In an important shift of scientific judgment, experts advising the world’s governments on climate change are saying 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 so naturally 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.
Even the string of very warm years in the 1980s and 1990s could have been just a natural swing of the climatic pendulum, the experts have said.
But a growing body of data and analysis now suggests that the warming of the last century, and especially of the last 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 like carbon dioxide under the 1992 treaty on climate change.
The panel’s draft summary, although intended for internal use, was recently made available 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.
The scientific community “has discovered the smoking gun,” said Dr. Michael Oppenheimer, an atmospheric scientist with the Environmental Defense Fund, who is familiar with the draft report. “This finding is of paramount importance. For many years, policy-makers have asked, ‘Where’s the signal?’ ” The intergovernmental panel, he said, “is telling us that the signal is here.”
But Wigley and others involved in the reassessment say it is not yet known how much of the last century’s warming can be attributed to human activity and how much is part of the earth’s natural fluctuation that leads to ice ages at one extreme and warm periods at the other.
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 stimulate 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 last 10,000 years, the panel says.
And it says that whatever action is taken in the future, the world still faces a further average temperature increase of 1 to 3.6 degrees.
By comparison, according to varying estimates, the average global temperature is 5 to 9 degrees warmer now than in the last ice age.
While a warmer world could be beneficial in some ways, the draft says, there would be many adverse effects. These include more extreme weather and possibly more intense tropical storms, destruction of some communities by rising seas, damage to and loss of natural ecosystems that cannot adapt rapidly enough, diminished agricultural output in some places and an increase in some tropical diseases.
Experts agree that the average surface temperature of the globe has already risen about 1 degree Fahrenheit in the last century, but there has long been debate over the cause.
Much of the argument has involved the computerized models of the atmosphere that have been climatologists’ main tools in analyzing the warming problem. The models have many imperfections, but the panel scientists say they have improved.