Science, Vol 288, April 28, 2000
By Richard A. Kerr
For the past several years, an international panel of climate scientists has been testing alternatives to the idea that people are affecting global climate. They examined climate's natural variability, changes in solar radiation, and volcanic outpourings, among others. But none of those factors fit the past century's observed warming as well as the explanation they suggested in 1995: an increase in greenhouse gases generated by human activity. So last week, the group, the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC), released the draft of a new report concluding "that there has been a discernible human influence on global climate." If those words hold up under further expert and governmental review, they would be the strongest official pronouncement yet that human-induced warming is real.
"Something definitely seems to have happened" to the climate, says climate researcher Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla, California, who reviewed part of an earlier draft. As this draft points out, "three of the last 5 years have been the warmest in the instrumental record," which goes back 140 years. And three different records of temperature preserved in tree rings and elsewhere have now revealed the large, abrupt 20th-century warming to be unique in the past 1000 years.
The confident recognition of an anthropogenic climate effect--which could bolster calls for action to curb global warming--is the draft report's only major shift since 1995, when the IPCC found that "the balance of evidence suggests a discernible human influence." The new report notes that there has been little progress in projecting the future of greenhouse warming, thanks to uncertainties about everything from climate models and the behavior of clouds to the vagaries of humans' burning of fossil fuels. Even so, the report, to be finalized later this year, should inform negotiations that culminate this fall on the implementation of the Kyoto Protocol for the reduction of greenhouse gas emissions.
The IPCC gained confidence in identifying the 0.6ºC warming of the past century as anthropogenic through a process of elimination. Since the previous report, researchers have run their improved climate models repeatedly and longer to look for alternatives--the natural ups and downs of temperature, solar variability, or volcanic emissions. None seems to suffice. And model simulations of the past century including rising greenhouse gases bear a strong resemblance to the actual warming.
Barnett is cautious about declaring complete certainty, but "we have a change we can't explain with natural variations. There aren't many other options."
Climatologist Gerald North of Texas A&M University in College Station, who does greenhouse detection work but has not been involved in the IPCC process, is more confident: "There are too many independent pieces of evidence, and there's not a single piece of contradictory evidence," he says. North is particularly impressed by the 1000-year temperature records. "The planet had been cooling slowly until 120 years ago, when, bam!, it jumps up," he says. "We've been breaking our backs on [greenhouse] detection, but I found the 1000-year records more convincing than any of our detection studies" using climate models.
Even greenhouse contrarians are tacitly going along with the IPCC's confident conclusion.
Rather than dispute the reality of the warming or its cause, they have lately emphasized its modest size and inferred minimal future negative effects. Much of the warming, they note, has come at night, in the winter, and in areas that might stand some warming, such as Siberia.
While the report seems to reflect broad support for the recognition of human-induced climate change, "we don't quite know what it means for the next 100 years," admits North. The report offers nothing new on how much temperatures might rise given an added shot of greenhouse gases. It cites the same possible warming from a doubling of carbon dioxide--2.5ºC with a range of 1.5º to 4.5ºC--as did the 1990 and 1995 reports. Indeed, that range goes back to a National Academy of Sciences report of 1979. Uncertainties in the magnitudes of complicating factors such as solar variations and the effects of pollutant hazes have changed little since 1995.
One change in the report -- a more prominent role for socioeconomic factors -- only increases the uncertainty. Depending on which of six possible scenarios for emission of greenhouse gases and cooling pollutant hazes is used, warming by 2100 could be between a modest 1ºC and a sizzling 5ºC. The range of warming created by economic, demographic, and policy assumptions in the scenarios "is similar to that due to uncertainty in models," the report observes. With so much up in the air, the IPCC should have no lack of grist for its next report in 2005.
Exceptional century. Temperatures recorded in tree rings and elsewhere (purple) reveal that the 20th century (red is instrumental record) was unique in the millennium. SOURCE: MANN ET AL., AMERICAN GEOPHYSICAL UNION