A new study of climate scientists' opinions shows 97 percent agree that global warming is driven mainly by human activity – emissions of carbon dioxide and other greenhouse gases.
The study, published this week by the National Academy of Sciences, involved 1,372 climate scientists, most considered top researchers in their field.
A globe is projected as people are seen in Town Hall Square on the opening day of the Climate Conference in Copenhagen, Denmark, Monday Dec. 7, 2009. The largest and most important U.N. climate change conference in history opened Monday, with organizers warning diplomats from 192 nations that this could be the best, last chance for a deal to protect the world from calamitous global warming. (AP Photo/Peter Dejong)
The study's authors took pains to separate true experts – active researchers with a large number of publications to their credit – from amateurs, or scientists from other fields who make statements about climate change.
They did not pose questions directly to the scientists. Instead, they surveyed public statements the scientists had signed indicating whether they accept that the planet is warming and that humans are the primary cause.
"We felt it would be helpful to put numbers to it, and really to bring in the expertise angle," said William R.L. Anderegg, a doctoral candidate at Stanford University who studies climate change and its effects on ecosystems, and who was the study's lead author. "We thought it was important to provide the average person with a roadmap on where the experts stand on climate change."
Culling out top experts in climate science by relying on their published work helped sharpen the findings, said UC Irvine professor Michael Prather, an expert in climate modeling and an author of climate reports from the Intergovernmental Panel on Climate Change.
"This is exactly what you would do, if you were smart, trying to pick a doctor to do some surgery on you," Prather said. "You'd want to know what the patient success rate is. You'd want to put all the statistics together. They did the same thing for science research."
The study grew in part from concern among scientists over increasing media focus on the assertions of groups that call themselves climate change skeptics – those that deny the planet is warming, or that humans are influencing the climate, or both.
The scientists, Anderegg said, had a growing sense that media reports "were becoming increasingly disconnected from the scientific discussion about climate change."
The skeptics included three percent of the top 200 scientists surveyed who remained unconvinced about climate change or humanity's role in it. But an analysis of their work showed far lower levels of publication in the climate field, as well as fewer citations of their work by other climate scientists.
Controversy erupted last year after climate scientists' emails were hacked from the University of East Anglia in England; critics contended that the emails showed exaggeration or attempts by the scientists to manipulate climate data.
The National Academy, however, points to several investigations that found no wrongdoing by the scientists, and that upheld the present scientific consensus on climate change.
Another recent survey, meanwhile, showed a decline in acceptance of human-caused warming among Americans, from 83 percent in 2007 to 75 percent today – still a high number, said the study's author, Jon Krosnick, of Stanford University's Woods Institute for the Environment.
The survey of scientists was published in the Proceedings of the National Academy of Sciences.