It’s time to stop debating the science behind whether humans contribute to rising temperatures, says U.S. Rep. Don Beyer, D-8th.
"Even while over 97 percent of the scientific community and the vast majority of Americans believe that humans are contributing to climate change, a small but well-heeled cohort continues to deny and even try to discredit what objective science tells us unambiguously," he wrote in a March 24 post on his congressional website.
This Truth-O-Meter will focus on Beyer’s claim that 97 percent of the scientific community believes humans are a cause of climate change - a figure widely cited by those urging efforts to stem global warming. Climate change skeptics repeatedly have said the figure is not credible.
We asked Beyer for the source of his statistic. His spokesman, Thomas Scanlon, pointed to several articles in scientific journals, information from NASA, and other sources about climate change.
Let’s start with NASA. Here’s what its website says:
"Multiple studies published in peer-reviewed scientific journals show that 97 percent or more of actively publishing climate scientists agree: Climate-warming trends over the past century are very likely due to human activities. In addition, most of the leading scientific organizations worldwide have issued public statements endorsing this position."
So the federal space agency notes that the 97 percent figure more narrowly focuses on "actively publishing climate scientists." That’s different from Beyer’s broad contention that 97 percent the entire "scientific community" thinks humans contribute to global warming.
Still, this leaves us with the question: What are the sources for the 97 percent? NASA points to several of them in a footnote on its website.
The first is a 2013 study by John Cook, a fellow at the Global Change Institute at the University of Queensland in Australia.
He and his co-authors examined 11,944 abstracts of climate science reports published from 1991 to 2011. Two-thirds of the abstracts didn’t express any opinion about whether man-made global warming was occurring. But among the roughly 4,000 studies that did, 97.1 percent of them "endorsed the consensus position that humans are causing global warming," the report says.
Cook also asked a sample of 1,200 authors of the climate reports to rate their own papers and say whether their research agreed that global warming was man-made. The result was that 97.2 percent of the papers that expressed an opinion about man-made global warming backed the position that it’s caused by human activity.
Cook’s methodology has been criticized.
Richard Tol, a professor with the Institute for Environmental Studies in Amsterdam, rejects the 97 percent figure. In a 2014 article, he said it was based on an unrepresentative sample of climate science papers that took into account only one-quarter of the climate research available.
Tol had other misgivings about the Australian researcher’s methodology. In re-running the figures, Tol found that some 91 percent of climate research papers that Cook’s team examined supported the consensus that man is the main reason for the Earth’s warming temperatures.
Despite taking aim at the specificity of the 97 percent figure, Tol said he agrees there indeed is a large consensus that warming temperatures are the result of human activity.
"There is no doubt in my mind that the literature on climate change overwhelmingly supports the hypothesis that climate change is caused by humans," Tol wrote. "I have very little reason to doubt that the consensus is indeed correct. Cook et al., however, failed to demonstrate this."
University of Illinois study
Beyond this debate, NASA also cites a January 2009 study by researchers at the University of Illinois. They surveyed 3,146 earth scientists and asked two questions: Was global warming occurring? Was human activity a "significant contributing factor" for changes in the Earth’s temperatures?
Nine out of 10 of the scientists said the Earth’s temperatures had risen, and 82 percent said human activities were a key reason.
Winnowing the field to climate change researchers who actively published their results, the survey found that 75 of 77 of them, or 97.4 percent, said human activity is a significant driver of climate change. That latter figure also has come under criticism because of its small sample size.
National Academy of Sciences
Another study published in 2010 by the National Academy of Sciences created a database of 908 climate researchers who had their work published at least 20 times in scientific journals.
The study found roughly 90 percent of the climate researchers were convinced that rising temperatures are being caused by human activity, while 10 percent were not convinced. But the more expertise the researchers had, the greater their consensus.
Looking at the top 200 researchers, based on the number of articles they published, just 2.5 percent of them were unconvinced climate change was mostly the result of human activity, the study found. That left "97–98% of the climate researchers most actively publishing in the field surveyed" convinced that rising temperatures were mostly due to humans, the report said.
Conversely, the study found that the expertise of researchers who weren’t convinced of man-made global warming was "substantially below that of the convinced researchers."
Bill Anderegg, the PNAS report’s lead author, told us in an email that "the quote from (Beyer) is absolutely consistent with the scientific literature on this."
We also found a 2013 poll of roughly 1,800 meteorologists by the American Meteorological Society found that 73 percent of those surveyed felt that humans contributed to global warming. But for the more active publishing climate science experts within that group, 93 percent believed that humans were contributing to global warming.
Beyer said "over 97 percent of the scientific community" believe humans are contributing to climate change.
The studies Beyer and others cite do not reflect the scientific community at large. They are surveys that focus on the conclusions of climatologists, earth scientists and meteorologists. The studies found that overwhelming majorities of these experts - sometimes, but not always as high as 97 percent - say humans are contributing to global warming.
Beyer’s statement is credible but needs elaboration. We rate it Mostly True.