Quantifying the Consensus on Anthropogenic Global Warming in the Scientific Literature

Quantifying the Consensus on Anthropogenic Global Warming in the Scientific Literature John Cook Dana Nuccitelli Sarah A. Green et al.

Tag layer of the DHSS Meta-Tagging Project. The article text is reproduced from the staged copy named in sourceDesc and is not altered; the tags are held separately in standOff so that overlapping and nested spans are preserved.

Quantifying the Consensus on Anthropogenic Global Warming in the Scientific Literature 2013 Environmental Research Letters 10.1088/1748-9326/8/2/024024 cook2013-climate-consensus cook2013-climate-consensus.txt, the project's staged copy 99953575cb7cf4ca5a8e9736e063bf30c391b1c0 open-access earth-science

the paper's own words

the tag layer; a sentence written by the tagger about the paper, always anchored to a quotation but not itself a quotation

141 spans located by searching the stored text for the quote's exact words, allowing any whitespace between them The position taken on anthropogenic global warming by 11,944 peer-reviewed climate abstracts published between 1991 and 2011. The public believes climate scientists are split - what proportion of the literature that takes a position on the cause of warming actually endorses the human one? 97.1% of abstracts stating a position endorse anthropogenic warming, confirmed at 97.2% by an independent author self-rating arm. existence scientific consensus on anthropogenic global warming corpus-analysiscorpus-analysis mechanismstwo independent blind raters per abstract, with identifying information stripped, so a rating cannot be anchored on the author or the journal mechanismsdisagreement is not hidden but reconciled in stages - raters see each other's ratings and revise, then a third party resolves the residue mechanismsa second, independent measurement of the same quantity: authors rate their own full papers against the same rubric, which tests whether abstract-level coding under-reads a paper's position mechanisms'no position' is separated from 'uncertain' by re-reading a random 1,000 of the former, because silence and doubt are different states that a coarse scheme would merge THE SAME PAPERS, TWO RATERS, TWO ANSWERS - AND THE SECOND RATER IS THE AUTHOR: `In a second phase of this study, we invited authors to rate their own papers. Compared to abstract ratings, a smaller percentage of self-rated papers expressed no position on AGW (35.5%)` against 66.4% by abstract rating. WHETHER A PAPER TAKES A POSITION DEPENDS ON WHO IS READING IT, and this paper measures the gap rather than choosing a side. AND `ENDORSEMENT` IS OPERATIONALISED WITH AN INFERENCE STEP BUILT INTO THE CATEGORY: level `(3) IMPLICIT ENDORSEMENT - IMPLIES humans are causing global warming. E.g., RESEARCH ASSUMES greenhouse gas emissions cause warming without explicitly stating humans are the cause.` A paper counts as endorsing when the rater judges that it assumes the conclusion, so the construct deliberately includes what an abstract does not say. most abstracts expressed no position; the 97% figure is computed among those that didwhat this limits: the 97% figure: it is computed among abstracts that took a position, and most ABSTRACTS EXPRESSED NO POSITION ON AGW Three named sources of uncertainty: sample representativeness, ambiguous abstract wording, and rater subjectivity.what this limits: the consensus estimate: THE PROCESS CONTAINS SEVERAL SOURCES OF UNCERTAINTY, INCLUDING THE REPRESENTATIVENESS OF THE SAMPLE, LACK OF CLARITY IN THE ABSTRACTS AND SUBJECTIVITY The sample, though the largest of its kind, covers only part of the climate literature.what this limits: the coverage claim: 11 944 PAPERS IS ONLY A FRACTION OF THE CLIMATE LITERATURE Rating abstracts is inherently partly subjective.what this limits: every rating in the study: SOME SUBJECTIVITY IS INHERENT IN THE ABSTRACT RATING PROCESS Scientific reticence and 'erring on the side of least drama' may still bias abstract ratings towards 'no position'.what this limits: the abstract ratings: SCIENTIFIC RETICENCE AND ESLD REMAIN POSSIBLE BIASES IN THE ABSTRACT RATINGS PROCESS, in the direction of 'no position' Rating abstracts alone undercounts papers that actually take a position.what this limits: any count of papers taking a position: CATEGORIZATION BASED ON THE ABSTRACT ALONE UNDERESTIMATES THE PERCENTAGE OF PAPERS TAKING A POSITION ON AGW Initial inter-rater disagreement was substantial - 27% on category and 33% on endorsement - so the headline figure depends on the reconciliation procedure, not on raw agreement.what this limits: the reliability of the ratings the result rests on: INITIALLY, 27% OF CATEGORY RATINGS AND 33% OF ENDORSEMENT RATINGS DISAGREED Two thirds of abstracts take no position at all, so the 97.1% is a proportion of the third that do.what this limits: the denominator of the headline percentage: 66.4% OF ABSTRACTS EXPRESSED NO POSITION ON AGW, 32.6% ENDORSED AGW, 0.7% REJECTED AGW AND 0.3% WERE UNCERTAIN - so the consensus figure is among the abstracts that took a position, as the paper states The abstract is a proxy for the paper: the self-rating arm found fewer papers taking no position (35.5% against 66.4%), which is the size of what abstract-level coding misses.what this limits: the same scope as limitations[5], with both numbers: A SMALLER PERCENTAGE OF SELF-RATED PAPERS EXPRESSED NO POSITION ON AGW (35.5%) than the 66.4% found by abstract rating Response is partial and self-selected: email addresses were obtainable for at least 60% of papers per year, and authors chose whether to reply.what this limits: the self-rating arm specifically: EMAIL ADDRESSES FOR 8547 AUTHORS WERE COLLECTED, so the responding sample is partial and self-selected a content-analysis coding methodology (independent raters categorizing texts) applied to quantify the scientific consensus on anthropogenic global warming a second phase in which the authors of the papers were invited to rate their own abstracts, alongside the volunteer raterseliminates: the consensus figure being a property of how the volunteers read the abstracts - the authors know what their own papers claimed, and their ratings are an independent check no coding protocol can supply · outcome (stated by the tagger, no quotation): held written criteria supplied to the raters for both the category and the level of endorsement of anthropogenic global warmingeliminates: each rater applying a private standard - the criteria are fixed and written before the rating rather than settled case by case · outcome (stated by the tagger, no quotation): held the article type restricted to 'article', excluding books, discussions, proceedings papers and other document typeseliminates: a consensus measured over documents that are not research reports at all · outcome (stated by the tagger, no quotation): held scientific consensusscientific consensus anthropogenic global warminganthropogenic global warming endorsementendorsement No PositionNo Position UncertainUncertain self-ratingself-rating ratersraters citizen sciencecitizen science Communicating the scientific consensusCommunicating the scientific consensus people’s acceptancepeople’s acceptance public support for climate policypublic support for climate policy ISI Web of ScienceISI Web of Science topic searchestopic searches research categoryresearch category degree of endorsementdegree of endorsement Explicit endorsement with quantificationExplicit endorsement with quantification Explicit endorsement without quantificationExplicit endorsement without quantification Implicit endorsementImplicit endorsement Implicit rejectionImplicit rejection Explicit rejection without quantificationExplicit rejection without quantification Explicit rejection with quantificationExplicit rejection with quantification web-based systemweb-based system rating disagreementsrating disagreements third partythird party four metricsfour metrics endorsementsendorsements rejectionsrejections abstract ratingabstract rating sources of uncertaintysources of uncertainty representativeness of the samplerepresentativeness of the sample lack of clarity in the abstractslack of clarity in the abstracts subjectivity in rating the abstractssubjectivity in rating the abstracts categorization based on the abstract alonecategorization based on the abstract alone percentage of papers taking a position on AGWpercentage of papers taking a position on AGW rating biasrating bias raters themselves endorsed the scientific consensus on AGWraters themselves endorsed the scientific consensus on AGW scientific reticencescientific reticence erring on the side of least dramaerring on the side of least drama ‘no position’ classification‘no position’ classification consensus gapconsensus gap campaigns designed to confuse the publiccampaigns designed to confuse the public opposing sides with equal attentionopposing sides with equal attention vocal minorityvocal minority systematic, comprehensive review of the literaturesystematic, comprehensive review of the literature quantitative evidencequantitative evidence accurate perception of the degree of scientific consensus → public support for climate policyAccurate consensus perception is presented as prerequisite for public support; prerequisite-for is not in the eight-type registry. four metrics → degree of endorsementThe four metrics quantify endorsement level; quantifies is not in the eight-type registry. ESLD → erring on the side of least dramaESLD expands to erring on the side of least drama; acronym-expansion is not in the eight-type registry. raters themselves endorsed the scientific consensus on AGW → rating biasRaters’ own endorsement may bias classification toward shared endorsement; biases-toward is not in the eight-type registry. scientific reticence → ‘no position’ classificationScientific reticence may bias ratings toward no position; biases-toward is not in the eight-type registry. campaigns designed to confuse the public → consensus gapPublic-confusion campaigns contribute to the consensus gap; contributes-to is not in the eight-type registry. consensus - as measured in the literature: the share of papers taking a position that endorse one explanation consensus - as measured by asking scientists directly, which earlier surveys put at 97–98% consensus - as perceived by the public, which is that scientists disagree, and which the paper treats as the thing to be corrected `the 97% consensus` - A NUMBER MADE BY A DENOMINATOR CHOICE, AND THE PAPER PRINTS BOTH: `66.4% of abstracts expressed no position on AGW, 32.6% endorsed AGW, 0.7% rejected AGW ... Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position.` The famous figure is computed on the third of the sample that took a position; two thirds are excluded BY DEFINITION, not by data. Degree of endorsement - seven-level code ranging from explicit quantified endorsement through no position to explicit quantified rejection Explicit quantified endorsement - statement that humans are the primary cause of recent global warming Explicit unquantified endorsement - statement of human causation without a percentage contribution Implicit endorsement - assumed human causation even when the abstract does not state it explicitly Implicit rejection - natural-main-cause implication that minimizes human impact Explicit unquantified rejection - explicit minimization or rejection of human causation Explicit quantified rejection - claim that humans caused less than half of global warming Blind abstract rating - web-distributed title and abstract with author, affiliation, journal, and date hidden Rating reconciliation - anonymous comparison and revision followed by third-party adjudication Four endorsement metrics - shares among all papers, positioned papers, all scientists, and positioned scientists Abstract-only categorization - method that underestimates how many full papers take an AGW position Scientific reticence / least-drama bias - possible pressure toward a no-position abstract classification Consensus gap - public under-perception of overwhelming scientific agreement on human-caused warming Equal-attention media norm - opposing-side balance that amplifies a vocal minority Systematic literature review - quantitative counter-evidence to claims that scientific rejection is growing claim_type: existencea second reading reaching the same value. Recorded: existence — note: a second reading reaching the same value from a different sentence. Not applied - a person decides. method: corpus-analysisa second reading DIFFERING from the recorded value. Recorded: corpus-analysis — note: a second reading, verbatim-verified, disagreeing with the value on the record. Not applied - a person decides. phenomenon: the gap between the level of scientific consensus on anthropogenic global warming and the public perception of ita second reading DIFFERING from the recorded value. Recorded: scientific consensus on anthropogenic global warming — note: a second reading, verbatim-verified, disagreeing with the value on the record. Not applied - a person decides. extends → Oreskes (2004), analysis of peer-reviewed climate papers replicates → Oreskes (2004), the finding that no papers in the 1993-2003 sample reject the consensus WITHDRAWN AND REPLACED (RETRACTED AND RE-ADDED WITH THE QUANTITY CORRECTED) · contradicts → Schulte (2008), which reported 6% rejection among 2004-2007 abstractsWITHDRAWN AND REPLACED: RETRACTED AND RE-ADDED WITH THE QUANTITY CORRECTED. The target said Schulte 'REPORTED 6% REJECTION'. The span says 'Schulte also found a significantly greater number of rejection papers, including 6 EXPLICIT REJECTIONS compared to our 0 explicit rejections.' A COUNT IS NOT A RATE. Recorded in the paper that is already this corpus's example of a denominator that does not travel - the neighbouring field in the same record carried the error. replicates → Bray (2010), the finding of a strengthening consensus over time extends → Oreskes (2004), which found no disagreeing papers among 928 abstracts replicates → direct surveys of climate scientists reporting 97–98% agreement contradicts → the public perception that climate scientists disagree about the cause of warming contradicts → Schulte (2008), which reported 6 explicit rejections among 2004-2007 abstractsdiffers in: THE AXIS IS THE COUNT OF EXPLICIT REJECTIONS, AND BOTH COUNTS ARE GIVEN. 'Schulte also found a significantly greater number of rejection papers, including 6 explicit rejections compared to our 0 explicit rejections.' Six against zero, over overlapping literatures - so the disagreement is about what counts as an explicit rejection, not about how many papers exist. This row's target was repaired earlier today: it had said '6% rejection' where the paper says SIX PAPERS. No Position —distinct-from→ Uncertainthe relation itself is separately evidenced · strength of the relation: 1 · the relation stated: Uncertain states the cause is not yet determined; No Position states nothing at allNo PositionUncertain EVIDENCE CORRECTED (RE-GROUNDED) · self-rating —contrasted-with→ ratersEVIDENCE CORRECTED: RE-GROUNDED. The old span named neither 'self-rating' nor 'raters'. This sentence names both and states the comparison the edge records - and states it as a partial answer to bias, which is the paper's own framing. — strength of the relation: 1 Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but abstract ratings were compared with author self-ratings; contrasted-with is loose.self-ratingraters Communicating the scientific consensus —causes→ people’s acceptancedirection on the measure: increases · the relation itself is separately evidenced · strength of the relation: 1 · note: The paper states that communicating consensus increases acceptance.Communicating the scientific consensuspeople’s acceptance Implicit endorsement —part-of→ endorsementsthe relation itself is separately evidenced · strength of the relation: 1 · note: The consolidation explicitly includes implicit and explicit categories within endorsements. Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span writes 'endorsements (including implicit and explicit ...)', not 'implicit endorsement'.Implicit endorsementendorsements Implicit rejection —part-of→ rejectionsthe relation itself is separately evidenced · strength of the relation: 1 · note: The consolidation explicitly includes implicit and explicit categories within rejections. Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span writes 'rejections (including implicit and explicit ...)', not 'implicit rejection'.Implicit rejectionrejections opposing sides with equal attention —causes→ vocal minoritydirection on the measure: increases · the relation itself is separately evidenced · strength of the relation: 1 · note: The media norm is said to have allowed amplification of the minority’s views. Audit 2026-09-29: DOUBTFUL - read against its full span and kept, but the span says equal attention allowed a vocal minority to have their VIEWS amplified; 'vocal minority' is the wrong object.opposing sides with equal attentionvocal minority 97.1% — share of position-taking abstracts endorsing that humans cause global warming 11,944 — peer-reviewed climate abstracts examined (1991-2011)about this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted. 66.4% — share of abstracts expressing NO position on AGW 32.6% / 0.7% / 0.3% — abstracts endorsing / rejecting / uncertain on AGW 11 944 abstracts, 1991–2011 — size and span of the rated corpus 97.1% / 97.2% — endorsement among abstracts, and among self-rated papers, that take a position 66.4% / 32.6% / 0.7% / 0.3% — no position / endorse / reject / uncertain, across all abstracts 27% and 33% initial disagreement; 11% and 16% after reconciliation — inter-rater disagreement on category and on endorsement, before and after raters compared ratings 23 061 ratings by 12 people — 97.4% of all ratings came from a team of twelve 97–98% — agreement on AGW found by earlier direct surveys of publishing climate expertsWHOSE RESULT THIS IS: ing climate experts (Doran and Zimmerman 2009 , Anderegg et al 2010 ). - THE RESULT IS THE CITED WORK'S AND THE CITATION TRAILS THE SENTENCE. A plain noun phrase is the subject of the result verb and the attribution is a bracket at the end - 'Neurons within the dorsal temporal pole demonstrated match suppression ... ( Ng et al., 2014 )'. Neither of my two cues could see this shape, so THESE WERE READ RATHER THAN SWEPT: 36 candidates of the shape, 20 read, 11 marked, 9 refused. THE REFUSALS ARE WHY IT IS NOT A CUE: 'Aru2012 findings[16]' is the paper's OWN result with a citation supporting the localisation, 'Natan2015 findings[3]' ends 'consistent with previous findings ( Szymanski et al., 2009 )' where the main clause is the paper's, and 'Drieu2019' x3 are 'One cannot entirely rule out...' - a limitation, not an attribution. About 55% precise, which is a list to read and not a sweep to run. Among abstracts expressing a position on AGW, 97.1% endorsed the consensus that humans are causing global warming. For both abstract ratings and author self-ratings, the share of endorsements among papers expressing a position marginally increased over time.about this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted. Of abstracts taking any position, 97.1% endorse human-caused warming. Authors rating their own full papers reached essentially the same figure, 97.2%, by a different route. The dominant category is silence: two thirds of abstracts state no position on the cause at all. A smaller share of self-rated papers (35.5%) than of abstract ratings expressed no position on AGW. The finding agrees with the earlier Oreskes analysis, which found no disagreeing papers among 928.WHOSE RESULT THIS IS: 8 papers disagreed with the consensus position on AGW (Oreskes 2004 ). - THE RESULT IS THE CITED WORK'S AND THE CITATION TRAILS THE SENTENCE. A plain noun phrase is the subject of the result verb and the attribution is a bracket at the end - 'Neurons within the dorsal temporal pole demonstrated match suppression ... ( Ng et al., 2014 )'. Neither of my two cues could see this shape, so THESE WERE READ RATHER THAN SWEPT: 36 candidates of the shape, 20 read, 11 marked, 9 refused. THE REFUSALS ARE WHY IT IS NOT A CUE: 'Aru2012 findings[16]' is the paper's OWN result with a citation supporting the localisation, 'Natan2015 findings[3]' ends 'consistent with previous findings ( Szymanski et al., 2009 )' where the main clause is the paper's, and 'Drieu2019' x3 are 'One cannot entirely rule out...' - a limitation, not an attribution. About 55% precise, which is a list to read and not a sweep to run. climate abstractsabout this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted.climate abstracts AGWAGW consensus positionconsensus position explicit endorsementexplicit endorsement confidence-intervalabout this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted.confidence-interval p-valueabout this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted.p-value regressionabout this quote: QUOTE RECOVERED FROM THE PAPER'S OWN CHARACTERS. The recorded quote was LETTERS-ONLY IDENTICAL to this stretch of the staged text but not an exact substring of it - spaces, hyphens, ligature glyphs or mathematical operators differed, so it could never anchor. This is the file's own text between the same two letters: not produced by a rule about the damage, copied out of the file. Round-trip checked (the recovered span is letters-only equal to the original) and length-checked. The text as originally recorded is kept in evidence as extracted.regression sample-sizesample-size inter-rater-reliabilityinter-rater-reliability proportionproportion random-samplingrandom-sampling

Quantifying the consensus on anthropogenic global warming in the scientific literature

John Cook , Dana Nuccitelli , Sarah A Green , Mark Richardson , Bärbel Winkler , Rob Painting , Robert Way , Peter Jacobs and Andrew Skuce

Environmental Research Letters , Volume 8 , Number 2 Citation John Cook et al 2013 Environ. Res. Lett. 8 024024 DOI 10.1088/1748-9326/8/2/024024

PDF Opens in a new tab.

Authors

John Cook AFFILIATIONS

Global Change Institute, University of Queensland, Australia

Skeptical Science, Brisbane, Queensland, Australia

School of Psychology, University of Western Australia, Australia

j.cook3@uq.edu.au

Dana Nuccitelli AFFILIATIONS

Skeptical Science, Brisbane, Queensland, Australia

Tetra Tech, Incorporated, McClellan, CA, USA

Sarah A Green AFFILIATIONS

Department of Chemistry, Michigan Technological University, USA

Mark Richardson AFFILIATIONS

Department of Meteorology, University of Reading, UK

Bärbel Winkler AFFILIATIONS

Skeptical Science, Brisbane, Queensland, Australia

Rob Painting AFFILIATIONS

Skeptical Science, Brisbane, Queensland, Australia

Robert Way AFFILIATIONS

Department of Geography, Memorial University of Newfoundland, Canada

Peter Jacobs AFFILIATIONS

Department of Environmental Science and Policy, George Mason University, USA

Andrew Skuce AFFILIATIONS

Skeptical Science, Brisbane, Queensland, Australia

Salt Spring Consulting Ltd, Salt Spring Island, BC, Canada

Article metrics

1519786 Total downloads 0 Video abstract views

Submit

Submit to this Journal

Share this article

Article information

Dates

Received 18 January 2013

Accepted 22 April 2013

Published 15 May 2013

Buy this article in print

Journal RSS

1748-9326/8/2/024024 Abstract

We analyze the evolution of the scientific consensus on anthropogenic global warming (AGW) in the peer-reviewed scientific literature, examining 11 944 climate abstracts from 1991–2011 matching the topics ‘global climate change’ or ‘global warming’. We find that 66.4% of abstracts expressed no position on AGW, 32.6% endorsed AGW, 0.7% rejected AGW and 0.3% were uncertain about the cause of global warming. Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position that humans are causing global warming. In a second phase of this study, we invited authors to rate their own papers. Compared to abstract ratings, a smaller percentage of self-rated papers expressed no position on AGW (35.5%). Among self-rated papers expressing a position on AGW, 97.2% endorsed the consensus. For both abstract ratings and authors’ self-ratings, the percentage of endorsements among papers expressing a position on AGW marginally increased over time. Our analysis indicates that the number of papers rejecting the consensus on AGW is a vanishingly small proportion of the published research.

Export citation and abstract BibTeX RIS

Previous article in issue

Next article in issue

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence . Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Supplementary data

1. Introduction

An accurate perception of the degree of scientific consensus is an essential element to public support for climate policy (Ding et al 2011 ). Communicating the scientific consensus also increases people’s acceptance that climate change (CC) is happening (Lewandowsky et al 2012 ). Despite numerous indicators of a consensus, there is wide public perception that climate scientists disagree over the fundamental cause of global warming (GW; Leiserowitz et al 2012 , Pew 2012 ). In the most comprehensive analysis performed to date, we have extended the analysis of peer-reviewed climate papers in Oreskes ( 2004 ). We examined a large sample of the scientific literature on global CC, published over a 21 year period, in order to determine the level of scientific consensus that human activity is very likely causing most of the current GW (anthropogenic global warming, or AGW).

Surveys of climate scientists have found strong agreement (97–98%) regarding AGW amongst publishing climate experts (Doran and Zimmerman 2009 , Anderegg et al 2010 ). Repeated surveys of scientists found that scientific agreement about AGW steadily increased from 1996 to 2009 (Bray 2010 ). This is reflected in the increasingly definitive statements issued by the Intergovernmental Panel on Climate Change on the attribution of recent GW (Houghton et al 1996 , 2001 , Solomon et al 2007 ).

The peer-reviewed scientific literature provides a ground-level assessment of the degree of consensus among publishing scientists. An analysis of abstracts published from 1993–2003 matching the search ‘global climate change’ found that none of 928 papers disagreed with the consensus position on AGW (Oreskes 2004 ). This is consistent with an analysis of citation networks that found a consensus on AGW forming in the early 1990s (Shwed and Bearman 2010 ).

Despite these independent indicators of a scientific consensus, the perception of the US public is that the scientific community still disagrees over the fundamental cause of GW. From 1997 to 2007, public opinion polls have indicated around 60% of the US public believes there is significant disagreement among scientists about whether GW was happening (Nisbet and Myers 2007 ). Similarly, 57% of the US public either disagreed or were unaware that scientists agree that the earth is very likely warming due to human activity (Pew 2012 ).

Through analysis of climate-related papers published from 1991 to 2011, this study provides the most comprehensive analysis of its kind to date in order to quantify and evaluate the level and evolution of consensus over the last two decades.

2. Methodology

This letter was conceived as a ‘citizen science’ project by volunteers contributing to the Skeptical Science website ( www.skepticalscience.com ). In March 2012, we searched the ISI Web of Science for papers published from 1991–2011 using topic searches for ‘global warming’ or ‘global climate change’. Article type was restricted to ‘article’, excluding books, discussions, proceedings papers and other document types. The search was updated in May 2012 with papers added to the Web of Science up to that date.

We classified each abstract according to the type of research (category) and degree of endorsement. Written criteria were provided to raters for category (table 1 ) and level of endorsement of AGW (table 2 ). Explicit endorsements were divided into non-quantified (e.g., humans are contributing to global warming without quantifying the contribution) and quantified (e.g., humans are contributing more than 50% of global warming, consistent with the 2007 IPCC statement that most of the global warming since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations).

Table 1. Definitions of each type of research category.

Category Description Example

(1) Impacts Effects and impacts of climate change on the environment, ecosystems or humanity ‘…global climate change together with increasing direct impacts of human activities, such as fisheries, are affecting the population dynamics of marine top predators’

(2) Methods Focus on measurements and modeling methods, or basic climate science not included in the other categories ‘This paper focuses on automating the task of estimating Polar ice thickness from airborne radar data…’

(3) Mitigation Research into lowering CO 2 emissions or atmospheric CO 2 levels ‘This paper presents a new approach for a nationally appropriate mitigation actions framework that can unlock the huge potential for greenhouse gas mitigation in dispersed energy end-use sectors in developing countries’

(4) Not climate-related Social science, education, research about people’s views on climate ‘This paper discusses the use of multimedia techniques and augmented reality tools to bring across the risks of global climate change’

(5) Opinion Not peer-reviewed articles ‘While the world argues about reducing global warming, chemical engineers are getting on with the technology. Charles Butcher has been finding out how to remove carbon dioxide from flue gas’

(6) Paleoclimate Examining climate during pre-industrial times ‘Here, we present a pollen-based quantitative temperature reconstruction from the midlatitudes of Australia that spans the last 135 000 years…’

Table 2. Definitions of each level of endorsement of AGW.

Level of endorsement Description Example

(1) Explicit endorsement with quantification Explicitly states that humans are the primary cause of recent global warming ‘The global warming during the 20th century is caused mainly by increasing greenhouse gas concentration especially since the late 1980s’

(2) Explicit endorsement without quantification Explicitly states humans are causing global warming or refers to anthropogenic global warming/climate change as a known fact ‘Emissions of a broad range of greenhouse gases of varying lifetimes contribute to global climate change’

(3) Implicit endorsement Implies humans are causing global warming. E.g., research assumes greenhouse gas emissions cause warming without explicitly stating humans are the cause ‘…carbon sequestration in soil is important for mitigating global climate change’

(4a) No position Does not address or mention the cause of global warming

(4b) Uncertain Expresses position that human’s role on recent global warming is uncertain/undefined ‘While the extent of human-induced global warming is inconclusive…’

(5) Implicit rejection Implies humans have had a minimal impact on global warming without saying so explicitly E.g., proposing a natural mechanism is the main cause of global warming ‘…anywhere from a major portion to all of the warming of the 20th century could plausibly result from natural causes according to these results’

(6) Explicit rejection without quantification Explicitly minimizes or rejects that humans are causing global warming ‘…the global temperature record provides little support for the catastrophic view of the greenhouse effect’

(7) Explicit rejection with quantification Explicitly states that humans are causing less than half of global warming ‘The human contribution to the CO 2 content in the atmosphere and the increase in temperature is negligible in comparison with other sources of carbon dioxide emission’

Abstracts were randomly distributed via a web-based system to raters with only the title and abstract visible. All other information such as author names and affiliations, journal and publishing date were hidden. Each abstract was categorized by two independent, anonymized raters. A team of 12 individuals completed 97.4% (23 061) of the ratings; an additional 12 contributed the remaining 2.6% (607). Initially, 27% of category ratings and 33% of endorsement ratings disagreed. Raters were then allowed to compare and justify or update their rating through the web system, while maintaining anonymity. Following this, 11% of category ratings and 16% of endorsement ratings disagreed; these were then resolved by a third party.

Upon completion of the final ratings, a random sample of 1000 ‘No Position’ category abstracts were re-examined to differentiate those that did not express an opinion from those that take the position that the cause of GW is uncertain. An ‘Uncertain’ abstract explicitly states that the cause of global warming is not yet determined (e.g., ‘…the extent of human-induced global warming is inconclusive…’) while a ‘No Position’ abstract makes no statement on AGW.

To complement the abstract analysis, email addresses for 8547 authors were collected, typically from the corresponding author and/or first author. For each year, email addresses were obtained for at least 60% of papers. Authors were emailed an invitation to participate in a survey in which they rated their own published papers (the entire content of the article, not just the abstract) with the same criteria as used by the independent rating team. Details of the survey text are provided in the supplementary information (available at stacks.iop.org/ERL/8/024024/mmedia ).

3. Results

The ISI search generated 12 465 papers. Eliminating papers that were not peer-reviewed (186), not climate-related (288) or without an abstract (47) reduced the analysis to 11 944 papers written by 29 083 authors and published in 1980 journals. To simplify the analysis, ratings were consolidated into three groups: endorsements (including implicit and explicit; categories 1–3 in table 2 ), no position (category 4) and rejections (including implicit and explicit; categories 5–7).

We examined four metrics to quantify the level of endorsement:

(1)

The percentage of endorsements/rejections/undecideds among all abstracts.

(2)

The percentage of endorsements/rejections/undecideds among only those abstracts expressing a position on AGW.

(3)

The percentage of scientists authoring endorsement/ rejection abstracts among all scientists.

(4)

The same percentage among only those scientists who expressed a position on AGW (table 3 ).

Table 3. Abstract ratings for each level of endorsement, shown as percentage and total number of papers.

Position % of all abstracts % among abstracts with AGW position (%) % of all authors % among authors with AGW position (%)

Endorse AGW 32.6% (3896) 97.1 34.8% (10 188) 98.4

No AGW position 66.4% (7930) — 64.6% (18 930) —

Reject AGW 0.7% (78) 1.9 0.4% (124) 1.2

Uncertain on AGW 0.3% (40) 1.0 0.2% (44) 0.4

3.1. Endorsement percentages from abstract ratings

Among abstracts that expressed a position on AGW, 97.1% endorsed the scientific consensus. Among scientists who expressed a position on AGW in their abstract, 98.4% endorsed the consensus.

The time series of each level of endorsement of the consensus on AGW was analyzed in terms of the number of abstracts (figure 1 (a)) and the percentage of abstracts (figure 1 (b)). Over time, the no position percentage has increased (simple linear regression trend 0.87% ± 0.28% yr −1 , 95% CI, R 2 = 0.66, p < 0.001) and the percentage of papers taking a position on AGW has equally decreased.

Zoom In Zoom Out Reset image size

Figure 1. (a) Total number of abstracts categorized into endorsement, rejection and no position. (b) Percentage of endorsement, rejection and no position/undecided abstracts. Uncertain comprise 0.5% of no position abstracts.

Download figure:

Standard image High-resolution image The average numbers of authors per endorsement abstract (3.4) and per no position abstract (3.6) are both significantly larger than the average number of authors per rejection abstract (2.0). The scientists originated from 91 countries (identified by email address) with the highest representation from the USA ( N = 2548) followed by the United Kingdom ( N = 546), Germany ( N = 404) and Japan ( N = 379) (see supplementary table S1 for full list, available at stacks.iop.org/ERL/8/024024/mmedia ).

3.2. Endorsement percentages from self-ratings

We emailed 8547 authors an invitation to rate their own papers and received 1200 responses (a 14% response rate). After excluding papers that were not peer-reviewed, not climate-related or had no abstract, 2142 papers received self-ratings from 1189 authors. The self-rated levels of endorsement are shown in table 4 . Among self-rated papers that stated a position on AGW, 97.2% endorsed the consensus. Among self-rated papers not expressing a position on AGW in the abstract, 53.8% were self-rated as endorsing the consensus. Among respondents who authored a paper expressing a view on AGW, 96.4% endorsed the consensus.

Table 4. Self-ratings for each level of endorsement, shown as percentage and total number of papers.

Position % of all papers % among papers with AGW position (%) % of respondents % among respondents with AGW position (%)

Endorse AGW a

62.7% (1342) 97.2 62.7% (746) 96.4

No AGW position b

35.5% (761) — 34.9% (415) —

Reject AGW c

1.8% (39) 2.8 2.4% (28) 3.6

a Self-rated papers that endorse AGW have an average endorsement rating less than 4 (1 =explicit endorsement with quantification, 7 = explicit rejection with quantification). b Undecided self-rated papers have an average rating equal to 4. c Rejection self-rated papers have an average rating greater than 4.

Figure 2 (a) shows the level of self-rated endorsement in terms of number of abstracts (the corollary to figure 1 (a)) and figure 2 (b) shows the percentage of abstracts (the corollary to figure 1 (b)). The percentage of self-rated rejection papers decreased (simple linear regression trend −0.25% ± 0.18% yr −1 , 95% CI, R 2 = 0.28, p = 0.01, figure 2 (b)). The time series of self-rated no position and consensus endorsement papers both show no clear trend over time.

Zoom In Zoom Out Reset image size

Figure 2. (a) Total number of endorsement, rejection and no position papers as self-rated by authors. Year is the published year of each self-rated paper. (b) Percentage of self-rated endorsement, rejection and no position papers.

Download figure:

Standard image High-resolution image A direct comparison of abstract rating versus self-rating endorsement levels for the 2142 papers that received a self-rating is shown in table 5 . More than half of the abstracts that we rated as ‘No Position’ or ‘Undecided’ were rated ‘Endorse AGW’ by the paper’s authors.

Table 5. Comparison of our abstract rating to self-rating for papers that received self-ratings.

Position Abstract rating Self-rating

Endorse AGW 791 (36.9%) 1342 (62.7%)

No AGW position or undecided 1339 (62.5%) 761 (35.5%)

Reject AGW 12 (0.6%) 39 (1.8%)

Figure 3 compares the percentage of papers endorsing the scientific consensus among all papers that express a position endorsing or rejecting the consensus. The year-to-year variability is larger in the self-ratings than in the abstract ratings due to the smaller sample sizes in the early 1990s. The percentage of AGW endorsements for both self-rating and abstract-rated papers increase marginally over time (simple linear regression trends 0.10 ± 0.09% yr −1 , 95% CI, R 2 = 0.20, p = 0.04 for abstracts, 0.35 ± 0.26% yr −1 , 95% CI, R 2 = 0.26, p = 0.02 for self-ratings), with both series approaching approximately 98% endorsements in 2011.

Zoom In Zoom Out Reset image size

Figure 3. Percentage of papers endorsing the consensus among only papers that express a position endorsing or rejecting the consensus.

Download figure:

Standard image High-resolution image

4. Discussion

Of note is the large proportion of abstracts that state no position on AGW. This result is expected in consensus situations where scientists ‘ …generally focus their discussions on questions that are still disputed or unanswered rather than on matters about which everyone agrees ’ (Oreskes 2007 , p 72). This explanation is also consistent with a description of consensus as a ‘spiral trajectory’ in which ‘ initially intense contestation generates rapid settlement and induces a spiral of new questions ’ (Shwed and Bearman 2010 ); the fundamental science of AGW is no longer controversial among the publishing science community and the remaining debate in the field has moved to other topics. This is supported by the fact that more than half of the self-rated endorsement papers did not express a position on AGW in their abstracts.

The self-ratings by the papers’ authors provide insight into the nature of the scientific consensus amongst publishing scientists. For both self-ratings and our abstract ratings, the percentage of endorsements among papers expressing a position on AGW marginally increased over time, consistent with Bray ( 2010 ) in finding a strengthening consensus.

4.1. Sources of uncertainty

The process of determining the level of consensus in the peer-reviewed literature contains several sources of uncertainty, including the representativeness of the sample, lack of clarity in the abstracts and subjectivity in rating the abstracts.

We address the issue of representativeness by selecting the largest sample to date for this type of literature analysis. Nevertheless, 11 944 papers is only a fraction of the climate literature. A Web of Science search for ‘climate change’ over the same period yields 43 548 papers, while a search for ‘climate’ yields 128 440 papers. The crowd-sourcing techniques employed in this analysis could be expanded to include more papers. This could facilitate an approach approximating the methods of Doran and Zimmerman ( 2009 ), which measured the level of scientific consensus for varying degrees of expertise in climate science. A similar approach could analyze the level of consensus among climate papers depending on their relevance to the attribution of GW.

Another potential area of uncertainty involved the text of the abstracts themselves. In some cases, ambiguous language made it difficult to ascertain the intended meaning of the authors. Naturally, a short abstract could not be expected to communicate all the details of the full paper. The implementation of the author self-rating process allowed us to look beyond the abstract. A comparison between self-ratings and abstract ratings revealed that categorization based on the abstract alone underestimates the percentage of papers taking a position on AGW.

Lastly, some subjectivity is inherent in the abstract rating process. While criteria for determining ratings were defined prior to the rating period, some clarifications and amendments were required as specific situations presented themselves. Two sources of rating bias can be cited: first, given that the raters themselves endorsed the scientific consensus on AGW, they may have been more likely to classify papers as sharing that endorsement. Second, scientific reticence (Hansen 2007 ) or ‘erring on the side of least drama’ (ESLD; Brysse et al 2012 ) may have exerted an opposite effect by biasing raters towards a ‘no position’ classification. These sources of bias were partially addressed by the use of multiple independent raters and by comparing abstract rating results to author self-ratings. A comparison of author ratings of the full papers and abstract ratings reveals a bias toward an under-counting of endorsement papers in the abstract ratings (mean difference 0.6 in units of endorsement level). This mitigated concerns about rater subjectivity, but suggests that scientific reticence and ESLD remain possible biases in the abstract ratings process. The potential impact of initial rating disagreements was also calculated and found to have minimal impact on the level of consensus (see supplemental information, section S1 available at stacks.iop.org/ERL/8/024024/mmedia ).

4.2. Comparisons with previous studies

Our sample encompasses those surveyed by Oreskes ( 2004 ) and Schulte ( 2008 ) and we can therefore directly compare the results. Oreskes ( 2004 ) analyzed 928 papers from 1993 to 2003. Over the same period, we found 932 papers matching the search phrase ‘global climate change’ (papers continue to be added to the ISI database). From that subset we eliminated 38 papers that were not peer-reviewed, climate-related or had no abstract. Of the remaining 894, none rejected the consensus, consistent with Oreskes’ result. Oreskes determined that 75% of papers endorsed the consensus, based on the assumption that mitigation and impact papers implicitly endorse the consensus. By comparison, we found that 28% of the 894 abstracts endorsed AGW while 72% expressed no position. Among the 71 papers that received self-ratings from authors, 69% endorse AGW, comparable to Oreskes’ estimate of 75% endorsements.

An analysis of 539 ‘global climate change’ abstracts from the Web of Science database over January 2004 to mid-February 2007 found 45% endorsement and 6% rejection (Schulte 2008 ). Our analysis over a similar period (including all of February 2007) produced 529 papers—the reason for this discrepancy is unclear as Schulte’s exact methodology is not provided. Schulte estimated a higher percentage of endorsements and rejections, possibly because the strict methodology we adopted led to a greater number of ‘No Position’ abstracts. Schulte also found a significantly greater number of rejection papers, including 6 explicit rejections compared to our 0 explicit rejections. See the supplementary information (available at stacks.iop.org/ERL/8/024024/mmedia ) for a tabulated comparison of results. Among 58 self-rated papers, only one (1.7%) rejected AGW in this sample. Over the period of January 2004 to February 2007, among ‘global climate change’ papers that state a position on AGW, we found 97% endorsements.

5. Conclusion

The public perception of a scientific consensus on AGW is a necessary element in public support for climate policy (Ding et al 2011 ). However, there is a significant gap between public perception and reality, with 57% of the US public either disagreeing or unaware that scientists overwhelmingly agree that the earth is warming due to human activity (Pew 2012 ).

Contributing to this ‘consensus gap’ are campaigns designed to confuse the public about the level of agreement among climate scientists. In 1991, Western Fuels Association conducted a $510 000 campaign whose primary goal was to ‘ reposition global warming as theory (not fact) ’. A key strategy involved constructing the impression of active scientific debate using dissenting scientists as spokesmen (Oreskes 2010 ). The situation is exacerbated by media treatment of the climate issue, where the normative practice of providing opposing sides with equal attention has allowed a vocal minority to have their views amplified (Boykoff and Boykoff 2004 ). While there are indications that the situation has improved in the UK and USA prestige press (Boykoff 2007 ), the UK tabloid press showed no indication of improvement from 2000 to 2006 (Boykoff and Mansfield 2008 ).

The narrative presented by some dissenters is that the scientific consensus is ‘ …on the point of collapse ’ (Oddie 2012 ) while ‘ …the number of scientific “heretics” is growing with each passing year ’ (Allègre et al 2012 ). A systematic, comprehensive review of the literature provides quantitative evidence countering this assertion. The number of papers rejecting AGW is a miniscule proportion of the published research, with the percentage slightly decreasing over time. Among papers expressing a position on AGW, an overwhelming percentage (97.2% based on self-ratings, 97.1% based on abstract ratings) endorses the scientific consensus on AGW.

Acknowledgments

Thanks to James Powell for his invaluable contribution to this analysis, Stephan Lewandowsky for his comments and to those who assisted with collecting email addresses and rating abstracts: Ari Jokimäki, Riccardo Reitano, Rob Honeycutt, Wendy Cook, Phil Scadden, Glenn Tamblyn, Anne-Marie Blackburn, John Hartz, Steve Brown, George Morrison, Alexander C Coulter, Martin B Stolpe (to name just those who are not listed as (co-)author to this paper).

Show