Isn't there a lot of disagreement among climate scientists about global warming?

February 3, 2020

No. By a large majority, climate scientists agree that average global temperature today is warmer than in pre-industrial times and that human activity is the most significant factor. 

Black and white comic of three buses loading riders to a scientific conference. The bus labelled "human-caused warming" is full, with a line stretching off the edge of the image.

Today, there is no real disagreement among climate experts that humans are the primary cause of recent global warming. NOAA Climate.gov cartoon by Emily Greenhalgh. 

Consensus of experts

The United States' foremost scientific agencies and organizations have recognized global warming as a human-caused problem that should be addressed. The U.S. Global Change Research Program has published a series of scientific reports documenting the causes and impacts of global climate change. NOAA, NASA, the National Science Foundation, the National Research Council, and the Environmental Protection Agency have all published reports and fact sheets stating that Earth is warming mainly due to the increase in human-produced heat-trapping gases.

On their climate home page, the National Academies of Sciences, Engineering, and Medicines says, "Scientists have known for some time, from multiple lines of evidence, that humans are changing Earth’s climate, primarily through greenhouse gas emissions," and that "Climate change is increasingly affecting people’s lives." 

Scientist in a climbing harness descends into a snow pit that shows different layers of soot mixed with snow

Soot from fires and air pollution contributes to global warming, and its impacts may be especially strong in the Arctic, where it darkens the snow and ice—as shown in this photo—and accelerates melting. Despite some uncertainty about just how much influence soot and other aerosol particles have played in climate change in the past century, there's little debate among climate scientists that the primary driver of recent global warming is carbon dioxide emissions. Photo from NOAA Ocean Today.

The American Meteorological Society (AMS) issued this position statement: "Scientific evidence indicates that the leading cause of climate change in the most recent half century is the anthropogenic increase in the concentration of atmospheric greenhouse gases, including carbon dioxide (CO2), chlorofluorocarbons, methane, tropospheric ozone, and nitrous oxide." (Adopted April 15, 2019)

The American Geophysical Union (AGU) issued this position statement: "Human-induced climate change requires urgent action. Humanity is the major influence on the global climate observed over the last 50 years. Rapid societal responses can significantly lessen negative outcomes." (Reaffirmed in November 2019)

The American Association for the Advancement of Science (AAAS) What We Know site states: "Based on the evidence, about 97 percent of climate scientists agree that human-caused climate change is happening."

Consensus of evidence

These scientific organizations have not issued statements in a void; they echo the findings of individual papers published in refereed scientific journals. The Institute for Scientific Information (ISI) maintains a database of over 8,500 peer-reviewed science journals, and multiple studies of this database show evidence of overwhelming agreement among climate scientists. In 2004, science historian Naomi Oreskes published the results of her examination of the ISI database in the journal Science. She reviewed 928 abstracts published between 1993 and 2003 related to human activities warming the Earth's surface, and stated, "Remarkably, none of the papers disagreed with the consensus position."

This finding hasn't changed with time. In 2016, a review paper summarized the results of several independent studies on peer-reviewed research related to climate. The authors found results consistent with a 97-percent consensus that human activity is causing climate change.

Probably the most definitive assessments of global climate science come from the Intergovernmental Panel on Climate Change (IPCC). Founded by the United Nations in 1988, the IPCC releases periodic reports, and each major release includes three volumes: one on the science, one on impacts, and one on mitigation. Each volume is authored by a separate team of experts, who reviews, evaluates, and summarizes relevant research published since the prior report. Each IPCC report undergoes several iterations of expert and government review. The 2007 IPCC report, for instance, received some 90,000 comments, and each comment received an individual response.

Trio of report covers for the three IPCC Fifth Assessment reports

Every five years, the Intergovernmental Panel on Climate Change convenes hundreds of international scientists and government representatives to review and assess peer-reviewed research on climate science. In each cycle, the panel publishes three key reports: one on the basic science, one on impacts, and one on mitigation. Work on the IPCC Sixth Assessment Report, slated for release in 2021, is already underway. 

The IPCC does not involve just a few scientists, or even just dozens of scientists. An IPCC factsheet explains: "Hundreds of leading experts in the different areas covered by IPCC reports volunteer their time and expertise as Coordinating Lead Authors and Lead Authors to produce these assessments. Many hundreds more are involved in drafting specific contributions as Contributing Authors and commenting on chapters as Expert Reviewers."

Governments and climate experts across the globe nominate scientists for IPCC authorship, and the IPCC works to find a mix of authors, from developed and developing countries, among men and women, and among authors who are experienced with the IPCC and new to the process. Published in 2014, the Fifth Assessment Report (AR5) involved 831 experts selected from 3,598 nominations. In other words, the IPCC reports themselves are a comprehensive, consensus statement on the state climate science.

The report states:

The evidence for human influence on the climate system has grown since AR4 . Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, and in global mean sea level rise; and it is extremely likely to have been the dominant cause of the observed warming since the mid- 20th century. In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Impacts are due to observed climate change, irrespective of its cause, indicating the sensitivity of natural and human systems to changing climate.

References

Cook, J., D. Nuccitelli, S.A. Green, M. Richardson, B. Winkler, R. Painting, R. Way, P. Jacobs, and A. Skuce (2013). Quantifying the consensus on anthropogenic global warming in the scientific literature. Environmental Research Letters, 8, 024024. https://doi.org/10.1088/1748-9326/8/2/024024.

Cook, J., Oreskes, N., Doran, P.T., Anderegg, W.R.L., Verheggen, B., Mailbach, E.W., Carlton, J.S., Lewandowsky, S., Skuce, A.G., Green, S.A., Nuccitelli, D., Jacobs, P., Richardson, M., Winkler, B., Painting, R., Rice, K. (2016). Consensus on consensus: A synthesis of consensus estimates on human-caused global warming. Environmental Research Letters, 11, 048002. https://doi.org/10.1088/1748-9326/11/4/048002.

Doran, P., and M.K. Zimmerman (2009): Examining the Scientific Consensus on Climate Change. Eos, 90(3), 22–23.

IPCC. (2013). Factsheet: How does the IPCC select its authors? Accessed January 3, 2020.

IPCC. (2014). Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva Switzerland. Accessed January 22, 2020.

Oreskes, N. (2004). The Scientific Consensus on Climate Change. Science, 306, 1686. https://doi.org/10.1126/science.1103618.

Oreskes, N. (2018). The scientific consensus on climate change: How do we know we're not wrong? Climate Modelling, pp. 31–64. https://doi.org/10.1007/978-3-319-65058-6_2.

Sherwood, S. (2011, May 10). Trust us, we're climate scientists: The case for the IPCC. The Conversation.