By James D. Agresti
May 20, 2013
“Heat-Trapping Gas Passes Milestone, Raising Fears,” declared a recent front page headline in the New York Times. The event that served as the catalyst for this article was the atmospheric concentration of carbon dioxide reaching 400 parts per million (ppm), up from 288 ppm or by 39% since the dawn of the industrial revolution in the mid-1800s.
Painting an ominous picture of the situation, the reporter, Justin Gillis, quoted scientists who proclaimed that:
• reaching this milestone “feels like the inevitable march toward disaster,”
• “we are quickly losing the possibility of keeping the climate below what people thought were possibly tolerable thresholds,”
• “we have failed miserably in tackling this problem,”
• “I feel like the time to do something was yesterday,” and
• “It’s scary.”
Amidst these dire assessments, Gillis quoted and quickly rebutted a lone dissenting voice to these scientists: Dana Rohrabacher, a Republican congressman from California. Simultaneously, Gillis alleged that “climate-change contrarians” have “little scientific credibility.” Based upon such reporting, one would think that no credible scientist doubts that manmade global warming is a grave threat to the future of the planet.
That narrative, however, is at odds with the fact that 3,805 scientists with degrees in atmospheric, earth, or environmental science and 9,029 Ph.D.’s in varying scientific fields have signed a petition stating, “There is no convincing scientific evidence that human release of greenhouse gasses is causing or will, in the foreseeable future, cause catastrophic heating of the Earth’s atmosphere and disruption of the Earth’s climate.”
In this article, Gillis made no attempt to support his allegation that “climate contrarians” have “little scientific credibility,” but in reply to an email from Just Facts, he referred to “the Anderegg study and several more.” These studies, however, do not substantiate his storyline. At best, they show that the most frequently published climate scientists think the earth has warmed over the past century, and human activity is responsible for most of this warming. Furthermore, even those relatively modest conclusions are undermined by significant flaws in the studies.
The Anderegg study
In 2010, the Proceedings of the National Academy of Sciences published a study by Bill Anderegg and others entitled “Expert credibility in climate change.” For this study, the authors compared the scientific expertise of 1,372 climate researchers that they labeled as either “convinced by the evidence” for manmade global warming or “unconvinced by the evidence.”
They defined “convinced” researchers as those who embrace the views that manmade “greenhouse gases have been responsible for ‘most’ of the ‘unequivocal’ warming of the Earth’s average global temperature over the second half of the 20th century.” Those words are paraphrased from a massive 2007 report on the science of climate change by the Intergovernmental Panel on Climate Change (IPCC).
The Anderegg study found that more than 90% of “convinced” researchers had at least 20 publications listed in Google Scholar under a search for the word “climate,” as compared to only 20% of “unconvinced” researchers. They also found that among those with at least 20 publications, convinced researchers had an average of 119 climate publications in Google Scholar, as compared to an average of 60 publications for unconvinced researchers.
If one uncritically accepts the results of this study, it shows clear numerical superiority in Google Scholar for convinced researchers, but this is a far cry from Gillis’ claim that “climate contrarians” have “little scientific credibility.” There are numerous Ph.D. scientists that Gillis could have cited in opposition to the scientists that he quoted, but instead, he selected a Republican congressman. This propagates a misleading narrative that all scientists are on one side of this issue, while Republican politicians are on the other side.
Moreover, the Anderegg study does not show that “convinced” researchers embrace the calamitous views of global warming quoted by Gillis. In fact, a number of the researchers categorized by the authors as “convinced” may not even embrace the more moderate views that the study ascribes to them. This is because the authors presumptuously assigned 619 researchers to the “convinced” category based solely on them being contributors to the above-mentioned 2007 IPCC report.
That methodology is questionable because being a contributor to this 996-page report does not necessitate accepting its conclusions. As the technical summary of the report states at the outset, “the material has not been subject to line-by-line discussion and agreement, but nevertheless presents a comprehensive, objective and balanced view of the subject matter.”
For example, John Christy, one of the authors of this report, has explicitly rejected the view that the Anderegg study attributed to the report’s contributors. As Christy explained, the IPCC report “says we are 90 percent certain that most of the warming in the last 50 years was due to human effects. I don’t agree with that. I think things are much more ambiguous.”
In addition to the IPCC report, Anderegg and company compiled their list of “convinced” researchers from four public statements about global warming that expressed clear positions on this issue. These statements were signed by 499 researchers (including duplicates). Why the authors felt the need to go beyond these people and incorporate all contributors to the 2007 IPCC report does not bode well for the study’s credibility. In short, it is highly dubious to assign views to scientists that they have not expressed.
The Anderegg study is also plagued by other issues, but this one alone is enough to call the results into question. And again, even if one blindly accepts the results, the study does not show that all credible scientists see global warming as a serious risk, which is the clear message of Gillis’ article.
The Doran study
Another frequently cited study about scientists’ views on global warming was published in Eos, “the premier international newspaper of the Earth and space sciences.” This article is entitled “Examining the Scientific Consensus on Climate Change” and was written by Peter Doran and Maggie Zimmerman of the University of Illinois. This study purports to show that “an unbiased survey of a large and broad group of Earth scientists” found that:
• 90% think average global temperatures “have generally risen” since the 1800s.
• 82% think “human activity is a significant contributing factor in changing mean global temperatures.”
The study also categorized a subgroup of respondents as “the most specialized and knowledgeable” if they reported that climate science was “their area of expertise,” and “more than 50% of their recent peer-reviewed papers” were about this subject. These stringent criteria restricted this subgroup to 2.5% of all respondents and less than 1% of the scientists who were sent the survey. Among these individuals, 96% and 97% agreed with the statements above.
Tacitly referring to this study, Gillis wrote in an April 2012 article that “polls say 97 percent of working climate scientists now see global warming as a serious risk.” As with his recent claim about “little scientific credibility,” he provided no link to substantiate this assertion. Nonetheless, this clearly references the Doran study, as evidenced by the “97 percent” figure and the prominence of this study.
Once again, Gillis stretched his reporting far beyond the study’s findings. The Doran study says absolutely nothing about global warming being a “serious risk.” Also, Gillis labeled every scientist not categorized by this study as “the most specialized and knowledgeable” as not being “working climate scientists.” In other words, he used a capricious definition of “working climate scientists” that includes only 2.5% of the scientists who responded to the survey.
This is especially problematic given that 8.5% of the scientists who responded to the survey indicated that “more than 50% of their peer-reviewed publications in the past 5 years have been on the subject of climate change.” Furthermore, there is no rational basis to assert that scientists don’t qualify as “working climate scientists” unless more than half of their papers are about climate change. It is not uncommon for scientists to have multiple areas of expertise and to be highly knowledgeable on a wide range of associated topics.
Beyond this, the Doran study has a major flaw, which is that the results are based on an internet survey that yielded a 31% response rate. The authors point out that this is “a typical response rate for Web-based surveys,” but this has no bearing upon whether the poll is credible. The issue is not how the Doran survey compares to other internet surveys but whether internet surveys are even reliable. As it turns out, internet and mail surveys often suffer from a phenomenon called selection bias or nonresponse bias, which frequently makes them untrustworthy.
Selection bias can take different forms, but for the type of poll used for the Doran study, it typically occurs because certain people are more likely to respond, specifically those who are opinionated or not very busy. As explained in the textbook Mind on Statistics, “Surveys that simply use those who respond voluntarily are sure to be biased in favor of those with strong opinions or with time on their hands.” Suitably, after stating this, the textbook analyzes a poll of scientists that had a 34% response rate, and it explains that “with only about a third of those contacted responding, it is inappropriate to generalize these findings” to most scientists.
In his recent article about CO2 levels reaching the numeric milestone of 400 ppm, New York Times environmental reporter Justin Gillis painted a picture of impending doom and casted all who differ as lacking in scientific credibility. However, thousands of Ph.D. scientists have explicitly stated there is “no convincing scientific evidence” that manmade greenhouse gases will “cause catastrophic heating of the Earth’s atmosphere and disruption of the Earth’s climate.”
Gillis has cited at least two popular studies to support his claim, but these studies don’t prove what he asserts. Instead, they show near-universal support among the most frequently published climate scientists for the more moderate views that the earth has warmed over the past century and human activity has contributed significantly to this. However, these studies have substantial flaws that cast doubt on their credibility, and thus, it is misleading to cite them without qualification.
Correction (3/10/17): The original version of this article incorrectly stated that “9,029 Ph.D. scientists, including 3,805 atmospheric, earth, and environmental scientists” signed the above-mentioned petition.” As shown in the current article, the figure of 3,805 scientists is independent of the figure of 9,029 Ph.D.’s, not a subset of it.
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