Climate Change Caused by Humans
The Scientific Consensus

Historian of science, Naomi Oreskes of UC San Diego, states: "Scientific knowledge is the intellectual and social consensus of affiliated experts based on the weight of available empirical evidence, and evaluated according to accepted methodologies." If we feel that a policy question deserves to be informed by scientific knowledge, then we have no choice but to ask, what is the consensus of experts on this matter."

Climate change has been extensively researched and the overwhelming majority of climate scientists agree that the observed modern day global warming is unprecedented and is very likely caused by humans. The 2007 IPCC reports detail this widespread consensus. Although there is little serious debate between climate experts, many in the general public still think that these scientists are unsure about climate change and the role that humans have played in modern day global warming (Doran & Zimmerman, 2009). There are some reasons why the general public may be confused and they are not accidental.

So whom do you trust? The statement below from Prof. Stephen Schneider answers the question quite well and can be considered a litmus test for the veracity of climate change claims:

As a rule of thumb, those working for an organization which conducts primary research on climate science (e.g. CSIRO or Universities), and publishes this work in peer-reviewed scientific journals (the industry gold standard), should have their theories taken seriously. This is because they are following the scientific process – the same process that underpins the massive literature reviews of the Intergovernmental Panel on Climate Change (IPCC) assessment reports, and indeed the same process that has taken man to the moon, decoded the genome, and given you digital watches, laptop computers and automobiles. In any research field there will, of course, be diverse opinions about causes and effects – the positing, testing and overturning of theory and hypotheses are at the very core of science. Provided such arguments are bound by empirical or experimental evidence, and have survived rigorous pre-publication scrutiny and review, then they should be considered a valid viewpoint.

Prof. Schneider is the Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies, Professor of Biological Sciences, and Professor by Courtesy of Civil and Environmental Engineering at Stanford University. He is Co-Director of the Center for Environmental Science and Policy in the Freeman-Spogli Institute and a Senior Fellow in the Woods Institute for the Environment. He received his Ph.D in Mechanical Engineering and Plasma Physics from Columbia University, USA, in 1971.

Examining the Scientific Consensus on Climate Change:

48% of Americans think most climate scientists do not agree that the Earth has been warming in recent years, and 53% think climate scientists do not agree that human activities are a major cause of that warming (Doran and Zimmerman, 2009). A poll performed by Peter Doran and Maggie Kendall Zimmerman at Earth and Environmental Sciences, University of Illinois at Chicago of 3,146 Earth scientists showed 96.2% of climatologists who are active in climate research agree that mean global temperatures have risen compared to pre-1800s levels, and 97.4% agree that human activity is a significant factor in changing mean global temperatures. Among all respondents, 90% agreed that temperatures have risen compared to pre-1800 levels, and 80% agreed that humans significantly influence the global temperature. Petroleum geologists were among the biggest doubters, with only 47 percent believing in human involvement.

Doran and Zimmerman conclude:

Debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes. The challenge, rather, appears to be how to effectively communicate this fact to policy makers and to a public that continues to mistakenly perceive debate among scientists.

Anderegg et al. (2010) in their PNAS paper, Expert credibility in climate change, used an extensive dataset of 1,372 climate researchers and their publication and citation data to show that:

  1. 97-98% of the climate researchers most actively publishing in the field support the tenets of ACC outlined by the Intergovernmental Panel on Climate Change
  2. The relative climate expertise and scientific prominence of the researchers unconvinced of ACC are substantially below that of the convinced researchers.

Furthermore, the authors note:

"A vocal minority of researchers and other critics contest the conclusions of the mainstream scientific assessment, frequently citing large numbers of scientists whom they believe support their claims-This group, often termed climate change skeptics, contrarians, or deniers, has received large amounts of media attention and wields significant influence in the societal debate about climate change impacts and policy."

"Despite media tendencies to present 'both sides' in ACC debates [anthropogenic climate change], which can contribute to continued public misunderstanding regarding ACC, not all climate researchers are equal in scientific credibility and expertise in the climate system. This extensive analysis of the mainstream versus skeptical/contrarian researchers suggests a strong role for considering expert credibility in the relative weight of and attention to these groups of researchers in future discussions in media, policy, and public forums regarding anthropogenic climate change."

In another smaller survey via email, Brown, Pielke, and Anaan (2007) contacted 1807 climate scientists and received responses from 140 of those scientists. In the poll scientists were asked to discuss their opinion about the role of human-caused radiative forcing of CO2 in climate change and how climate science was represented in the IPCC's WG1 Report. The response is summarized below:

Intergovernmental Panel on Climate Change, IPCC:

The IPCC is a scientific intergovernmental body set up by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP). Its constituency is made of :

The governments: the IPCC is open to all member countries of WMO and UNEP. Governments participate in plenary sessions of the IPCC where main decisions about the IPCC work program are taken and reports are accepted, adopted and approved. They also participate in the review of IPCC reports.

The scientists: hundreds of scientists all over the world contribute to the work of the IPCC as authors, contributors and reviewers.

Its role is to assess on a comprehensive, objective, open and transparent basis the latest scientific, technical and socio-economic literature produced worldwide relevant to the understanding of the risk of human-induced climate change, its observed and projected impacts and options for adaptation and mitigation.

The IPCC 4th Assessment Report (2007) is the result of 2500+ scientific expert reviewers, 800+ contributing authors, and 450+ lead authors from 130+ countries.

Below are some excerpts from the IPCC WGI 4th Assessment Report (IPCC, 2007) which leaves little doubt that the present climate is experiencing an unprecedented global warming rate which is primarily due to human (anthropogenic) activities:

For more information please see: IPCC Website

Climate Change: Global Risks, Challenges and Decisions:

Since the IPCC 2007 reports, new research has been reviewed by the International Alliance of Research Universities (IARU). IARU organized an international scientific congress on climate change, Climate Change: Global Risks, Challenges and Decisions, which was held in Copenhagen from 10-12 March 2009. Participation in the Congress was open to all. Most of the approximately 2500 people attending the Congress were researchers, many of whom have also been contributors to the IPCC reports. Participants came from nearly 80 different countries and contributed with more than 1400 scientific presentations. The Synthesis Report (Richardson, et al., 2009) contains six key messages:

  1. Climatic Trends
    Recent observations show that greenhouse gas emissions and many aspects of the climate are changing near the upper boundary of the IPCC range of projections. Many key climate indicators are already moving beyond the patterns of natural variability within which contemporary society and economy have developed and thrived. These indicators include global mean surface temperature, sea-level rise, global ocean temperature, Arctic sea ice extent, ocean acidification, and extreme climatic events. With unabated emissions, many trends in climate will likely accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts.
  2. Social and Environmental Disruption
    The research community provides much information to support discussions on "dangerous climate change". Recent observations show that societies and ecosystems are highly vulnerable to even modest levels of climate change, with poor nations and communities, ecosystem services and biodiversity particularly at risk. Temperature rises above 2oC will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond.
  3. Long-term Strategy : Global Targets and Timetables
    Rapid, sustained, and effective mitigation based on coordinated global and regional action is required to avoid "dangerous climate change" regardless of how it is defined. Weaker targets for 2020 increase the risk of serious impacts, including the crossing of tipping points, and make the task of meeting 2050 targets more difficult and costly. Setting a credible long-term price for carbon and the adoption of policies that promote energy efficiency and low-carbon technologies are central to effective mitigation.
  4. Equity Dimensions
    Climate change is having, and will have, strongly differential effects on people within and between countries and regions, on this generation and future generations, and on human societies and the natural world. An effective, well-funded adaptation safety net is required for those people least capable of coping with climate change impacts, and equitable mitigation strategies are needed to protect the poor and most vulnerable. Tackling climate change should be seen as integral to the broader goals of enhancing socioeconomic development and equity throughout the world.
  5. Inaction is Inexcusable
    Society already has many tools and approaches "economic, technological, behavioral, and managerial" to deal effectively with the climate change challenge. If these tools are not vigorously and widely implemented, adaptation to the unavoidable climate change and the societal transformation required to decarbonize economies will not be achieved. A wide range of benefits will flow from a concerted effort to achieve effective and rapid adaptation and mitigation. These include job growth in the sustainable energy sector; reductions in the health, social, economic and environmental costs of climate change; and the repair of ecosystems and revitalization of ecosystem services.
  6. Meeting the Challenge
    If the societal transformation required to meet the climate change challenge is to be achieved, then a number of significant constraints must be overcome and critical opportunities seized. These include reducing inertia in social and economic systems; building on a growing public desire for governments to act on climate change; reducing activities that increase greenhouse gas emissions and reduce resilience (e.g. subsidies); and enabling the shifts from ineffective governance and weak institutions to innovative leadership in government, the private sector and civil society. Linking climate change with broader sustainable consumption and production concerns, human rights issues and democratic values is crucial for shifting societies towards more sustainable development pathways.

No Scientific Body of National or International Standing Holds a Dissenting Opinion:

"A strong, credible body of scientific evidence shows that climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems. Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of subsequently being found to be wrong is vanishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities." (U.S. National Academy of Sciences, 2010)

"Decades of scientific research have shown that climate can change from both natural and anthropogenic causes. The Geological Society of America (GSA) concurs with assessments by the National Academies of Science (2005), the National Research Council (2006), and the Intergovernmental Panel on Climate Change (IPCC, 2007) that global climate has warmed and that human activities (mainly greenhouse gas emissions) account for most of the warming since the middle 1900s. If current trends continue, the projected increase in global temperature by the end of the twentyfirst century will result in large impacts on humans and other species. Addressing the challenges posed by climate change will require a combination of adaptation to the changes that are likely to occur and global reductions of CO2 emissions from anthropogenic sources." (The Geological Society of America [GSA], 2010)

"The Earth's climate is now clearly out of balance and is warming. Many components of the climate system-including the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasons-are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century." (American Geophysical Union, 2007)

"Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research demonstrates that the greenhouse gases emitted by human activities are the primary driver." (Union of Concerned Scientists, 2009)

"Observations show that warming of the climate is unequivocal. The global warming observed over the past 50 years is due primarily to human-induced emissions of heat-trapping gases. These emissions come mainly from the burning of fossil fuels (coal, oil, and gas), with important contributions from the clearing of forests, agricultural practices, and other activities." (U.S. Global Change Research Program, 2009)

"Human activity is most likely responsible for climate warming. Most of the climatic warming over the last 50 years is likely to have been caused by increased concentrations of greenhouse gases in the atmosphere." (European Academy of Sciences and Arts, 2007)

"Scientific evidence is clear: global climate change caused by human activities is occurring now, and it is a growing threat to society....The pace of change and the evidence of harm have increased markedly over the last five years. The time to control greenhouse gas emissions is now." (American Association for the Advancement of Science, 2006)

More official statements can be viewed at: Wiki: Scientific Opinion on Climate Change.

These two quotes also show the consensus:

"I think we understand the mechanisms of CO2 and climate better than we do of what causes lung cancer...In fact, it is fair to say that global warming may be the most carefully and fully studied scientific topic in human history."

"Consensus as strong as the one that has developed around global warming is rare in science."
--- Donald Kennedy, editor of Science

Most-Cited Authors on Climate Science:

Jim Prall, from the University of Toronto, has compiled a list of homepages and some citation stats for all the authors from Working Group 1 of the 2007 Fourth Assessment Report (AR4) from the Intergovernmental Panel on Climate Change (IPCC), as well as a longer list including other active climate science researchers. Please see his Most-Cited Authors on Climate Science table for an excellent representation of those that agree there is AGW and those that are skeptical.

What is Scientific Consensus Climate Consensus: A Cautionary Tale, Part 1/2
Prof. Barry Bickmore examines the following questions. 1) What does it mean to have a scientific consensus? 2) Is there a strong scientific consensus about whether humans are significantly affecting the climate? 3) How do people convince themselves to go against the consensus?

What About Scientists and Papers That Challenge the Consensus?

Some claim that there are 31,000 U.S. scientists and over 850 peer-reviewed papers that challenge the consensus that humans are causing climate change. Skeptical Science gave these claims the greatest benefit of the doubt and came up with this interesting factoid:

  1. 31,000 scientists represents 0.1% of US scientists that hold a BS degree or higher
  2. 850 papers represents 0.1% of the peer-reviewed papers on the subject of climate change

It is quite foolish to hang one's hat on those statistics!

Next: Determining the Climate Record

Scott A. Mandia
Professor - Physical Sciences
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Last Updated: 3/5/18