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LETTER
Greater than 99% consensus on human caused climate change
in the peer-reviewed scientic literature
Mark Lynas1,∗, Benjamin Z Houlton2and Simon Perry3
1Visiting Fellow, Cornell University, Global Development, Alliance for Science, B75 Mann Library, Ithaca, NY 14850,
United States of America
2Cornell University, Department of Ecology and Evolutionary Biology and Department of Global Development, Cornell University,
Ithaca, NY 14850, United States of America
3Alliance for Science, Ithaca, NY 14850, United States of America
∗Author to whom any correspondence should be addressed.
E-mail: ml866@cornell.edu
Keywords: global warming, climate change, scientific consensus
Supplementary material for this article is available online
Abstract
While controls over the Earth’s climate system have undergone rigorous hypothesis-testing since
the 1800s, questions over the scientific consensus of the role of human activities in modern climate
change continue to arise in public settings. We update previous efforts to quantify the scientific
consensus on climate change by searching the recent literature for papers sceptical of
anthropogenic-caused global warming. From a dataset of 88125 climate-related papers published
since 2012, when this question was last addressed comprehensively, we examine a randomized
subset of 3000 such publications. We also use a second sample-weighted approach that was
specifically biased with keywords to help identify any sceptical peer-reviewed papers in the whole
dataset. We identify four sceptical papers out of the sub-set of 3000, as evidenced by abstracts that
were rated as implicitly or explicitly sceptical of human-caused global warming. In our sample
utilizing pre-identified sceptical keywords we found 28 papers that were implicitly or explicitly
sceptical. We conclude with high statistical confidence that the scientific consensus on
human-caused contemporary climate change—expressed as a proportion of the total
publications—exceeds 99% in the peer reviewed scientific literature.
1. Introduction
The extent of the scientific consensus on human-
caused climate change is of great interest to society.
If there remains substantial genuine scientific doubt
about whether modern climate change is human-
caused, then the case for mitigation of greenhouse
gas emissions is weakened. By contrast, a widely-held
consensus view in the peer-reviewed literature inval-
idates alternative arguments which claim that there
is still significant debate in the scientific community
about the reality of anthropogenic climate change
(ACC).
The question of the cause of observed and pre-
dicted global warming and precipitation change is
still highly politically salient. A Gallup poll published
in April 2021 found that there has been a deepening
of the partisan divide in American politics on whether
observed increases in the planet’s temperature since
the Industrial Revolution are primarily caused by
humans [1]. Among elected U.S. politicians the divide
is similarly stark: according to the Center for Amer-
ican Progress there were 139 elected officials in the
117th Congress (sitting in 2021), including 109 rep-
resentatives and 30 senators, ‘who refuse to acknow-
ledge the scientific evidence of human-caused climate
change’ [2]. In 2016 Pew Research found that only
27% of U.S. adults believed that ‘almost all’ scientists
agreed that climate change is due to human activity
[3].
Many efforts have been made over the years to
quantify the extent of the scientific consensus on ACC
[4,5]. These are comprehensively reviewed in a paper
published in 2016 entitled ‘Consensus on consensus’
© 2021 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
[6]. It has additionally been argued that perception
of scientific consensus is a ‘gateway belief ’ motivating
wider public support for mitigation of climate change
[7]. While scientific consensus does not sensu stricto
prove a statement about the physical world, science-
based anlaysis and hypothesis testing is capable of dis-
proving alternative constructs, which could explain a
given observation, either absolutely or relatively [8].
Hence, quantifying the scientific consensus clarifies
the extent of any dissent in the scientific community
in the process of disproval, and the plausible validity
of alternative hypotheses in the face of scientific scru-
tiny, observations, and testing over time.
The most recent well-known effort to quantify
the consensus was published in 2013, encompassing
papers appearing in the peer-reviewed literature
between 1991 and 2012, and sparked the famous
headline that 97% of the world’s science supported
the climate change consensus [9]. The ‘97% con-
sensus’ view (published by Cook et al 2013, referred
to hereafter as C13) had a big impact on global aware-
ness of the scientific consensus on the role of green-
house gases in causing climate change and was extens-
ively covered in the media. Our primary motivation
for this current study was to re-examine the literature
published since 2012 to ascertain whether any change
in the scientific consensus on climate change is dis-
cernible.
2. Method
Previous attempts to quantify the consensus on cli-
mate change have employed many different method-
ologies, varying from expert elicitation to examina-
tion of abstracts returned by a keyword search. We
base our methodology on C13 with some import-
ant refinements. We searched the Web of Science for
English language ‘articles’ added between the dates of
2012 and November 2020 with the keywords ‘climate
change’, ‘global climate change’ and ‘global warm-
ing’. C13 used the latter two phrases but not ‘cli-
mate change’ without the preceding ‘global’. (As dis-
cussed below, this was justified post-facto in our study
because the majority of sceptical papers we found
would not have been returned had we used the same
search phrases as C13.) This wider set of search terms
yielded a total of 88125 papers, whereas C13 identi-
fied a total of 11944 abstracts from papers published
over the years 1991 and 2011. (Using our expanded
search terms over the same 1991–2011 time period as
C13 would have yielded 30627 results.)
Given the large number of papers found using our
approach we randomly sub-sampled 3000 abstracts
out of the 88125 total papers identified in our search,
and subsequently categorized them in accordance
with C13 (See table 1).
As per C13 we rated the abstracts of papers,
assigning them numbers according to their level of
implicit or explicit endorsement or rejection of ACC
(table 2). Abstracts were rated with only the title and
abstract visible; information about authors, date and
journal were hidden at this stage.
To further extend our approach for identifying
as many sceptical papers as possible within the full
dataset, we created an algorithm to identify keywords
within the papers rated by C13 as sceptical that had
appeared more often in sceptical papers than con-
sensus papers. The software counted the appearance
of every word in the title, author list and abstract of
every sceptical paper. For each word that appeared in
at least two papers, the algorithm counted the num-
ber of sceptical and consensus papers it appeared
in to calculate its predictive power. We took the
150 most predictive words, then manually reviewed
them to remove words that appeared to be there
by chance (e.g. ‘walk’ and ‘nearest’) leaving those
we believed could be predictively useful (e.g. ‘cos-
mic’ and ‘rays’). A second algorithm then scored all
88125 papers (including the 3000 sampled separately
earlier) based on the appearance of the predictive
words. (See supplementary info for precise details of
this exercise (available online at stacks.iop.org/ERL/
16/114005/mmedia)). We then rated and categorized
the 1000 papers with the highest score using the same
approaches from C13 as detailed in tables 1and 2.
As stated earlier, this approach was taken in order to
increase the chances of us finding sceptical papers in
the full dataset, allowing for a robust assessment and
inclusion of any dissent.
In contrast to C13, we did not perform an author
elicitation survey asking authors to carry out a self-
rating of their papers.
3. Results
3.1. Results of random sampling
Our random sample of 3000 papers revealed a total
of 282 papers that were categorized as ‘not climate-
related’. These false-positives occurred because, even
though the climate keywords occurred in their
title/abstracts, the published articles dealt with social
science, education or research about people’s views
on climate change rather than original scientific
work. Hence, we excluded these papers in accordance
with C13’s approach. We then assessed the remain-
ing total of 2718 papers in the data set and found
four that argued against the scientific consensus
of ACC.
The ratings and categorizations for the 3000 ran-
domly sampled papers are shown in table 3. Note that
‘not climate-related’ papers are displayed in table 3for
completeness. Figure 1shows the same data, but with
‘not climate-related’ papers excluded.
Our estimate of the proportion of consensus
papers was 1 −(4/2718) =99.85%. The 95% con-
fidence limits for this proportion are 99.62%–99.96%
2
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
Table 1. Categorization of climate papers, as per C13.
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 modelling
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 CO2emissions or
atmospheric CO2levels
‘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 multi-
media 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 quant-
itative temperature reconstruction from
the midlatitudes of Australia that spans
the last 135 000 years…’
Table 2. Rating of climate papers, as per C13.
Level of endorsement Description Example
(1) Explicit endorsement
with quantification
Explicitly states that humans are the
primary cause of recent global warm-
ing
‘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 anthropo-
genic global warming/climate change as a
known fact
‘Emissions of a broad range of green-
house gases of varying lifetimes contrib-
ute to global climate change’
(3) Implicit endorsement Implies humans are causing global
warming. e.g. research assumes green-
house gas emissions cause warming
without explicitly stating humans are
the cause
‘…carbon sequestration in soil is import-
ant for mitigating global climate change’
(4a) No position Does not address or mention the cause of
global warming
—
(4b) Uncertain Expresses position that humans’ role in
recent global warming is uncertain/un-
defined
‘While the extent of human-induced
global warming is inconclusive…’
(5) Implicit rejection Implies humans have had a minimal
impact on global warming without say-
ing 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 cata-
strophic 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 CO2
content in the atmosphere and the
increase in temperature is negligible in
comparison with other sources of carbon
dioxide emission’
3
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
Table 3. Results of rating and categorization of 3000 abstracts.
Rating/categorization # of abstracts
1—Explicit endorsement with
quantification
19
Impacts 7
Methods 9
Mitigation 3
2—Explicit endorsement without
quantification
413
Impacts 204
Methods 78
Mitigation 124
Not climate-related 4
Opinion 1
Paleoclimate 2
3—Implicit endorsement 460
Impacts 95
Methods 119
Mitigation 199
Not climate-related 43
Paleoclimate 4
4a—No position 2104
Impacts 915
Methods 790
Mitigation 60
Not climate-related 235
Opinion 3
Paleoclimate 101
5—Implicit rejection 2
Methods 2
6—Explicit rejection without quanti-
fication
1
Paleoclimate 1
7—Explicit rejection with
quantification
1
Methods 1
Grand Total 3000
(see R code in supplementary info), therefore it is
likely that the proportion of climate papers that
favour the consensus is at least 99.62%.
Recalculating at the 99.999% confidence level
gives us the interval 99.212%–99.996%, therefore it
is virtually certain that the proportion of climate
papers that do not dispute that the consensus is above
99.212%.
If we repeat the methods of C13 and further
exclude papers that take no position on AGW (i.e.
those rated 4a), we estimate the proportion of con-
sensus papers to be 99.53% with the 95% confidence
interval being 98.80%–99.87%.
3.2. Keywords indicating scepticism
We reviewed the 1000 studies that our keyword
matching software identified as most likely to be scep-
tical out of the entire 88125 dataset. After manual
review, 28 sceptical papers within the most likely 1000
papers were identified, with the majority being in the
top rows of the dataset. The first paper was sceptical,
as were 12 out of the first 50, and 16 out of the first
100. (See supplementary data for the full list.) Table 4
shows how the 1000 studies that the keywords found
to be most likely to be sceptical were rated.
In other words, the predictive keywords success-
fully allowed us to identify a total of 28 papers from
the full dataset of 88125 which appeared implicitly or
explicitly sceptical of ACC. Only one of these papers
had already appeared in the first 3000 randomized
sample. While we are aware that this approach does
not reveal all sceptical papers that exist in the full
dataset, it provides an absolute upper bound to the
percentage of papers that agree with the consensus.
Knowing that at least 31 (including the 3 additional
papers found in the random sample) out of the full
88125 dataset are sceptical, we can say the consensus
on ACC is at most 99.966%.
4. Discussion
Our analysis demonstrates >99% agreement in the
peer-reviewed scientific literature on the principal
role of greenhouse gas (GHG) emissions from human
activities in driving modern climate change (i.e.
since the Industrial Revolution). This result further
advances our understanding of the scientific con-
sensus view on climate change as evidenced by the
peer reviewed scientific literature, and provides addi-
tional evidence that the statements made by the
Intergovernmental Panel on Climate Change (see
below) accurately reflect the overwhelming view of
the international scientific community. We conclude
that alternative explanations for the dominant cause
of modern (i.e., post-industrial) climate change bey-
ond the role of rising GHG emissions from human
activities are exceedingly rare in the peer-reviewed
scientific literature.
Previous researchers have debated how to define
and therefore quantify ‘consensus’ in the scientific lit-
erature on an array of issues. While C13 define con-
sensus rather narrowly as explicit or implicit agree-
ment, a broader definition can be employed which
defines consensus as lack of objection to a prevailing
position or worldview. In 2015 James Powell argued
for this broader definition, pointing out that the C13
methodology, if applied to other scientific research
areas such as plate tectonics or evolution, would fail
to find consensus because few authors of papers in the
expert literature feel the need to re-state their adher-
ence in both cases to what has long been universally-
accepted theory [10].
In a rejoinder to this critique, several C13 authors
argued that their narrower definition of consensus
was still relevant in other well-established fields if
both implicit and explicit agreement was included
[11]. Therefore in plate tectonics, for example, sea-
floor spreading, mountain-building by means of con-
tinental collision, subduction etc, could be implicitly
supportive of a consensus on the reality of the theory
of plate tectonics without this necessarily being expli-
citly stated.
4
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
Figure 1. Ratings and categorizations given to 2718 randomly-sampled climate abstracts.
In our paper, for the sake of clarity and comparab-
ility with previous literature, we present results using
both approaches. However, having reviewed, rated
and categorized several thousand papers we believe
that there is now a stronger case for the broader
approach given how widely accepted ACC has become
in the peer-reviewed literature. For example, a major-
ity of the papers we categorized as being about
‘impacts’ of climate change did not state a position
on whether the phenomenon they were studying—
the changing climate—was human-caused. It seems
highly unlikely that if researchers felt sceptical about
the reality of ACC they would publish numerous
studies of its impacts without ever raising the ques-
tion of attribution.
In other words, given that most 4a (‘no posi-
tion’) ratings do not either explicitly or implicitly dif-
fer from the consensus view of GHG emissions as
the principal driver of climate change it does not fol-
low in our view that these analyses should be a priori
excluded from the consensus. In another example, we
gave rating ‘2’ (‘explicit endorsement without quan-
tification’) to all papers referencing future emissions
scenarios in their abstracts, because emissions scen-
arios by definition imply an evaluation of humanity’s
role in GHG emissions and their subsequent impact
on climate. Thus the authors choice of wording on
emissions scenarios or other issues implying human
causation to climate change in the abstracts of their
climate impact studies might lead to arbitrariness if
these were taken as the sole indicators of the authors’
adherence to the consensus on ACC.
In addition, decisions about whether to give rat-
ing ‘3’ (‘implicit endorsement’) are subjective in that
the rating of a position on ACC is considered to
be implied by the authors without this being expli-
citly stated in the abstract of a paper. Thus subject-
ive judgements by those doing the ratings about the
implicit meanings communicated by abstract word-
ing choices of paper authors are critical to the numer-
ical consensus result obtained using C13’s method,
potentially introducing a source of bias. It is unclear
to us why this is preferable to defining consensus
in a clearer and more objectively transparent way as
simply the absence of clearly-stated rejection or dis-
agreement.
We also note that our keyword choices, in particu-
lar not requiring the word ‘global’ in front of ‘climate
change’ led to our discovery of many sceptical papers
that would not have been identified by searches only
of ‘global warming’ and ‘global climate change’. This
suggests—but does not prove—that a number of
5
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
Table 4. Ratings and categorizations for the 1000 abstracts most
likely to be sceptical.
Rating/categorization # of abstracts
1—Explicit endorsement with
quantification
7
Impacts 4
Methods 3
2—Explicit endorsement without
quantification
69
Impacts 24
Methods 35
Mitigation 7
Not climate related 1
Paleoclimate 2
3—Implicit endorsement 134
Impacts 32
Methods 55
Mitigation 33
Not climate related 9
Paleoclimate 5
4a—No position 760
Impacts 156
Methods 276
Mitigation 61
Not climate related 70
Paleoclimate 197
4b—Uncertain 2
Methods 2
5—Implicit rejection 18
Methods 17
Paleoclimate 1
6—Explicit rejection without
quantification
6
Methods 5
Paleoclimate 1
7—Explicit rejection with quantifica-
tion
4
Methods 4
Grand Total 1000
sceptical papers may have been missed in the ori-
ginal C13 study. However, these minor disagreements
aside, we are indebted to C13 for the rigor of their
methodology, much of which we re-employ directly
here.
4.1. Review of sceptical papers
In supplementary table 1we present the full list of all
31 sceptical papers we found in our dataset. An in-
depth evaluation of their merits is outside the scope
of this paper, and could be an interesting area for
further work. We note some recurring themes how-
ever, such as the hypothesis that changes in cosmic
rays are significantly influencing the Earth’s changes
in climate, that the Sun is driving modern climate
change, or that natural fluctuations are somehow
involved. An additional area of research might invest-
igate how far these themes in the published literature
are reflected in popular discourse outside of the sci-
entific community.
5. Conclusion
Our results confirm, as has been found in numerous
other previous studies of this question, that there is
no significant scientific debate among experts about
whether or not climate change is human-caused. This
issue has been comprehensively settled, and the real-
ity of ACC is no more in contention among scientists
than is plate tectonics or evolution. The tiny num-
ber of papers that have been published during our
time period which disagree with this overwhelming
scientific consensus have had no discernible impact,
presumably because they do not provide any con-
vincing evidence to refute the hypothesis that—in
the words of IPCC AR5—‘it is extremely likely that
human influence has been the dominant cause of the
observed warming since the mid-20th century’ [12],
and, most recently in IPCC AR6—‘it is unequivocal
that human influence has warmed the atmosphere,
ocean and land’ [13].
Our finding is that the broadly-defined scientific
consensus likely far exceeds 99% regarding the role
of anthropogenic GHG emissions in modern cli-
mate change, and may even be as high as 99.9%. Of
course, the prevalence of mis/disinformation about
the role of GHG emissions in modern climate change
is unlikely to be driven purely by genuine scientific
illiteracy or lack of understanding [14]. Even so, in
our view it remains important to continue to inform
society on the state of the evidence. According to the
IPCC AR6 summary and many other previous stud-
ies, mitigating future warming requires urgent efforts
to eliminate fossil fuels combustion and other major
sources of anthropogenic greenhouse gas emissions.
Our study helps confirm that there is no remaining
scientific uncertainty about the urgency and gravity
of this task.
Data availability statement
All data that support the findings of this study are
included within the article (and any supplementary
files).
Acknowledgments
The authors would like to thank David Colquhoun
for helpful discussions about the statistical methodo-
logy. We would also like to thank John Cook for use-
ful comments on an early draft, and Sarah Evanega
at the Alliance for Science. Support for the Alliance
for Science is provided by the Bill & Melinda Gates
Foundation.
Author contributions
M L conceived the paper. S P wrote software and
algorithms for data extraction and performed data
analysis. M L performed ratings and categorizations.
B H, S P and M L wrote the paper.
6
Environ. Res. Lett. 16 (2021) 114005 M Lynas et al
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