Study Finds 97% Consensus on Human-Caused Global Warming in the Peer-Reviewed Literature

By Dana Nuccitelli and John Cook via Skeptical Science.

A new survey of over 12,000 peer-reviewed climate science papers by our citizen science team at Skeptical Science has found a 97 percent consensus in the peer-reviewed literature that humans are causing global warming.

Lead author John Cook created a short video abstract summarizing the study.

The Abstracts Survey

The first step of our approach involved expanding the original survey of the peer-reviewed scientific literature in Oreskes (2004). We performed a keyword search of peer-reviewed scientific journal publications (in the ISI Web of Science) for the terms ‘global warming’ and ‘global climate change’ between the years 1991 and 2011, which returned over 12,000 papers. John Cook created a web-based system that would randomly display a paper’s abstract (summary). We agreed upon definitions of possible categories: explicit or implicit endorsement of human-caused global warming, no position, and implicit or explicit rejection (or minimization of the human influence).

Our approach was also similar to that taken by James Powell, as illustrated in the popular graphic below. Powell examined nearly 14,000 abstracts, searching for explicit rejections of human-caused global warming, finding only 24. We took this approach further, also looking at implicit rejections, no opinions, and implicit/explicit endorsements.

We took a conservative approach in our ratings. For example, a study which takes it for granted that global warming will continue for the foreseeable future could easily be put into the implicit endorsement category; there is no reason to expect global warming to continue indefinitely unless humans are causing it. However, unless an abstract included (either implicit or explicit) language about the cause of the warming, we categorized it as ‘no position’.

Note that John Cook also initiated a spinoff from the project with a survey of climate blog participants re-rating a subset of these same abstracts. However, this spinoff is not a part of our research or conclusions.

The Team

A team of Skeptical Science volunteers proceeded to categorize the 12,000 abstracts — the most comprehensive survey of its kind to date. Each paper was rated independently at least twice, with the identity of the other co-rater not known. A dozen team members completed most of the 24,000+ ratings. There was no funding provided for this project; all the work was performed on a purely voluntary basis.

Once we finished the 24,000+ ratings, we went back and checked the abstracts where there were disagreements. If the disagreement about a given paper couldn’t be settled by the two initial raters, a third person acted as the tie-breaker.

The volunteers were an internationally diverse group. Team members’ home countries included Australia, USA, Canada, UK, New Zealand, Germany, Finland, and Italy.

The Self-Ratings

As an independent test of the measured consensus, we also emailed over 8,500 authors and asked them to rate their own papers using our same categories. The most appropriate expert to rate the level of endorsement of a published paper is the author of the paper, after all. We received responses from 1,200 scientists who rated a total of over 2,100 papers. Unlike our team’s ratings that only considered the summary of each paper presented in the abstract, the scientists considered the entire paper in the self-ratings.

The 97% Consensus Results

Based on our abstract ratings, we found that just over 4,000 papers expressed a position on the cause of global warming, 97.1% of which endorsed human-caused global warming. In the self-ratings, nearly 1,400 papers were rated as taking a position, 97.2% of which endorsed human-caused global warming.

We found that about two-thirds of papers didn’t express a position on the subject in the abstract, which confirms that we were conservative in our initial abstract ratings. This result isn’t surprising for two reasons: 1) most journals have strict word limits for their abstracts, and 2) frankly, every scientist doing climate research knows humans are causing global warming. There’s no longer a need to state something so obvious. For example, would you expect every geological paper to note in its abstract that the Earth is a spherical body that orbits the sun?

This result was also predicted by Oreskes (2007), which noted that scientists

“… generally focus their discussions on questions that are still disputed or unanswered rather than on matters about which everyone agrees”

However, according to the author self-ratings, nearly two-thirds of the papers in our survey do express a position on the subject somewhere in the paper.

We also found that the consensus has strengthened gradually over time. The slow rate reflects that there has been little room to grow, because the consensus on human-caused global warming has generally always been over 90% since 1991. Nevertheless, in both the abstract ratings and self-ratings, we found that the consensus has grown to about 98% as of 2011.

Percentage of papers endorsing the consensus among only papers that express a position endorsing or rejecting the consensus. From Cook et al. (2013).

Our results are also consistent with previous research finding a 97 percent consensus amongst climate experts on the human cause of global warming. Doran and Zimmerman (2009) surveyed Earth scientists, and found that of the 77 scientists responding to their survey who are actively publishing climate science research, 75 (97.4%) agreed that “human activity is a significant contributing factor in changing mean global temperatures.” Anderegg et al. (2010) compiled a list of 908 researchers with at least 20 peer-reviewed climate publications. They found that:

“≈97% of self-identified actively publishing climate scientists agree with the tenets of ACC [anthropogenic climate change]”

In our survey, among scientists who expressed a position on AGW in their abstract, 98.4% endorsed the consensus. This is greater than 97% consensus of peer-reviewed papers because endorsement papers had more authors than rejection papers, on average. Thus there is a 97.1% consensus in the peer-reviewed literature, and a 98.4% consensus amongst scientists researching climate change.

Why is this Important?

Several studies have shown that people who correctly perceive the scientific consensus on human-caused global warming are more likely to support government action to curb greenhouse gas emissions. This was most recently shown in McCright et al. (2013), recently published in the journal Climatic Change. People will defer to the judgment of experts, and they trust climate scientists on the subject of global warming.

However, research has also shown that the public is misinformed on the climate consensus. For example, a 2012 poll from US Pew Research Center found less than half of Americans thought that scientists agreed that humans were causing global warming. One contributor to this misperception is false balance in the media, particularly in the US, where most climate stories are “balanced” with a “skeptic” perspective. However, this results in making the 3 percent seem much larger, like 50 percent. In trying to achieve “balance”, the media has actually created a very unbalanced perception of reality. As a result, people believe scientists are still split about what’s causing global warming, and therefore there is not nearly enough public support or motivation to solve the problem.\

Such false balance has long been the goal of a dedicated misinformation campaign waged by the fossil fuel industry. Just as one example, in 1991 Western Fuels Association conducted a $510,000 campaign whose primary goal was to “reposition global warming as theory (not fact).” These vested interests have exploited the media desire to appear “balanced.”

Open Access for Maximum Transparency

We chose to submit our paper to Environmental Research Letters because it is a well-respected, high-impact journal, but also because it offers the option of making a paper available by open access, meaning that for an up-front fee, the paper can be made free for anybody to download. This was important to us, because we want our results to be as accessible and transparent as possible.

To pay the open access fee, in keeping with the citizen science approach, we asked for donations from Skeptical Science readers. We received over 50 donations in less than 10 hours to fully crowd-fund the $1,600 open access cost.

Human-Caused Global Warming

We fully anticipate that some climate contrarians will respond by saying “we don’t dispute that humans cause some global warming.” First of all, there are a lot of people who do dispute that there is a consensus that humans cause any global warming. Our paper shows that their position is not supported in the scientific literature.

Second, we did look for papers that quantify the human contribution to global warming, and most are not that specific. However, as noted above, if a paper minimized the human contribution, we classified that as a rejection. For example, if a paper were to say “the sun caused most of the global warming over the past century,” that would be included in the less than 3% of papers in the rejection categories.

Many studies simply defer to the expert summary of climate science research put together by the Intergovernmental Panel on Climate Change (IPCC), which states that most of the global warming since the mid-20th century has been caused by humans. According to recent research, that statement is actually too conservative.

Of the papers that specifically examine the human and natural causes of global warming, virtually all conclude that humans are the dominant cause over the past 50 to 100 years.

Net human and natural percent contributions to the observed global surface warming over the past 50-65 years according to Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, light green), Lean and Rind 2008 (LR08, purple), Huber and Knutti 2011 (HK11, light blue), Gillett et al. 2012 (G12, orange), Wigley and Santer 2012 (WS12, dark green), and Jones et al. 2013 (J12, pink).

Most studies simply accept this fact and go on to examine the consequences of this human-caused global warming and associated climate change.

Another important point is that once you accept that humans are causing global warming, you must also accept that global warming is still happening; humans cause global warming by increasing the greenhouse effect, and our greenhouse gas emissions just keep accelerating. This ties in to our previous posts noting that global warming is accelerating; but that over the past decade, most of that warming has gone into the oceans (including the oft-neglected deep oceans). If you accept that humans are causing global warming, as over 97% of peer-reviewed scientific papers do, then this conclusion should not be at all controversial. With all this evidence for human-caused global warming, it couldn’t simply have just stopped, so the heat must be going somewhere. Scientists have found it in the oceans.

Spread the Word

Awareness of the scientific consensus on human-caused global warming is a key factor in peoples’ decisions whether or not to support action to reduce greenhouse gas emissions. However, there is a gap here due to the public’s lack of awareness of the consensus. Thus it’s critical that we make people aware of these results. To that end, design and advertising firm SJI Associates generously created a website pro-bono, centered around the results of our survey. The website can be viewed at, and it includes a page where relevant and useful graphics like the one at the top of this post can be shared. You can also follow The Consensus Project on Twitter @ConsensusProj.

Quite possibly the most important thing to communicate about climate change is that there is a 97 percent consensus amongst the scientific experts and scientific research that humans are causing global warming. Let’s spread the word and close the consensus gap.

– By Dana Nuccitelli and John Cook. This piece was originally published at Skeptical Science and was reprinted with permission.

52 Responses to Study Finds 97% Consensus on Human-Caused Global Warming in the Peer-Reviewed Literature

  1. Sou says:

    Fantastic effort and timely.

    Keeps the pressure on while fake sceptics are running out of steam.

  2. D. R. Tucker says:

    If the planet’s atmosphere hits a milestone of 400 ppm of green house gases and hardly anyone notices, does it still count?
    Uhhh…..yes says David Turnbull, Campaigns Director for International Oil Change. We’ll hear why this parts-per-million
    threshold better be a pivot point or else. And a wake-up call in the form of a new documentary is our next focus. We’ll
    speak with the film maker of “Unacceptable Levels” to learn whats at stake from exposure to everyday chemicals and what
    can be done about it. Ed Brown says it starts with awareness and the film, and this interview, tells us what we need to know.

  3. Merrelyn Emery says:

    On what grounds does this small percentage reject or dispute the hypothesis? ME

  4. BillD says:

    Then we had the recent report by James Taylor (of Heartland) stating that a survey showed that only about 35% of scientists and engineers agree that humans are causing climate warming. He posted an elaborate analysis on the Forbes (financial magazine) web site. However, he neglected to mention that his survey represented the opinions of the members of the Alberta (CA) society of petroleum engineers. Thus, it’s really important that the news media understand and report the nature of the scientific consensus.

  5. Adam R. says:

    3% don’t agree!


  6. Mulga Mumblebrain says:

    Wishful thinking, I’m afraid. As far as I can see the hardcore, moron, denialists and their Thought Controllers in the Rightwing MSM are as belligerent as ever. And the denialist industry, which has the blessing of the Right everywhere, is developing new tactics of ‘soft denialism’. Concepts like ‘adaptation’ and ‘resilience’ are getting trotted out by various confidence-tricksters and the ‘concern trolls’, to actively downplay the disaster. Also useful is the tactic of admitting the problem, but pretending that it will only threaten us in 2050 or 2100, thereby effectively consigning it to the never-never. Moreover, as we can see in the UK, Rightist regimes are growing openly hostile to every environmental concern, group or calamity, as if doubling-down on their denial and actively increasing the environmental destruction will somehow cow the forces of Nature into submission. There is also an element of proving who is the biggest thug amongst thugs, like in an elite boarding school, and one must never underestimate just how visceral, and dangerous, is their hatred for environmentalists, the new ‘communists’.

  7. Mulga Mumblebrain says:

    I think Taylor achieves all the categories-‘Lies, damned lies and statistics’. They really have no shame nor sense of guilt.

  8. David Smith says:

    At what approximate date was this 97% consensus reached?

  9. BillD says:

    This is survey of publications from 1993 to 2012, I think. It’s always been about 97% with a slight increase in recent times. My only climate change publication dealt with the effects of 20 years of warming on the food chain of an alpine lake. Not surprisingly, the paper did not say much about the cause of the warming, but focused on what happened in the lake. Since science articles only focus on the data and evidence they present, it makes good sense that most climate studies do not directly address the cause of climate change. Many study look at consequences, not causes.

  10. Lore says:

    Galileo is actually a good example of where the scientific observable evidence triumphed over the fake skeptics of the day.

  11. Paul Klinkman says:

    It’s May 16 and tropical storm Alvin has 50 mph winds. It’s kind of early for hurricane season but it’s Eaarth.

  12. Adam R. says:

    Yes, but that doesn’t stop the deniers from misusing his memory.

  13. Superman1 says:

    “Many study look at consequences, not causes.” The medical literature is not much different. There, many causes are due to a ‘product’, in one form or another. Behind each of these products is an industry that has interest in any adverse effects from its product being suppressed, and, depending on its wealth and influence, can downplay these adverse effects, or causes. Not very different from what’s happening with climate change.

  14. Superman1 says:

    And if it found 100%, so what? If a similar survey on smoking found 100% consensus on relation between smoking and lung cancer, how many chain smokers would stop smoking? The Surgeon General’s Report of 1964 had little effect, and a similar climate change finding would have similar little effect.

  15. Superman1 says:

    Who cares; it’s irrelevant to addressing the real problem!

  16. kelly anspaugh says:

    Well, one of the talking point of the denialists is that more and more people are seeing through the global warming “hoax” and that the consensus opinion that AGW is a reality (the product, we are supposed to believe, of a world-wide conspiracy among climate scientists to defraud the public) is falling apart. This shows that’s all bs. This study confirms what Oreskes discovered a few years back, what Jim Powell re-established more recently: that the consensus grows stronger.

  17. It would be really useful to understand what the 3% said. What is their beef? We always cite this 97%, but in arguments or public presentation, I often need a way to address the 3%. Even just out of curiosity I’d like to know this.

  18. kelly anspaugh says:

    On the grounds of wishful thinking? As has been before noted, if you were to eliminate from that three percent those cynical so-and-sos on the payroll of the energy companies (or on the payroll of “think tanks” supported by the energy industry)and saying what their what their meal tickets want to hear (think Willie Soon), and eliminate those free-market, anti-regulatory ideologues who find the spectre of the government intervening in the private sector to regulation greenhouse gas emissions unthinkable (they’d see the earth burn to a cinder before they would admit this necessity), then the consensus would probably be 99.5%, the remaining five percent being those who just can’t bear to think of that catastrophe may be in the offing.

  19. kelly anspaugh says:

    On the grounds of wishful thinking? As has been before noted, if you were to eliminate from that three percent those cynical so-and-sos on the payroll of the energy companies (or on the payroll of “think tanks” supported by the energy industry)and saying what their what their meal tickets want to hear (think Willie Soon), and eliminate those free-market, anti-regulatory ideologues who find the spectre of the government intervening in the private sector to regulation greenhouse gas emissions unthinkable (they’d see the earth burn to a cinder before they would admit this necessity), then the consensus would probably be 99.5%, the remaining five percent being those who just can’t bear to think of that catastrophe may be in the offing.

  20. Superman1 says:

    “remaining five percent”. Remaining 1/2%.

  21. kermit says:

    Not every paper discussing the development of lung cancer is an example of pro-tobacco disinformation, nor is every climatology paper on glacier melt or changes in a lake avoiding discussing AGW. Tthis article clearly points out that most scientists in this field (and other fields of course) accept the consensus model as a given.

  22. Merrelyn Emery says:

    Kelly, the ideologues, pretenders and Soon usually do not make it through peer review in scientific journals – I’ve read them complaining about it. I think the 3% is more likely to be the failure rate of peer review, ME

  23. Aussie John says:

    The essential reason the AGW denial propaganda industry is so well resourced is that from the moment any corporation admits that the pollution their industry produces contributes to climate change, they are liable for damages claims.
    They would immediately be sued to cease such pollution and pay for subsequent remedial action.

    Democratic process of government is being subverted at its base – the mis-informed voter – being used as a corporate tool.

    All in total irresponsible disregard of Earth’s environmental degradation.

  24. BobbyL says:

    Even if the 3% somehow defied all odds and turned out to be right we have to act based on the best available knowledge. It would be nice if we had time to wait for another 14,000 papers to be published but clearly we don’t. Actually we should have acted not yesterday but more than 20 years ago when the risks were defined in the first IPCC report. Or even earlier in the 1980s when alarms were raised. We blew it but are compelled socially and politically to act as if we didn’t and pretend there is still time. Fine, there is still time, let’s get going.

  25. Jan Freed says:

    Never underestimate the agility of the denier. My wife’s aunt, a loyal contrarian, writes, “Consensus doesn’t mean it is true. Look at Galileo, etc.” Feh!

  26. Aussie John says:

    A highly recommended article from Australian comedian Rod Quantok is here:

    The embedded videos demonstrate his dedicated efforts to spread the AGW truth.

    He couldn’t get this article published on Australia’s mainstream media -a familiar tale of corporate media control.

  27. Superman1 says:

    There’s no general rule on these issues; they need to be handled on a case-by-case basis. Consider the issue of the effects of cell phones and WiFi.

  28. Superman1 says:

    Probably the majority of articles say there is little or no impact, but they are merely reflecting the heavy hand of industry. Lennart Hardell, perhaps the world’s premier epidemiology oncologist on EMF effects, has shown that heavy cell phone users (>30 minutes/day) who start as children have five times the odds of getting brain cancer after a decade of use.

  29. Superman1 says:

    Interestingly, many of those on this site who complain most vociferously about climate change deniers are the most vociferous in denying the reality of the adverse effects of cell phones and WiFi. That’s why I believe that, for the most part, personal agendas are dominating the denial business in both climate change and EMF effects, not some imagined lack of information.

  30. Superman1 says:

    Continued at #15 below.

  31. BobbyL says:

    Do you believe in the wind turbine syndrome?

  32. kelly anspaugh says:

    Yea, point five percent, I meant to write, obviously.

  33. kelly anspaugh says:

    While it may be true that distingushed climate scientists such as James Hansen, Michael Mann, and Lonnie Thompson all agree that anthropogenic global warming is a reality, the fact remains that 65% of people with undergraduate science degrees from community colleges and who live in their parents’ basements have serious doubts about global warming. So I guess that means the jury is still out. In the meantime, let’s teach the controversy!

  34. kelly anspaugh says:

    Right. One of the more ludicrous moments of the last presidental campaign season was when Rick Perry compared himself and other global warming deniers to Galileo. What was required at that moment was a Lloyd Benson to step up and say “Governor, with all due respect, I knew Galileo, I worked with Galileo, Galileo was a friend of mine. Governor, believe me, you’re no Galileo.”

  35. kelly anspaugh says:

    I’m sure you’re right. Peer review not faultless. Just as the tenure review system is not faultless. Accounts for the fact that a strident global warming denier got tenured at my former workplace, OSU-Lima, and for the next ten years spewed denialist hogwash with absolute impunity, sent hundreds of students out the door of his climate classroom with the idea in their heads that global warming was all a “liberal hoax.” Sorta gave the lie to our campus’s advertising slogan “Study with the Best!”

  36. James says:

    Ask the American Meteorological Society, the American Chemical Society, the National Academy of Sciences, the American Geophysical Union, and many, many others. Climate change is a reality.

  37. SecularAnimist says:

    Superman1 wrote: “the most vociferous in denying the reality of the adverse effects of cell phones and WiFi”

    This is the same pseudoscience that got you laughed off of RealClimate, when you claimed that passengers in electric cars were getting cancer from proximity to the batteries, or some similar nonsense.

  38. Superman1 says:

    I don’t think Hardell’s subjects who had five times the odds of getting brain cancer would find your denialist comments at all humorous. Like your climate denialist counterparts, you’ve thrown in your lot with cell phone heavy use, and like them, you’re convincing yourself catastrophe won’t happen.

  39. Superman1 says:

    I haven’t looked into it, so I can’t comment. But, I have looked at cell phones and WiFi, and the cancer latency period for heavy cell phone use (~ten years) is slightly less than half that of smoking (~20-30 years). We will see a Tsunami of brain cancers within a decade from now, about the same time that the denied climate change impacts make themselves felt.

  40. Superman1 says:

    Moved to #16 below.

  41. BobbyL says:

    I’m surprised you haven’t checked out wind turbine syndrome because it is so relevant to the global warming issue. From my reading it appears that most experts think that the symptoms people are experiencing have a psychological basis and are not actually due to any physical effects from wind turbines. In any event it has become a NYMBY issue in a number of places.

  42. Superman1 says:

    From a report by State of Israel, Ministry of the Environment: “The Ministry of Environmental Protection published the results of ELF magnetic fields in cars. It was found that the exposure in the hybrid cars models sold in Israel, mainly in rear seats, is higher, and for some models, much higher than the exposure in non hybrid cars.”

  43. Superman1 says:

    I don’t know what the health impacts would be, but if you and your fellow cell phone denialists want to spend hours in a high EMF environment, here’s a wonderful opportunity.

  44. Superman1 says:

    From an article by Ariel Schwartz, commenting on the study: “According to The International Agency for Research on Cancer (IARC), any extended exposure to electromagnetic fields higher than 2 mG can possibly cause cancer, while the Israeli Ministry of Health believes 4 mG is the maximum amount of allowable radiation. In a study conducted last year by Israeli Web site Walla! Cars, the Prius, Honda Insight, and Civic Hybrid all released 100 mG of radiation during acceleration. Normal driving of the Prius emitted between 14 and 30 mG of radiation.”

  45. Mulga Mumblebrain says:

    The hardcore denialists, most linked openly to the mining industry and the extreme Right, used to meet in telephone booths and call themselves the ‘Lavoisier Society’, for some arcane reason but probably because they identified with private tax collecting. They seem to have disappeared up their own fundamental orifices, but we have their names, on The List, at The Defarge Institute for Joyful Knitting.

  46. Superman1 says:

    To place these numbers in context, researchers at Kaiser-Permanente placed gauss-meters on expectant Mothers, and tracked their magnetic field exposures. They found that once-a-day exposures above a threshold of 16mg resulted in a factor of six increase in miscarriage. Maybe you find that humorous as well.

  47. Raul M. says:

    Electromagnetic shielding seat covers?

  48. Superman1 says:

    BobbyL, There’s a lot I haven’t looked into relative to climate change. It’s enough of a challenge to keep up with first-order effects; at some later time, I can incorporate second and third-order effects.

  49. MH says:

    It is being stated by the ‘deniers” that research results follow the money — that gov’t grant $ for research on CO2-warming relationship is so abundant that scientists flock to that line of inquiry, and the implication is that they inquire in biased ways, I guess. This surprised me, since I asumed the fossil fuels industry had more than enough $$ to have overbalanced this in the opposite direction. Any comment? I foprget where I saw this – I probably just googled it. Any way to research this? Did Skep Sci researchers happen to collect info on funding source?

    I think we (those of us who are convinced of AGW) will be better able to get the ear of those who are noit ‘morons’ and yet are not convinced of AGW, if we changer the rhetoric – denier is apt in a way and in a way it is perjorative and noit so apt. People read mainstream media and think they can trust it — eg., the Wall ST J. recent article 5/9 Op Ed by Happer PhD physics and an engineer/Senator/astronaut and a previous article during Pres campaign by Happer and 15 others – In that year, 2012 I guess it was, WSJ didn’t publish the rebuttal op ed by many many more scientists, which ws eventually published in (?) Nature or Science ? peer periodical. So, less intense rhetoric and flood with facts, references, links to research….CP does a pretty good job of the refs and links, thanks, keep it up and give us more of that – again, less rhetoric. As in, “walk softly and carry a biog (hockey) stick.” (espertly accurately reproduced to minimize accusations of flattening the Medieval/Little Ice Age periods).

  50. Duncan Noble says:

    For those trying to understand the 3%, consider that humans are much more likely to “reason” like lawyers than “objective scientists”. We tend to start with our preferred answer (e.g., guilty or not guilty) and then build a case to support that answer. For a good overview article about this tendency, check out the following piece from Mother Jones:

    It’s worth reading. Since reading it, I have become more aware of my own tendencies in this direction, as well as others’. We humans are very good at rationalizing our behavior.

  51. Sarah Griffith says:

    The IPCC publishes some of the world’s most authoritative reports on climate change, which are written and reviewed by thousands of scientists, and approved by governments around the world.