospheric methane at the University of Victoria in British Columbia, regards the new findings as "spectacular data," yet she emphasizes that most scientists are so far at a loss to understand what is behind the change. "Ed has presented for us a tremendous challenge," she notes.

But Dlugokencky and his colleagues did more than just offer up data. They suggested that the slowing growth rate of methane can be explained by viewing the atmosphere as a chemical system approaching equilibrium on a timescale that is on par with the lifetime of methane--about a decade (it is broken down both by ultraviolet light and by certain chemical reactions).

Aslam K. Khalil, an atmospheric scientist at Portland State University, has a different interpretation. He believes that methane is accumulating more slowly now because the long-standing link between methane sources and human population is weakening. But whatever the cause, if the current pattern holds, then stabilization between production and destruction is not far off.

In contrast, the IPCC had projected that atmospheric methane would continue to rise, roughly doubling (in one typical forecast) by 2100. Thus, many of the scenarios for global warming provided in their last assessment may have been overly gloomy. Nevertheless, Fung thinks it is premature to celebrate, because this slowing in the rise of methane might not persist. When pressed for how long one must wait to be optimistic, she answers: "If it goes on for another 10 years, I would be ecstatic."

David S. Schimel of the National Center for Atmospheric Research, one of the authors of the 1995 IPCC report on the topic, admits that the assumptions used at that time were "based on an understanding of methane that was five to 15 years old." He, too, notes that climate researchers are disquieted by their inability to forecast such changes. Still, he regards the recent downturn in the growth of this greenhouse gas as "definitely good news."