Scientific consensus on climate change

• 1. Introduction

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  Scientific consensus on climate change is not a single document or vote. It is the accumulated weight of tens of thousands of peer-reviewed studies, stretching from surveys in the 1970s through a 2021 analysis of 80,000 climate-related papers. And across all of that research, the conclusion keeps arriving at the same place: human activities have been warming the planet, and the rate of that warming is largely unprecedented in modern history.

  The headline figure is striking. A 2019 review found the consensus on the human cause of climate change had reached 100 percent. A separate 2021 study found it exceeded 99 percent. That near-unanimity did not happen overnight. It was built through decades of surveys, replication studies, and synthesis reports assembled by bodies ranging from the Intergovernmental Panel on Climate Change to the national science academies of the world's major industrialized nations.

  What drove that convergence? How was it measured? And what happened to the small fraction of papers that pushed back? Those are the questions this documentary will answer.

• 2. What the Consensus Actually Says

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  Carbon dioxide in the atmosphere is now at its highest level in at least 2 million years, and possibly 3.2 million years. Methane and nitrous oxide have reached their highest concentrations in at least 800,000 years. The record from those 800,000 years also shows that increases of this speed and scale would take millennia if driven by natural processes alone.

  The decade of the 2010s was 1.1 degrees Celsius warmer than the late 19th century. The warming of the past 50 years has occurred faster than any other warming period in at least the past 2,000 years. Global sea level has risen since 1900, with half of that increase occurring after 1980. Scientists describe that rate of rise as the fastest in at least 3,000 years.

  The mechanisms are well understood. As warming heats the ocean, seawater expands, accounting for roughly half of recent sea level rise. The other half comes from warming that melts ice sheets and glaciers in Greenland and Antarctica. IPCC reports have had to raise their sea level estimates repeatedly in response to newer research on those ice sheets.

  Severe and extreme weather events have always existed. Tropical cyclones, droughts, and heat waves predate human industry. But the scientific consensus holds that climate change has made many of them more severe, more frequent, or more likely to co-occur across every region of the globe. The California Governor's Office has listed nearly 200 worldwide scientific organizations that hold the position that human activity is the cause.

• 3. Surveying the Scientists

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  In 1978, the National Defense University of the United States surveyed 24 experts about near-term climate change and its effects on agriculture. Most expected some warming between 1970 and 2000, and identified human carbon dioxide emissions as the primary driver. When the survey team combined their predictions, they estimated a 25 percent likelihood of moderate warming of around 0.4 degrees Celsius. That scenario proved closest to what actually happened: roughly 0.4 degrees of warming occurred between 1970 and 2000.

  By 1989, David H. Slade had surveyed 21 climate scientists, and 17 expressed strong belief in the reality of significant climate change. A year later, in March 1990, the Cutter Information Corporation sent questionnaires to 1,500 researchers who had attended climate change conferences and received 331 responses from 41 countries. Only 1.9 percent of those respondents predicted overall cooling in the next 100 years.

  The surveys grew larger through the 1990s. In 1991, the Center for Science, Technology, and Media sent questions to around 4,000 ocean and atmospheric scientists from 45 countries. Of the 118 who responded, 73 agreed or strongly agreed that mean global temperatures would increase. When the same researchers were asked about the 1990 IPCC warming estimate of 0.3 degrees per decade, 42 percent agreed with it. As of 2023, the actual observed rate was 0.2 degrees per decade or less.

  In 1996, Dennis Bray and Hans von Storch at the Helmholtz Research Centre surveyed 1,000 climate scientists in Germany, the United States, and Canada. On a scale where higher numbers meant more disagreement, the statement "global warming will occur in the future" scored 2.6, indicating strong agreement. What the scientists doubted was not the warming itself but the accuracy of models making 100-year predictions, which scored 5.2 on the disagreement scale. Even so, they rejected the idea that uncertainty was sufficient reason to delay action, giving that notion a mean rating of 5.6 out of 7.

• 4. Counting the Papers

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  In 2004, the geologist and historian of science Naomi Oreskes examined the abstracts of 928 scientific papers on global climate change published between 1993 and 2003. Of those, 75 percent explicitly endorsed the scientific consensus or accepted it as a given while evaluating impacts or proposing mitigation approaches. The remaining 25 percent focused on research methods or paleoclimate analysis. Not one abstract explicitly rejected the consensus.

  In 2013, John Cook examined 11,944 abstracts from peer-reviewed literature published between 1991 and 2011. Among those that expressed a position on human-caused warming, 97.1 percent endorsed the consensus. Cook and his co-authors also invited the original paper authors to rate their own work: 97.2 percent of those expressing a position endorsed the consensus. Economist Richard Tol criticized the methodology for lacking controls and reliability measurements. Cook and other researchers responded that the results were consistent with studies published before and since.

  Also in 2013, James L. Powell, a former member of the National Science Board, analyzed 13,950 peer-reviewed articles published between 1991 and 2012. Only 24 of them, less than 0.2 percent, rejected anthropogenic global warming. A follow-up covering November 2012 through December 2013 found that of 9,136 authors, only one rejected the human cause.

  By 2017, Powell had combined five separate literature surveys covering 1991 through 2015, totaling 54,195 articles. The average consensus across that combined body came to 99.94 percent. Then in November 2019, his survey of over 11,600 articles published in just the first seven months of 2019 showed the consensus had reached 100 percent.

  In 2021, a team led by Mark Lynas found 80,000 climate-related studies published between 2012 and 2020 and analysed a random subset of 3,000. Of those, only four were skeptical of the human cause. The team estimated 99.85 percent of papers were not skeptical of human causation.

• 5. Expertise and Dissent

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  In 2009, Peter Doran and Maggie Kendall Zimmerman at the University of Illinois at Chicago polled 10,257 earth scientists from various specialties and received 3,146 replies. Among the 79 respondents who were climatologists with over half their peer-reviewed work on climate change specifically, 76 agreed that mean global temperatures had risen compared to pre-1800s levels, and 75 described human activity as a significant factor. Among all respondents the agreement was lower: 90 percent on rising temperatures, 82 percent on significant human influence. Economic geologists and meteorologists were the biggest doubters, with only 47 percent and 64 percent, respectively, believing in significant human involvement.

  Doran and Zimmerman drew a clear conclusion from that pattern: the debate on the authenticity of global warming and the role of human activity was, in their words, "largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes."

  A 2010 paper in the Proceedings of the National Academy of Sciences reviewed publication and citation data for 1,372 climate researchers, of whom 908 had authored 20 or more publications on climate. It found that 97 to 98 percent of the most actively publishing researchers supported the tenets of anthropogenic climate change. It also found that the climate expertise and scientific prominence of researchers unconvinced by those tenets was substantially below that of convinced researchers.

  The 2021 study by Krista Myers surveyed 2,780 earth scientists. Agreement that human activity is causing climate change ran from 91 percent among all scientists to 100 percent among climate scientists with more than 20 peer-reviewed papers on the subject. The group showing the lowest agreement was scientists who identified Economic Geology as one of their fields, at 84 percent.

  A 2016 paper titled Learning from mistakes in climate research followed up on the small fraction of published work that had rejected the consensus. The researchers found a pattern of common methodological flaws across those papers, flaws that were not visible when looking at individual cases in isolation.

• 6. Institutions and Joint Statements

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  The Intergovernmental Panel on Climate Change was formed by the United Nations in 1988 to synthesize the state of scientific knowledge on climate change for UN member states. Its major reports have appeared at intervals of 5 to 7 years beginning in 1990, and each successive report has had to update estimates, particularly on sea level rise, as new research on the Greenland and Antarctic ice sheets came in.

  In 2001, science academies from 17 countries, including Australia, Brazil, China, India, and the United Kingdom, issued a joint statement endorsing the IPCC's work. That statement is notable partly for being among the first to explicitly use the word "consensus" and for concluding that "Business as usual is no longer a viable option."

  In 2005, science academies from 11 major nations, including the United States, Japan, and Russia, described the IPCC conclusions as "the international scientific consensus" and urged prompt action on both mitigation and adaptation. By 2007, the Network of African Science Academies had joined that position, followed by the International Union for Quaternary Research in 2008.

  The United States' own National Climate Assessment has been released periodically since 2000 under the auspices of the United States Global Change Research Program. The fourth NCA, released in 2017, involved thirteen federal agencies led by the National Oceanic and Atmospheric Administration, along with around 1,000 contributors, including 300 leading scientists, roughly half from outside government.

  A small number of organizations with members in extractive industries have held non-committal positions. Some have attempted to persuade the public that climate change is not happening, or that human activity is not its cause. No scientific body of national or international standing disagrees with the consensus view. The studies showing errors among dissenting papers, the 97 to 100 percent figures across dozens of separate literature surveys, and the growing expertise gradient between convinced and unconvinced researchers all point toward the same endpoint that Dennis Bray and Hans von Storch documented across their five surveys from 1996 through 2016: by 2016-75.7 percent of active climate scientists said that the level of risk associated with climate change had increased considerably since 1996, while only 5 percent said it had decreased.