In an important shift of scientific judgment, experts advising the world's governments on climate change say for the first time that human activity is a likely cause of the warming of the global atmosphere.
While many climatologists have thought this to be the case, all but a few have held until now that the climate is naturally so variable that they could not be sure they were seeing a clear signal of the feared "greenhouse effect" -- the heating of the atmosphere because of the carbon dioxide released by burning coal, oil and wood.
But a growing body of data and analysis now suggests that the warming of the past century, and especially of the past few years, "is unlikely to be entirely due to natural causes and that a pattern of climatic response to human activities is identifiable in the climatological record," says a draft summary of a new report by the Intergovernmental Panel on Climate Change.
The panel's role is to advise governments now negotiating reductions in emissions of greenhouse gases such as carbon dioxide under the 1992 treaty on climate change.
The panel's draft summary, although intended for internal use, recently appeared on the Internet. The draft has been through at least one round of scientific review, but its wording may change, since it is now being reviewed by governments.
Scientists who prepared the full chapter on which the summary statement is based say they do not expect any substantial change in their basic assessment. The chapter has gone through extensive review by scientists around the world.
"I think the scientific justification for the statement is there, unequivocally," said Dr. Tom M. L. Wigley, a climatologist at the National Center for Atmospheric Research in Boulder, Colo., one of the chapter's authors.
Nevertheless, the panel's conclusion marks a watershed in the views of climatologists, who -- with the notable exception of Dr. James E. Hansen of the NASA Goddard Institute for Space Studies in New York -- have until now refused to declare publicly that they can discern the signature of the greenhouse effect.
The new consensus, as represented by the intergovernmental panel, seems likely to stir more public debate over how seriously the threat of climate change should be taken.
As for the future, the draft summary forecasts an increase in the average global temperature of 1.44 degrees to 6.3 degrees Fahrenheit by the year 2100 if there is no further action to curb emissions of greenhouse gases.
But that represents only 50 percent to 70 percent of the eventual warming, it says. These changes would be more rapid than any in the past 10,000 years, the period in which civilization developed, the panel says.