When I was a graduate student, I attended a conference session on the sediment record of the Arctic Ocean. Over the course of the conference, I heard experts debate, vigorously, the pros and cons of using the chemical elements protactinium and thorium to estimate the age of Arctic sediments, where the region’s sedimentary features preclude a more conventional dating approach. Let’s just say there were some diverse viewpoints.
This anecdote illustrates a point that seems absent from some of the discourse around yesterday’s repeal of the Endangerment Finding. Namely, that reaching agreement in science is extremely difficult. When virtually all of a large body of scientific evidence consistently points to the same conclusion, that conclusion is no longer up for debate—it becomes scientific consensus. The Endangerment Finding that “the current and projected concentrations of . . . greenhouse gases . . . in the atmosphere threaten the public health and welfare of current and future generations,” is the scientific consensus on climate change.
That consensus was not reached without debate. Debate is built into the very fabric of scientific research. Consider the peer review process: authors submit a draft paper to a journal where an editor—typically an established, senior scientist—conducts an initial review. If the editor decides the draft could be an appropriate publication in the journal, they send it out to multiple other experts for review. These peer reviewers go on to poke as many holes as possible in the ideas and logic presented in the draft and recommend to the editor whether to reject or accept it, subject to major or minor revisions. An individual paper may undergo several rounds of debate and discussion between the authors and reviewers, often over a period of months or even years, before it is published.
Despite this robust vetting process, not everything that is published ends up being accurate. Science evolves over time. New results may challenge old paradigms and, ultimately, pave the way for more accurate theories. So, if everything is subject to change, how can anybody—scientists and non-scientists alike—have confidence in our current best understanding of a given topic?
Let’s take a step back. What do scientists mean by “confidence?”
Over many years, scientists have developed a rigorous definition. Confidence in a particular finding depends on two things: the scientific evidence that underpins the finding, and the level of agreement among different lines of evidence (Fig. 1).
Figure 1. Framework for evaluating confidence in climate-science findings. Source: Working Group I, Fifth Assessment Report, IPCC (2014)
An analogy can be made to online shopping. Before making a purchase, you might check two things: the average customer rating and the number of reviews. A five-star rating doesn’t provide a lot of confidence in a product when the rating is based on only a handful of reviews. Ideally, you want a high rating (the evidence) based on a lot of reviews (the agreement). Put the two together, and you feel confident about your purchase.
While straightforward in theory, this framework gets more complicated when applied to climate science. That’s because the climate system is itself complicated. Climate science necessarily relies on many different disciplines, and scientists spend their entire careers developing expertise and advancing knowledge within their specialized (often very narrow) area of research. A comprehensive understanding of the Earth’s climate requires input from atmospheric physicists and oceanographers with expertise in fluid dynamics, biologists with expertise in ecosystems and biogeochemical cycles that govern the fate of greenhouse gases in the atmosphere, paleoclimatologists who combine geological archives of the distant past with computer models to understand mechanisms of long-term change, astronomers who study orbital dynamics and monitor solar activity, statisticians who develop mathematical techniques to analyze observational data, and many more. The scale and complexity of our planet’s climate system is humbling. No single scientist can, honestly, purport to have expertise in the whole thing.
And yet, because of the scientific process, we—collectively, as the scientific community—understand the science of the climate system extremely well. Nowhere is this more visible than in the periodic scientific assessments by the Intergovernmental Panel on Climate Change (IPCC). Every 6 to 8 years, the IPCC convenes hundreds of scientists to synthesize the existing body of scientific literature on climate change and prepare a report that consists of three parts: one on the physical science of climate change, one on climate impacts to people and nature, and a final part on potential ways to avoid or mitigate the impacts of climate change.
This exercise is a massive interdisciplinary undertaking, and one that is only made possible by the generosity of scientists who volunteer their time and expertise. The latest IPCC assessment report, released between 2021 and 2023, was written by more than 800 authors from all over the world (Figure 2). It is an example—the most ambitious one—of a consensus report.
Figure 2. Number of authors of the IPCC’s Sixth Assessment Report by region. Data retrieved from IPCC.
What goes into consensus reports?
Consensus reports, like those of the IPCC, do not produce new scientific research. Rather, they synthesize the totality of expert knowledge, painstakingly detailed in many thousands of independent peer-reviewed studies, related to the causes of climate change, its impacts, and possible solutions.
To ensure that this synthesis is comprehensive, balanced, and faithfully represents the state of the science, the IPCC process involves a careful selection of its many authors, considering the full range of expert backgrounds required to take on such a synthesis. In practice, this means that each of the three parts of an IPCC report (physical science, impacts, and mitigation) is divided into multiple chapters, with each chapter authored by multiple experts with backgrounds specifically relevant to the scope of the chapter at hand. These authors are required to evaluate and summarize the full body of relevant scientific literature, including when that literature contains divergent views.
But the authors are not the final word. Consensus reports are subject to rigorous review processes that make the peer review of journal articles described above look like a breeze. For example, the first draft of an IPCC report undergoes review by independent “expert reviewers.” Anyone can register to be an expert reviewer and submit comments to challenge the content within the draft report. In response to these comments, authors produce a second draft, subject to an additional round of review by both expert reviewers and government delegates from the 195 member nations of the IPCC.
All comments, in addition to the authors’ responses to them, are publicly available, and must be addressed before the final publication of the report. This is no small undertaking—the most recent report contained over 200,000 comments. This process ensures that a wide range of scientific, technical, and socioeconomic perspectives are considered in the final product.
What is the result?
The latest IPCC report identifies, in no uncertain terms, the reality and human cause of the climate problem: “[i]t is unequivocal that human influence has warmed the atmosphere, ocean and land[,]” and that “[h]uman-induced climate change is already affecting many weather and climate extremes in every region across the globe.”
That finding has been echoed by numerous other consensus reports on climate science. Here’s an excerpt from the U.S. Fifth National Climate Assessment, which undergoes a similarly rigorous review process that involves comments from the expert scientific community, the public, and the National Academy of Sciences (NAS): “The more the planet warms, the greater the impacts. Without rapid and deep reductions in global greenhouse gas emissions from human activities, the risks of accelerating sea level rise, intensifying extreme weather, and other harmful climate impacts will continue to grow. Each additional increment of warming is expected to lead to more damage and greater economic losses compared to previous increments of warming, while the risk of catastrophic or unforeseen consequences also increases.”
What about non-consensus views?
Not every individual scientist agrees with the scientific consensus. In any field of science, one can find a handful of contrarians. Climate science is no exception. Recently, some of these voices have been elevated in the discourse around the Endangerment Finding. Last summer the U.S. Department of Energy’s (DOE) issued a report, “A Critical Review of Impacts of Greenhouse Gas Emissions on The U.S. Climate.” The title gives away the plot—in the DOE’s own words, the report was intended to be “critical” of the mainstream scientific understanding that human activity warms the climate, including of the U.S., and contributes to numerous well-documented harms.
As an initial matter, the DOE report was authored by just five individuals, which should raise an obvious question: how can five authors credibly purport to review such a massive and interdisciplinary body of scientific literature? Recall that the IPCC’s effort to comprehensively evaluate this body of literature required 803 authors (Fig. 2).
It seems these five authors were assigned a specific task: in the DOE’s own words, the report was “published ... as part of the U.S. Environmental Protection Agency’s (EPA) proposed rule repealing the 2009 Endangerment Finding.” It’s unsurprising that a report convened to end the Endangerment Finding would arrive at a conclusion divergent from that of the scientific consensus on climate change.
Following the announced plan to end the Finding, the NAS organized a 16-person expert committee to conduct fresh review of the state of climate science, focusing on developments since the Finding was made in 2009. That review received input from more than 200 individuals and organizations, incorporated over 600 peer-reviewed articles, and was reviewed by an additional fifteen experts. Here’s the topline: “[T]he evidence for current and future harm to human health and welfare created by human-caused GHGs [greenhouse gases] is beyond scientific dispute.”
That wasn’t the only response to the DOE report. Independently, over 85 climate scientists authored an additional 450-page rebuttal which they summarized:
“Our review reveals that the DOE report’s key assertions—including claims of no trends in extreme weather and the supposed broad benefits of carbon dioxide—are either misleading or fundamentally incorrect. The authors reached these flawed conclusions through selective filtering of evidence (‘cherry picking’), overemphasis of uncertainties, misquoting peer-reviewed research, and a general dismissal of the vast majority of decades of peer-reviewed research.
No one should doubt that human-caused climate change is real, is already producing potentially dangerous impacts, and that humanity is on track for a geologically enormous amount of warming.”
What is the takeaway?
The legal community is accustomed to burdens of proof. The highest burden of proof, applied in criminal cases, is “beyond a reasonable doubt.” Scientists speak in a different dialect of the same language—probability.
The IPCC uses calibrated “likelihood” statements to describe the probability of a certain phenomenon being true, in accordance with the weight of available scientific evidence. In IPCC parlance, “virtually certain,” corresponds to a likelihood of >99%.
Individual scientists may disagree, but the basic facts of climate change—that it is real, human-caused, and will have mostly negative impacts on people and nature—are virtually certain. In other words, these basic facts are established at a level that can be considered beyond a reasonable doubt. They are reality, and should not be lost amidst a policy controversy over greenhouse gas regulation.