Is there a scientific consensus that Global Warming is happening and humans are causing it?

Here we consider the extent of scientific consensus on the evidence for and against this hypothesis:

There has been a measurable increase in the temperature of the earth since about the Industrial Revolution
Humans are causing it, primarily by burning fossil fuels.

This hypothesis is known in the scientific literature as anthropogenic global warming (AGW).

Some quotes that define the popular controversy about this hypothesis:

“Ninety-seven percent of scientists agree: #climate change is real, man-made, and dangerous.”
-President Barack Obama (@BarackObama), May 16, 2013. Tweet.

“I’m still open-minded. Nobody really knows. Look, I’m somebody that gets it, and nobody really knows. It’s not something that’s so hard and fast.”
-President-elect Donald Trump, quote from interview with Chris Wallace, Fox News Sunday, December 11, 2016.

So, which of the above quotes is right? It it true that experts studying the earth’s climate generally disagree with one another on our hypothesis and “nobody really knows?” Or is it true that the vast majority of experts, as many as 97%, agree Global Climate Change is real and caused by us? Is it possible both statements are true? That is, might it be true that the vast majority of experts think climate change is real and caused by us, but there is yet credible evidence that a minority of experts who don’t believe this are actually correct?

Evidence of expert consensus on Anthropogenic Global Warming (AGW)

Fortunately, due to the nature of the scientific method and the scientific practice of peer-reviewed publication of research results, this question can be (and has been) considered in a fact-based way.

Let’s start with Obama’s quote, which has become one of the most often stated claims by proponents of our hypothesis that global warming is happening and caused by us. Is there evidence backing up the claim of 97% agreement with the hypothesis?

The claim is primarily derived from a 2013 research paper co-authored by John Cook and an international and interdisciplinary group of scientists. You can read the paper, which is freely available on the internet. Briefly, the group of researchers used standard search tools to find peer-reviewed articles involving “global warming” or “global climate change” published in scientific journals during the 21-year period between 1991 and 2012. This search identified 11,944 papers written by 29,083 authors and published in 1980 different journals. They then recruited an anonymous group of scientists to read the abstracts and rate them according to whether they “endorsed AGW,” “expressed no position on AGW,” or “rejected AGW.” Here is a graph of the results over the 1991-2012 time period:

Figure A1.  Graph from Figure 1 of Cook, et al., Environmental Research Letters 8 (2013) 024024.

Of the papers during this time period deemed to express a position on AGW (the green and red data in the graph above), 97.1% endorsed AGW (that is, the papers expressed a position that the hypothesis at the top of this page is true). Of the scientists contributing to those papers, 98.4% endorsed AGW.

A 2015 paper by James Powell similarly examined the conclusions of 24,210 peer-reviewed scientific articles authored by 69,406 scientists during the more recent period of 2013-2014. This paper is particularly well-written and easy to read (it’s only 4 pages long and not very technical); I recommend reading it. Powell was critical of the method used by Cook and coworkers in the earlier 2013 paper, in that Cook et al. only classified an article as “endorsing AGW” if the abstract of the article (that is, the very short summary conventionally included right below the title of the article) contained a direct expression of opinion that global warming is occurring and caused by humans. If no direct opinion was expressed in the abstract, the article was categorized as expressing “no position on AGW,” regardless of the rest of the content of the article, and omitted from the calculation of the percentage of articles “endorsing AGW.” This accounts for the significant number of articles appearing as “no position,” the black line in the graph above.

Scientists are generally not given to expressing direct opinions in abstracts, usually preferring their conclusions to be expressed by the data itself. Powell argued persuasively (and demonstrated through analysis of articles in other areas of scientific research in which a consensus emerged over time) that scientists are particularly unlikely to express in an abstract a direct opinion endorsing the consensus belief (the consensus belief since the 1990’s being, as evident from the graph above, a growing endorsement of AGW). Imagine, for example, that you are a scientist writing a paper on the optical mechanism giving the sky a blue color. Would you think of writing in the abstract of your paper, “It is my opinion that the sky is blue?” Surely not! Your paper is about why the sky is blue, and you would assume you don’t need to explain to readers the obviously accepted belief that it appears blue.

With this in mind, Powell’s analysis of 24,210 scientific articles by 69,406 authors in the years 2013-2014 concluded that a total of 5 articles written by 4 authors rejected AGW (one of the 4 authors wrote two articles in that time period). This corresponds to 99.99% support of our hypothesis among tens of thousands of scientists all over the world. It would appear the scientists are virtually unanimous in the belief that global warming is occurring and is caused by humans.

To me, the two studies described above offer the most compelling evidence of consensus among scientists on AGW, since they are very recent and involve reviewing large samplings of the data-supported conclusions of peer-reviewed (that is, the most reliable) scientific studies of the subject by tens of thousands of experts. But a number of other studies have revealed a similar consensus. If you’re interested, I’ve added a partial list of other such studies, including links to the original papers, here.

Based on these studies, analyzing the research conclusions of tens of thousands of expert scientists studying the issue all over the world, I am persuaded that Obama’s quote above is factually true, at least insofar as there is a consensus among scientists that Climate Change is real and man-made (whether there is evidence that it is dangerous will be considered on another page of this site). In fact, based on Powell’s more recent analysis of the scientific literature, Obama’s quoted figure of 97% is probably on the low side of the proportion of peer-reviewed scientists agreeing with our hypothesis. It is more likely around 99.99%, as close to unanimous as you are ever likely to get if you poll tens of thousands of people about any subject.

So, what about the 0.01%? Is the evidence presented in their peer-reviewed papers compelling? Apparently not, at least to other scientists. In scientific literature, a key measure of a research article’s influence is how many later articles cite, or reference, that article. If the ideas or findings in an article turn out to be useful, it stands to reason they will lead to further work by other scientists, resulting in citations. As determined by Powell, the 1,789 scientific articles on “global warming” published in 2014 have led to 7,215 citations (excluding self-citations), an average of just over 4 citations per article. The 5 dissenting articles mentioned above, altogether, have been cited a total of … once. By this measure, then, those authors’ rejections look quite like fringe science.

No recognized national or international scientific body has a formal opinion dissenting with AGW. The last hold-out was the American Association of Petroleum Geologists, which revised its dissenting opinion in 2007 (a decade ago!) to state its belief in AGW in the face of the overwhelming evidence.

Mr. Trump may have much to offer as our nation’s leader, but it’s difficult to square his position on this issue, “Nobody really knows . . . It’s not something that’s so hard and fast,” with the evidence detailed above. It’s quite clear the scientists who have been studying Global Climate Change for the past 20-30 years are virtually unanimous in believing they do know Global Climate Change is real and caused by humans. In fact, such a high degree of agreement is remarkable given that we are talking about tens of thousands of scientists, from a wide variety of educational backgrounds and using an enormous assortment of experimental techniques and analytical methods, hailing from different research groups funded by different sources in various nations all over the world. With respect to Mr. Trump’s quote, then, I can only conclude…

  • Mr. Trump is not aware the scientific community has come to a virtually unanimous consensus that Global Climate Change is occurring and we are doing it; or
  • Mr. Trump has considered the evidence carefully and come to the conclusion that the handful (perhaps around 0.01%) of dissenting scientists are more insightful than the others and likely right; or
  • Mr. Trump doesn’t believe the scientists; or
  • Mr. Trump has heard the scientists but doesn’t value their opinion any more highly than the opinions of others (lobbyists, corporate interests) who have a dissenting view; or
  • Mr. Trump believes Global Climate Change is happening and we’re doing it, but he thinks it’s not sufficiently dangerous to be a governmental priority (whether it appears to be dangerous will be considered on a future page of this site); or
  • Mr. Trump believes Global Climate Change is happening, we’re doing it, and it’s dangerous, but he doesn’t want to talk about it because it’s not a priority for him and he thinks we can “kick the can down the road” a bit.

Mr. Trump is certainly not the only nonscientist who appears to dissent with our hypothesis.

How the opinions of scientists, the general population, and government representatives compare

The New York Times reported the results of a survey by three prominent Republican pollsters that was administered in 2015 to 1200 U.S. registered voters. A total of 73% of respondents believed that global warming is happening and humans are at least contributing (45% believed humans are “contributing a lot to the change” while 28% believed humans are “probably contributing a little to the change”).

The Center for American Progress (CAP) recently published the results of a study counting Global Climate Change dissenters in the U.S. 114th Congress. CAP is clearly a partisan organization with a liberal agenda. However, the study counted congressional members as dissenters only if they had expressed their dissent in public statements. The statements are available on a website reporting the study; I checked a few of them and they appear to be accurate. Based on their own self-declarations, 144 of 435 House members and 38 of 100 Senate members say global warming is either not clearly happening or not caused by human activities.

So, here’s the current picture of our collective beliefs about anthropogenic global warming:

Figure A2. Attitudes of scientists, the general public, and lawmakers about anthropogenic global warming.

The group most at odds with the scientists is our elected leaders, over a third of whom dissent! Why? Perhaps they are ignorant of the scientific consensus and need to be educated. Perhaps, like our hapless frog on my Home page, they are consumed by shorter-term challenges and objectives and have trouble finding time to grapple with a danger, however perilous, that is developing so slowly compared to the duration of an election cycle. Perhaps they are influenced by powerful corporate interests which benefit from our current energy infrastructure. Let’s see, can we think of any past example where a growing scientific consensus was ignored, or even actively suppressed, due to the influence of powerful corporations?

Figure A3. Graph from (click image for original article). Superimposed timeline was added by me with reference to Proctor, Tobacco Control 21 (2012), 87-91.

The history of the development of the scientific consensus on the link between tobacco use and lung cancer has been documented well by Proctor. I’ve superimposed a timeline of key points in this history on the graph above.

  • In 1898, a German medical student proposed tobacco dust might be behind the strangely high incidence of lung tumors (previously a rarely diagnosed medical problem) among German tobacco workers.
  • In 1912, a book by Adler referenced scientific evidence that lung cancer was caused by inhalation of particulates including tobacco smoke.
  • In 1939, a German epidemiologist published evidence from a controlled study proving a correlation between smoking and lung cancer.
  • By 1950, five separate epidemiological studies in the U.S. and U.K. corroborated the correlation.
  • By 1953, a multitude of scientific studies involving different scientific disciplines had shown not only a correlation, but determined the causal connection between smoking and lung cancer. Research after the fact shows scientists within the tobacco industry were aware of the connection through their own (unpublished) research. Life and Time magazine both published reports of a study showing that tumors could be generated on mice by painting cigarette smoke on their backs. This has been credited for the brief downturn in tobacco use around 1953 which you can see in the graph above.
  • In December, 1953, CEO’s of the 6 largest American tobacco companies met in New York City to plan a coordinated response. This resulted in a well-funded industry effort to refute the scientific evidence with advertisements, “white papers” written by industry-funded “experts,” industry-funded scientific research into the “controversy,” and propaganda films. The effort was successful, eliminating the 1953 downturn in tobacco use. It was also long-lived; 20 years later, in 1973, the Tobacco Institute hired 2 marketing firms to quantify the impact of its 1972 film, “Smoking and Health: The Need to Know,” which was shown to high school students and portrayed scientists and doctors suggesting that stress and genetics, not smoking, were the primary causes of illness. (They were pleased to note a significant shift in film viewers’ perspectives.)
  • Beginning in 1965, the U.S. Congress required the ubiquitous warning on all tobacco product packaging.
  • A Congressional ban on tobacco advertising went into effect in 1970.

When we “knew” that cigarettes cause cancer can be debated. At the latest, I would say it was around 1953. On the graph above, I have labeled the 12 years between 1953 and 1965, then, as the action gap – the gap between our knowledge that cigarettes were killing people and any meaningful government action to do something about it. The action gap was certainly unfortunate for the many people who died as a result of still-growing tobacco use during that period.

I have spent considerable words on this episode of our history because it is instructive to understand the willingness, resources, and effectiveness of corporations to contribute to the action gap by influencing both lawmakers and public opinion.

Figure A4. Gallup poll data showing public opinion on the link of smoking to lung cancer and heart disease (click image for original article).

In the graph above, you can see the slow nature of the transition of scientific consensus to public opinion (note that a year earlier than this graph, in 1954, the American Cancer Society had issued an official statement “without dissent” endorsing the link between smoking and cancer). This was due in significant part to industry-funded activities as evidenced by their own data from market studies.

While the smoking “debate” is largely considered “settled” in the U.S. (even though the above graph suggests that, as recently as 2011, 9% of respondents were still skeptical about the link), it is far from settled in some parts of the world, and industrial activities to fuel the “controversy” continue. Proctor presents an interesting analysis on the value of a human life to the tobacco industry. Tobacco companies make approximately one cent of profit for each cigarette sold. Based on the known rates of cancer death per cigarette, it’s easy to calculate that the tobacco industry makes about $10,000 in profit for each person killed by cigarettes. As a scientist, I am sometimes astounded by how inventive businesses can be in devising ways of protecting the profits earned from old ideas that have been found to be harmful. It would seem everyone, including the businesses, would be better served if that same innovative energy were simply directed to doing something better. As our most celebrated innovators and entrepreneurs have repeatedly shown, there have been ample opportunities since the 1950’s to earn 10 grand while enriching the human condition, and certainly without presiding over a person’s death.

OK, back to Global Climate Change. Compare Figure A3, above, to the graph below.

Figure A5. Atmospheric carbon dioxide concentration (blue) and mean global temperature (red) over the 1000 years preceding the year 2000. Graph by Hanno as published on Wikimedia; data from multiple sources (click image for source image including references).

It’s an eerie comparison, right?! If you think so, you’re not alone. The American Association for the Advancement of Science, the largest general scientific society in the world, released a report in 2014 stating its position that, “The science linking human activities to climate change is analogous to the science linking smoking to lung and cardiovascular diseases.”

Based on the evidence presented on this page, I think it’s irrefutable that scientists have reached a robust consensus that Global Climate Change is real and caused by humans. As shown above in Figure A2, though, that scientific consensus has failed to translate to both lawmakers and the general public. I attribute this to the following:

  • The issue is complex, and most of us are too busy doing our own jobs and taking care of family responsibilities to spend time wading into the scientific details. (This website aims to make some of the science a bit more accessible!)
  • Our leaders are subject to an out-sized influence by corporations with significant financial resources and an interest in maintaining the current energy infrastructure. (We need to help rescue them from this influence by getting in their faces with the truth!)
  • The opinions of most people are influenced both by what they hear from the scientists and what they hear from their leaders. And, while scientists are often forced by their professional standards of accuracy to state their conclusions in complex and nuanced language, politicians can get by with short, attention-grabbing exclamations and tweets that may be inaccurate but are easy to understand and remember.
  • The “balanced reporting” format often used by the media in an attempt at impartiality, in which opposing viewpoints are given equal time, has created an appearance of false balance with respect to Global Climate Change. “Balanced reporting” is an effective format for cultural issues requiring subjective value judgments, like the appropriate distribution of taxes. It is not appropriate for issues of scientific fact when the evidence for one conclusion far outweighs the evidence for the other. If a 5-minute news segment on Global Climate Change were properly balanced by the weight of scientific consensus, the person expressing skepticism about AGW should only be allowed to talk for 3 one-hundredths of a second! Try spitting out even a tweet in that time!

Determining when we “knew” Global Climate Change was real and caused by humans is highly subjective. I think it might be around 2007, when the last nationally or internationally recognized scientific body, the American Association of Petroleum Geologists formally admitted the evidence was overwhelming. Thus, if it turns out that Global Climate Change is dangerous (which I will explore on a future page of this site) and we are not taking sufficient measures to mitigate it, then we are a decade into the action gap on Global Climate Change.

While there are many similarities between the “debates” about whether smoking causes lung cancer and whether humans are causing Global Climate Change, there are a couple of very important differences. First, while the action gap related to tobacco undeniably resulted in many unnecessary deaths, people who chose to become informed on the subject at least had the opportunity to avoid the worst risks of tobacco by not smoking cigarettes. In contrast, the skeptics on climate change – our leaders who argue that it’s not real, not proven, or not caused by humans – wish for the rest of us, and our kids, and our grandchildren, to roll the dice with them. There is only one Earth and, if Global Climate Change turns out to have the potential to destroy its ability to support our lives, then, as Martin Luther King, Jr. said, “We may have all come on different ships, but we’re in the same boat now.”

Second, once the problem of tobacco use was recognized, the solution to the problem was relatively straightforward: discourage smoking. If we recognize that Global Climate Change is a problem, the solution will be relatively complex. We will need to develop and implement sustainable energy production technologies on a large scale. Many of those technologies already exist at small-to-moderate but commercial scales, and we have shown the ability in the past to implement highly complex technological and economic infrastructures. After all, the current energy system has been developed in only a little over 150 years – around two human lifespans. We are certainly capable of mitigating the effects of Global Climate Change, but it will take the ingenuity of all of us.

At the very least, we need to first agree that it’s real. I hope this page has convinced you it is. As long as we are stuck arguing about whether it’s even happening (it is), we will not have begun debating about how to address it. The consensus of our leaders on this issue needs to be made to match the scientific consensus. I hope you will join me in taking action to make that happen.

(Note: The comparison between the development of scientific consensus related to smoking and Global Climate Change has been made by many, including a particularly good article by Seth Miller which inspired my consideration of it on this page.)


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