Over half of Americans believe that there’s considerable disagreement among climate scientists about human-caused climate change—perhaps because they've heard that from industry advocacy campaigns and politicians. With so much controversy in the media many assume that the same controversy must exist in the scientific community.

In most situations people agree that it’s sensible to go with the majority of relevant experts whether that's in accepting that protons are real or a given medical treatment is effective. Those decisions depend critically on an accurate understanding of expert consensus.

Several attempts have been made to shine a light on expert opinions relating to global warming. One such study surveyed about 1,000 active climate scientists, finding that 97 percent of them accepted the evidence for the consensus position that anthropogenic greenhouse gas emissions are largely responsible for the warming observed over the last century.

Expert opinion is interesting and valuable, but the evidence the opinion is based on is the real arbiter. That evidence resides in the peer-reviewed scientific literature. Peer review isn’t perfect but it represents a critical standard that studies must meet. If there are good reasons to doubt that the human influence on climate is unimportant they should be found in the scientific literature.

In a 2004 study UC San Diego historian and geologist Naomi Oreskes surveyed published studies instead of scientists. Of the 928 papers published in the previous decade in which “global climate change” appeared in the abstracts, Oreskes found none that bucked the consensus. Both of these studies had their limitations so the folks behind SkepticalScience.com undertook an ambitious effort to perform a more rigorous survey of the scientific literature.

The group searched for abstracts containing the phrases “global climate change” or “global warming” published between 1991 and 2011 in peer-reviewed journals, netting nearly 12,000 papers. A group of 24 people (including 12 volunteers recruited through the Skeptical Science website) reviewed each and every one of them.

The abstracts were served up randomly without identifying information like author names. The degree to which each paper endorsed the consensus view was rated on a seven point scale from “explicit endorsement with quantification” to “explicit rejection with quantification”. Two raters categorized each abstract, with disagreements resolved between them and a tie-breaker if necessary.

About 33 percent of abstracts were categorized as endorsing the consensus, with 0.7 percent rejecting it. The remainder made no statement discernible as either. So among the abstracts with a clearly-stated position, 97.1 percent backed the consensus.

But what about the others? Did those abstracts not state a position because the consensus is so well-accepted as to make doing so unnecessary? Or was the human impact on climate often presented as uncertain in these papers? To answer this question (and further verify the ratings of the other abstracts) the group sent a survey to the authors whose email addresses were listed with the papers—over 8,500 in total. The survey was completed by 1,200 of them, who rated their own abstracts using the same criteria as the research group.

Of the abstracts that the research group had rated as not expressing a position the authors rated more than half of the papers as endorsing the consensus. Overall, 62.7 percent were self-rated as endorsing the consensus, 1.8 percent as rejecting the consensus, and 35.5 percent as having given no position.

So of those that expressed a position, 97.2 percent endorsed the consensus and 2.8 percent rejected it according to the authors of those papers.

The results aren't surprising but they do test the claim of rising and substantial disagreement among climate scientists. The study found that if anything, there was a very slight increase in the proportion of papers endorsing the consensus over this time. More importantly it helps people see that while the public controversy over climate change remains fairly strong in the United States, a scientific one barely exists.

There are also several interesting citizen science aspects of this study. There were the 12 volunteers who participated directly in the work, but the authors also raised the money needed to make the paper open-access with a crowd-funding campaign. In addition, they recently invited the public to help independently survey the abstracts—the results of which will be released in the near future.

Environmental Research Letters, 2013. DOI: 10.1088/1748-9326/8/2/024024 (Open Access) (About DOIs).