Reckoning The Scientific Consensus on Global Warming: A Beginner's Primer

I was initially motivated by the intellectual torpor of close acquaintances to produce this little primer. I now offer it to the larger community as a public service.

My basic thesis is that it should take about ten minutes for any Limbaugh-listening but intellectually honest person (a hopeless oxymoron?) to be convinced that scientists who are experts on the subject overwhelmingly accept that the earth is warming, and that warming is largely due to human activities. (Such warming is called anthropogenic climate change.) The ease with which you can convince yourself is important, because the climate-change-denying right, and its principal media outlets, have been extraordinarily effective at sowing doubt and confusion in the public's mind about what scientists believe. Even the mainstream media hasn't done a very good job of communicating the scientific consensus. That leaves the average consumer of news believing there's an uncertainty among scientists that does not in fact exist.

My objective here is not to explain the actual science of climate change, or to get you to believe that the science as it is now understood accurately describes reality. Those are worthy further goals, and well within the reach of much of the populace, but they are not what this particular piece is about.  At the moment, all I want is for you to understand that there is absolutely no doubt about what scientists believe: about what the greater scientific enterprise has collectively concluded, and by a very large margin. I want you to understand that the consensus is quite real even if you have seen an authoritative-sounding scientist on television who said otherwise, or if your favorite media outlet has been working to get you to think it's all just a big liberal hoax.

Where to start? I like to start with NASA—the world's premier space exploration enterprise—because it's an organization with high exposure and esteem in the public consciousness. The scientists and engineers at NASA put a man on the moon way back in the 1960s, and have sent probes to every planet in the solar system, plus many moons, comets, and asteroids. More recently, NASA has landed a number of highly successful rovers on Mars. A conservative relative of mine who's a global warming denier was enthralled by this spectacular NASA video depicting a previous mission to Mars, and rightfully so. NASA's accomplishments have been amazing.

The scope and expanse of scientific study under NASA's purview is breathtaking. You may not be surprised to learn that NASA studies the climate of Mars, but did you know that NASA is heavily involved in studying the earth's climate, and has been for decades? Did my NASA-loving climate-change-denying relative know that? It's true. Until very recently one of the world's leading climate scientists, James Hansen, was the head of NASA's Goddard Institute for Space Studies, a position he held for for more than three decades.

So let us begin by taking a quick look at what NASA has to say about the scientific consensus on climate change. This page on NASA's web site gives us the overview we're looking for. It begins by telling us that there is, indeed, overwhelming scientific consensus; that "97% of scientists agree" (a figure we'll come back to later) about anthropogenic climate change. It gives us a quick rundown of the consensus statements of the major American scientific societies, plus American and international scientific academies, bodies, and agencies. This is the ten-minute look that I referred to. It would seem that NASA has no doubt about the matter. If NASA is sure about the scientific consensus, why aren't you?

NASA's summary highlights the statements of major scientific bodies and organizations. That's an appropriate thing to do when trying to gauge consensus, because scientific organizations speak collectively for thousands of members, and the consensus statements are arrived at through active involvement of those members, often after much consideration and debate.

Perhaps the most prestegious scientific organization of all is the U.S. National Academy of Sciences (NAS), which was founded in 1863 by President Abraham Lincoln, and is commonly asked to assess the most weighty scientific questions for the U.S. government. This 2008 NAS report (pdf) says that "most scientists agree that the warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere." And "there is no doubt that climate will continue to change throughout the 21st century and beyond...." And that "the scientific understanding of climate change is now sufficiently clear to begin taking steps to prepare for climate change and to slow it."

The National Academy of Sciences was a signatory to a joint statement (pdf) with international academies saying that "climate  change is real" and that "it is likely that most of the warming in recent decades can be attributed to human activities." This was back in 2005, and both the evidence and consensus have grown much stronger since then. The statement concluded that all nations should "take prompt action to reduce the causes of climate change, adapt to its impacts and ensure that the issue is included in all relevant national and international strategies."

In this NPR story NAS president Ralph Cicerone says that "the consensus statement is that the climate changes that are being observed are certainly real, they seem to be increasing and that humans are mostly likely the cause of all or most of these changes." He speaks in the careful, conservative language of the scientist, which eschews exaggeration and hyperbole. Cicerone goes on to mention the consensus statements of other major U.S. scientific organizations: "The American Chemical Society, the American Physical Society has a, I think, a fairly clear statement on climate change. The American Geophysical Union and the American Meteorological Society, and then any other number of other societies—Geological Society of America, quite a few others."

In the same NPR story, Yale University's Anthony Leiserowitz, who has conducted polling on the topic, said that most Americans don't know that "about 97 percent of American scientists say that climate change is happening." There's that "97 percent" figure again. It turns out that when serious attempts are made to gauge scientific consensus on anthropogenic climate change, the general agreement is quite high—consistently in the 97-98 percent range. The big disconnect is between what scientists understand and what the public at large believes about that scientific understanding.

The same overwhelming consensus that is expressed by all the major U.S. scientific societies is also shared by international societies and academies. Most people will not want to read in its entirety this exhaustive Wikipedia (an online encyclopedia) article entitled Scientific opinion on climate change, but it is worth a quick scan. Dozens of the world's national scientific academies share the opinion of the U.S. National Academy of Sciences on climate change. So do all the world's major scientific societies. Most have issued official consensus statements. Indeed, there is not a single scientific body of national or international standing with a dissenting statement. The last dissenting statement by a major scientific society was withdrawn in June 2007 when the American Association of Petroleum Geologists changed its position to non-committal, saying, in effect, that its members were not subject matter experts and would no longer advance an opinion.

You should scroll through the Wikipedia article beginning with the heading "Concurring". Click here to go there directly, and then scroll down through the article. The number of major concurring societies is breathtaking. Are they all wrong, while Rush Limbaugh is right? To reiterate: while scores of scientific academies and societies concur about anthropogenic climate change, not a single society of national or international standing dissents.

I like to chide Catholics to whom I'm related that even the Vatican's Pontifical Academy of Sciences endorses the scientific consensus. In this report (pdf), the academy's working group says: "We call on all people and nations to recognise the serious and potentially irreversible impacts of global warming caused by the anthropogenic emissions of greenhouse gases and other pollutants, and by changes in forests, wetlands, grasslands, and other land uses. We appeal to all nations to develop and implement, without delay, effective and fair policies to reduce the causes and impacts of climate change on communities and ecosystems, including mountain glaciers and their watersheds, aware that we all live in the same home." [my emphasis -mb]

There are other ways to gauge scientific consensus than through the statements of major scientific academies and societies. One is to directly poll individual scientists. This can be difficult for a variety of reasons, but attempts to survey scientists have consistently shown strong support for the reality of anthropogenic climate change. This Wikipedia article entitled Surveys of scientists' views on climate change  describes the most prominent surveys, and begins with the statement that the surveys "have generally concluded that the majority of scientists are convinced that human activity is causing global warming."

The degree of climate specialization strongly affects the level of certainty among individual scientists. According to one report described here, "it seems that the 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."  According to a report (pdf) published in the Proceedings of the National Academy of Sciences of the United States, 97-98% of actively publishing climate scientists support the reality of anthropogenic climate change. There's that number again.

Another way to gauge scientific consensus is through surveys of the published scientific literature. The technique is to review abstracts of scientific papers to see if an opinion is expressed regarding the reality and cause of global warming. Among papers where an opinion is expressed, around 97% concur with the reality of human caused global warming. For example, here's a recently published analysis (pdf) in the journal Environmental Research Letters of 12,000 scientific papers. You probably don't want to read the actual scientific paper, but here's a short summary by NPR.

By far, the world's largest consensus document on climate change is produced by the Intergovernmental Panel On Climate Change (IPCC) under the auspices of the United Nations. The following information comes from the book The Hockey Stick And The Climate Wars by climate scientist and past IPCC lead author Michael Mann.

Every five to seven years the IPCC produces an extraordinarily exhustive assessment of the state of science's understanding of climate change. Four assessments have been produced since 1990. Each assessment is a monumental undertaking that results in a three-volume report. Each volume contains multiple chapters of 50 to 100 pages. Each chapter is written by a dozen scientific experts in a particular subdiscipline, along with 50 or more contributing authors. According to Mann (pp. 90-91),

The IPCC review process remains the most rigorous, comprehensive, and transparent of any major scientific assessment. IPCC reports are subject to three distinct rounds of peer review, each of which takes place over roughly two months. First, there is an initial round of expert review, wherein several thousand scientists from all disciplines and with a wide range of backgrounds and perspectives, drawn from academic, government, and nongovernmental organizations and industry, are called upon to provide detailed comments on the content of the report. Lead authors are required to consider and respond to all comments and make appropriate revisions, all of which are documented and available online. The revision process is overseen by two independent review editors with expertise in the specific subject areas of the chapter, to ensure that any legitimate issues reviewers raise are dealt with in a satisfactory manner.

The revised draft report is then subject to the next round of review, the so-called government review, which includes rereview by the original expert reviewers and additional review by government representatives from all participating United Nations member nations. Each government may choose how it implements its review. The U.S. government, for example, solicits public comments through a notice in the Federal Register, in essence allowing anyone at all to serve as a reviewer of the IPCC report. The revision process is again repeated. Finally, national governments are again invited to comment on the report.

The final wording of the all-important summary for policy makers for each of the three working groups is agreed upon word-by-word in a final plenary meeting in which government delegations are present and can propose variant terms or make objections to specific wording, but the scientists are in charge. The final version reflects a consensus on the precise wording of the report between the scientists involved in the writing of the report and the representatives of the government delegations. It is difficult to imagine a more open, inclusive, and responsible assessment process than that which the IPCC follows.

The IPCC reports remain the gold standard for evaluating the state of scientific understanding of climate change. They are intended to inform but, importantly, not prescribe policies for avoiding so-called dangerous anthropogenic interference with the climate system. The actual policy prescriptions, as they should be, are left for policy makers and their constituents to decide.

These reports are consensus documents in every respect. They emphasize conclusions that are based on results that have been replicated by independent studies, using differing approaches and assumptions. They highlight where there is true uncertainty. They put greatest weight on findings that are widely accepted by the scientific community, and downplay the more tentative findings of individual articles departing from conventional wisdom that, as we have seen, frequently—but not always—do not hold up when subject to further scrutiny by subsequent independent efforts.

The IPCC's fourth assessment report was completed in 2007. According to the Wikipedia article entitled Intergovernmental Panel on Climate Change, the assessment concluded most notably that warming of the climate system is unequivocal; that there is high confidence that recent warming is due to anthropogenic emissions of greenhouse gasses; that anthropogenic warming and sea level rise will continue for centuries; that the world's temperatures could rise between 2.0 and 11.5 °F over the current century; that the world will experience more frequent heat waves, droughts, tropical cyclones, and extreme high tides; and so forth. See the conclusions here.

The opinion on climate change held by the scientific enterprise writ large is simply not in dispute. As should now be clear, it is the consensus distillation of the work of many thousands of expert scientists, and thousands of papers published in the world's top peer-reviewed scientific journals. It has the concurrence of all the world's top scientific academies and societies. The notion that the global warming "scare" is the work of a small cabal of rogue scientists, who have somehow managed to hijack the scientific process, is absurd. By every possible measure, the scientific consensus is unambiguous and overwhelming. By every possible measure, the dissent is vanishingly small (and often comes from sources burdened by conflict of interest). Regardless of whether or not you think the scientists have got it right (and on what basis would you think otherwise?), there can be no doubt about what they believe.

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More Information and Further Reading

[1] The U.S. Environmental Protection Agency (EPA) has this overview page which says that the "Earth's climate is changing" and that "most of the warming of the past half century has been caused by human emissions of greenhouse gasses." See also the references at the bottom of the EPA's page.

[2] Here's more from NASA, which describes the evidence for climate change.

[3] The Union of Concerned Scientists has a page on the scientific consensus for climate change.

[4] If you want to dig deeply into the IPCC's fourth assessment, start here.

[5] See the book Advancing the Science of Climate Change by the U.S. National Research Council, and published by the National Academies Press. The book can be purchased in paperback, or downloaded for free.

[6] The U.S. National Oceanic and Atmospheric Administration (NOAA) issued a report on the climate-driven weather of 2011. NOAA, which includes the National Weather Service, is more commonly known as the government agency that studies and tracks severe weather events such as hurricanes. See NOAA's State of the Climate - 2011 supplement to the July 2012 Bulletin of the American Meteorological Society. In 2011 NOAA Administrator Jane Lubchenco issued a statement welcoming the National Research Council's report entitled America's Climate Choices, saying that it "not only re-affirms the broad international scientific consensus about the causes and consequences of climate change, but makes clear that comprehensive, sustained efforts must begin today to deal with those consequences."

[7] Two interesting articles here and here in The Guardian newspaper on the 97% consensus figure.

Copyright (C) 2013 James Michael Brennan, All Rights Reserved

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