Is there a scientific consensus on global warming?


By promoting the idea that climate scientists are all in agreement on man-made global warming theory, it might create the impression that there is scientific consensus. But, it hides the wide range of different views that are actually held by the scientific community. 'Mimi and Eustace' cartoons by Nina Paley, shared under Creative Commons, CC-BY-SA.

Many people (including many scientists) believe that there is a strong scientific consensus that increases in atmospheric CO2 concentrations cause dangerous man-made global warming, and that if we don’t urgently reduce our carbon footprint, it will get much worse. But, while it is true that a substantial fraction of climate scientists hold this view, there is actually a wide range of opinions on man-made global warming amongst the scientific community.

For instance, some scientists believe there has been man-made global warming, but that the media descriptions are seriously exaggerated, and that it isn’t an urgent issue. Other scientists believe that global warming is probably due to natural climate variability.

In this essay, we present examples of some of the different views actually held by climate researchers.

1. Is the science settled?

We are constantly inundated with claims in the media, and on the blogs that more than 97% of scientists are all in agreement that humans are warming the planet by using fossil fuels, and that this is dangerous, e.g., see here, here, here, here, here, here, here, here and here.

We are also told that the science on man-made global warming is “settled”, and instead of debating the science, we should be focusing on how to urgently reduce our carbon dioxide emissions:

The science behind climate change is settled, and human activity is responsible for global warming. That conclusion is not a partisan one. – U.S. Environmental Protection Agency Administrator, Lisa Jackson, February 2010

The science is settled. The data is clear. Man-made carbon pollution is real. – Al Gore, 23rd July 2013

For our research, we reviewed and analysed thousands of scientific papers on global warming and climate change. Instead of the alleged “settled science”, we found evidence for many different views on man-made global warming theories.

We agree that there are a lot of scientists who seem to believe that humans are causing dangerous global warming. But, there also seem to be quite a few scientists who don’t believe that.

Some scientists believe in man-made global warming theory, but think that the natural variability of climate has been seriously underestimated, i.e., man-made global warming is not as urgent a crisis as is commonly believed.

Other scientists believe that global warming is a natural process that has nothing to do with human activity.

In this essay, we will give examples of some of the different views of prominent climate researchers on man-made global warming theory. However, first, we should point out that scientists can, and do, change their minds on many scientific issues… and that includes global warming!

2. Scientists can and do change their mind

When the economist John Maynard Keynes was criticised for changing his opinions over time, he would apparently reply,

When my information changes, I alter my conclusions. What do you do, sir? – attributed to John Maynard Keynes by Paul Samuelson

If we discover new information we should be encouraged to change our mind if the new information affects our earlier conclusions.

For this reason, climate scientists are perfectly entitled to change their views on man-made global warming theory, or at the very least keep an open mind on the matter, when they find out new information. Indeed, over the years, a number of prominent climate researchers have openly changed their position on man-made global warming.

For instance, Dr. James Lovelock is a prominent environmentalist, and independent researcher, who in the 1960s was the first scientist to notice that human activity was increasing the concentration of CFCs in the atmosphere, and he also developed the popular Gaia hypothesis.

Until recently, he was a firm believer in the theory that humans are causing catastrophic global warming. In 2006, he even claimed that “…before this century is over billions of us will die and the few breeding pairs of people that survive will be in the Arctic where the climate remains tolerable”.

However, recently he has decided that he had probably been “extrapolating too far”:

“The problem is we don’t know what the climate is doing. We thought we knew 20 years ago. That led to some alarmist books – mine included – because it looked clear-cut, but it hasn’t happened…
The climate is doing its usual tricks. There’s nothing much really happening yet. We were supposed to be halfway toward a frying world now…
– James Lovelock, April 2012 interview for MSNBC

Lovelock still believes in man-made global warming theory, but he believes man-made global warming is a more gradual problem that we can take our time dealing with, and that the claims that we are causing catastrophic climate change are wrong.

Similarly, Prof. Hans von Storch recently stated that the lack of global warming in recent years is causing him to question the reliability of the current climate models:

If things continue as they have been, in five years, at the latest, we will need to acknowledge that something is fundamentally wrong with our climate models. A 20-year pause in global warming does not occur in a single modeled scenario. But even today, we are finding it very difficult to reconcile actual temperature trends with our expectations. – Prof. Hans von Storch in an interview with der Spiegel in June 2013

On the other hand, Prof. Richard Muller has said that he used to be sceptical of man-made global warming theory, but has recently converted to believing that most of the global warming since the Industrial Revolution is man-made:

Call me a converted skeptic. Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming. Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I’m now going a step further: Humans are almost entirely the cause. – Prof. Richard Muller, 28th July, 2012

In other words, climate scientists can (and do) change their opinions on man-made global warming (or any other scientific issue, for that matter).

Also, as we will see in this essay, there is actually quite a wide range of different views held by climate researchers on global warming.

This makes it hard to categorise climate scientists by their views on man-made global warming – not only is there a risk of pigeonholing a scientist’s views in too simplistic a manner, but their views may change over time.

Nonetheless, both of these potential problems are in keeping with the main point of this essay, which is that the science is not “settled” and there is actually quite a lot of debate over man-made global warming theory within the scientific community.

3. Examples of the views of climate scientists on global warming

We have loosely grouped the global warming views of several prominent climate researchers along a scale ranging from those who are convinced global warming is a man-made crisis (“1”) to those who believe that global warming can be entirely explained by natural variability (“5”). We then provide a fairly recent video interview/presentation for each of the researchers we mention.

As we discussed in the previous section, any attempt to categorise groups of scientists is going to be subjective, and some of these researchers may have changed their views since the videos were made. Nonetheless, we have tried our best to ensure that the clips we have selected for each researcher accurately represents their current, publicly-stated, views on global warming.

The video clips range from 2 minutes to half-an-hour, and you may find it too intensive to watch all of the clips in the one sitting. But, we recommend that you view some clips from each of the different groups. Many of these researchers are quite passionate and convincing in their beliefs. So, if you only listen to researchers with similar views, you might be tempted to just agree with them and ignore the others.

By the way, the 1-5 scale bar below is clickable, so if you want to skip to the clips for a particular group, just click on the appropriate number.

Prof. Richard Alley.

Richard Alley is a professor of geosciences at Pennsylvania State University, and has contributed to several of the IPCC reports.

In the 1990s, he and his colleagues found evidence in ice cores which suggested that the climate has sometimes gone through very rapid and abrupt climate changes. This has made him very worried that man-made global warming could cause similarly abrupt changes in the near future. For this reason, he believes that society needs to immediately reduce our fossil fuel usage, or else there could be disastrous consequences.

He has written several books on climate, e.g., “The Two-Mile Time Machine: Ice cores, abrupt climate change, and our future” (2002), and has made a number of TV appearances.

Here is a 7 minute TV interview with him from August 2012:

Prof. Christopher Field

Christopher Field is the founding director of the Carnegie Institution’s Department of Global Ecology, a professor of environmental studies at Stanford University, and Faculty Director of Stanford’s Jasper Ridge Biological Preserve. He is also co-chair of Working Group II of the IPCC.

He believes that humans are the main cause of climate change since the Industrial Revolution, and that man-made global warming is dangerously affecting the planet.

In the video below (5 minutes), he was interviewed by climate blogger, Dr. Joe Romm, in February 2010:

Prof. Stefan Rahmstorf

Stefan Rahmstorf is a professor at Potsdam University, and the head of Earth System Analysis at the Potsdam Institute for Climate Impact Research. He has also contributed to the IPCC reports.

He frequently writes on the KlimaLounge blog (in German) and is a contributor to the RealClimate blog.

He believes that man-made global warming is real, that most of the climate change since at least the 1950s is man-made, and that if we keep going business-as-usual it could destroy human civilisation itself!

He thinks that 97/98% of climate scientists agree with him on this, and that the only scientists who disagree with him on this are “climate deniers”, who are either being funded by the fossil fuel industry, or are basing their decisions on emotions, rather than science.

Below, is a 9 minute interview with him by an Icelandic TV program in October 2013:

Dr. Richard Betts.

Dr. Richard A. Betts is Head of the Climate Impacts strategic area at the U.K Meteorological Office, and has contributed to several of the IPCC reports.

He believes that man-made global warming is real, and is responsible for much of the climate change of the last century. However, he recognises that critics of man-made global warming make valid points, and frequently debates the science with man-made global warming critics in blog discussions and on Twitter.

Here is a clip of him talking at the Climate Week stakeholder reception on 21st November 2011 (5 minutes):

Dr. Eduardo Zorita

Zorita is a Spanish paleoclimatologist who is currently is the head of the Department of Paleoclimate at the GKSS Research Centre in Germany.

He is on the Editorial Board of Wiley Interdisciplinary Reviews Climate Change, is an Editor of Climate of the Past, and a review editor of the journal Climate Research. He has also contributed to the IPCC reports.

He is a founding member of the Klimazwiebel blog which promotes debate between both critics and supporters of man-made global warming. An interesting e-mail interview with him was posted on that blog in October 2012.

Zorita believes that man-made global warming is real, and a serious problem, but he has spoken out about the pressure amongst the climate science community to downplay the uncertainties in the science and to create a public perception of unanimity amongst climate scientists.

In particular he believes that the role of natural variability in recent climate change is hard to quantify, and that the claims that various extreme weather events are due to man-made global warming are unjustified.

He has been critical of some of the paleoclimate studies which have claimed that 20th century temperatures are the hottest in a thousand years. But, he argues that the existence of earlier warm periods (such as the “Medieval Warm Period”) does not in itself disprove man-made global warming:

Prof. Richard Muller.

Muller is a physics professor at the University of California, Berkeley (USA) and a senior scientist at the Lawrence Berkeley National Laboratory. His research has mainly focused on astrophysics, and he has worked with several Nobel laureates in the field. However, he is also very interested in climate science, and developed an alternative theory to the Milankovitch theory for explaining the glacial-interglacial transitions between “ice ages” and non-ice ages every 100,000 years or so. He has also served on national review panels assessing various climate issues, and written several popular science books for non-scientists.

He recently became concerned about the quality of the weather station datasets used for calculating global temperatures, and set up the Berkeley Earth group to develop a more comprehensive, and open-access dataset.

In the past, he has been sceptical about man-made global warming theory. But, he currently believes that man-made global warming is real.

Below is an interview with him by from August 2012 (23 minutes):

Prof. Mike Hulme.

Hulme is a Professor of Climate and Culture in the Department of Geography at King’s College London (UK), and was the founding director of the Tyndall Centre for Climate Change Research from 2000 until 2007. He has contributed to the IPCC reports. In the 1990s, he was a major advocate for “climate action” (i.e., policies to urgently reduce our carbon footprint). However, he has since decided that those policies were not a good idea.

He still believes that man-made global warming is real, and that we should be concerned about it. But, he argues that advocates for climate action and the media overly simplify the science, leading to sensationalist and unrealistically alarming exaggerations.

He is concerned about the way the global warming debate has become so polarised, and worries that the world has become too focused on CO2, when there are many other world problems we should be focusing on. He talks of his main views in this interview for the Canadian TV station, Studio 4 by Fanny Kiefer (recorded in February 2012, 13 minutes):

Prof. Bjorn Lomborg.

Lomborg is a Danish political scientist who researches on enviromental and other global issues. He assumes that man-made global warming theory is valid, although he finds many of the claims attributed to man-made global warming by the media are exaggerated. He believes that we should be somewhat concerned about our carbon footprint, but that it is not as urgent a problem as people think.

When he assesses the various proposals to reduce our carbon footprint from a cost-benefit analysis, he finds none of them are effective. Instead, he argues that there are many more urgent issues which we should be focusing on, e.g., malaria prevention, reducing malnutrition, trying to prevent HIV/AIDS.

In addition, he believes that if we want to genuinely reduce our carbon footprint, the current renewable energy sources are too expensive and unreliable to work as genuine alternatives to fossil fuels. He argues that we should not be pushing to use the current technologies. Instead, we should be investing in more research & development into improving renewable energy sources, so that in the future, they will be viable.

Here is an interview with him by David Letterman on 12th April 2011:

Prof. Judith Curry

Judith Curry is a professor and Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. Her research interests include hurricanes, remote sensing, atmospheric modeling, polar climates, air-sea interactions, and the use of unmanned aerial vehicles for atmospheric research. She is a member of the National Research Council’s Climate Research Committee, and has also contributed to the IPCC reports.

She believes that we should get rid of the consensus seeking approach to climate assessments, try and better understand natural climate variability, and openly discuss the uncertainties of climate science rather than trying to hide them. To help promote this debate, she runs a popular climate blog called Climate Etc.

Below is an interview with her in November 2011 (8 minutes) (unfortunately, there is some background noise):

Prof. John Christy.

Christy is a professor of Atmospheric Science and the director of the Earth System Science Center at the University of Alabama, Huntzville (USA), and together with Dr. Roy Spencer developed the first estimates of global atmospheric temperature trends from satellite data. He has contributed to the IPCC reports.

He believes that there probably should be some man-made global warming from increasing CO2, but he finds that the experimental data shows that there isn’t much. In addition, current climate models make very specific predictions about how man-made global warming should be distributed throughout the atmosphere, but his analysis of the atmospheric data measurements contradict these predictions.

For these reasons, he questions the reliability of man-made global warming theory. Therefore, he believes we shouldn’t be particularly bothered about our carbon footprint.

He summarised his main views on man-made global warming in this interview at an exhibition hosted by Cretaquarium (a public aquarium in Crete, Greece), June 2012 (15 minutes), although unfortunately the sound quality is a bit poor:

Prof. Richard Lindzen.

Lindzen is a recently retired professor of Meteorology at the Massachusetts Institute of Technology (USA), who studies climate and atmospheric science. He has contributed to the IPCC reports.

Lindzen believes that human CO2 emissions have caused some man-made global warming, but that the actual amount is quite modest, tolerable, and even beneficial. He argues that the current computer models which make more alarming predictions are unreliable, and based on the assumption of large positive “water vapour feedbacks”.

According to the greenhouse effect theory, water vapour is a more important greenhouse gas than CO2. But, if global temperatures increase from man-made global warming, this could increase water vapour concentrations, potentially leading to more warming. The current climate models dramatically increase the predicted warming from CO2 through this “positive feedback”. However, Lindzen’s analysis of the experimental data suggests that “negative feedbacks” (e.g., changes in cloud cover) act to decrease the amount of warming from increases in CO2.

His main views on man-made global warming theory were briefly summarised in this 2012 interview by Andrew Bolt (5 minutes):

Prof. Vincent Courtillot.

Courtillot is a French geophysicist, is currently the director of the Institut de Physique du Globe de Paris (France), and from 1998-2001 served as director of research for the French Ministry of National Education.

He recently became interested in the effects of changes in the Earth’s magnetic field on climate, and his studies suggest that much of the recent climate trends of the 20th century could be explained by natural variability. This has prompted him to reconsider the theory of man-made global warming, and he now no longer believes it is necessary to assume CO2 influences climate.

He presented some of his views and findings on global warming during the following talk at the Internationale Klimakonferenz in Berlin, Germany, December 2010 (32 minutes):

Prof. Robert Carter.

Professor Bob Carter is an Emeritus Fellow at the Institute of Public Affairs in Melbourne, Australia. He is a palaeontologist, stratigrapher, marine geologist, environmental scientist and writer.

He agrees that there was some global warming over the 20th century, although he suspects much of the reported global warming is due to urbanization bias. However, he finds that there is no actual experimental evidence that it was “man-made”. It could just as easily have been “natural global warming”.

For that reason, he argues that we should recognise that climate is always changing, regardless of human activity. In other words, “climate change” is a natural phenomenon. Instead of mistakenly trying to “stop” the climate from changing by worrying about our “carbon footprint”, we should be investing more time and money into adapting to the natural climate changes that are always happening.

Here is an interview with him by a New Zealand news programme in April 2008 (9 minutes):

1. Prof. S. Fred Singer.

Singer is an atmospheric and space physicist, the first director of the U.S. Weather Satellite Center, a research professor at George Mason University (USA) and professor emeritus of environmental science at the University of Virginia (USA). He directs the nonprofit Science and Environmental Policy Project, and also acts as a consultant for several think tanks, such as the Heartland Institute. He is a reviewer for the IPCC reports, and also is involved in producing a set of rival reports, the NIPCC reports.

He agrees with man-made global warming supporters that global temperatures are warmer now than they were in the 19th century. However, he argues that most of this global warming occurred during the first half of the 20th century, i.e., when carbon dioxide concentrations were still relatively low. He believes that the claims of unusual late-20th century global warming are mostly an artefact of biases in the weather station records, e.g., urbanization bias.

He argues that climate is always changing, due to natural variability. He has found evidence which suggests to him that the planet naturally alternates between warm and cool periods every 1000-1500 years. He believes that 20th century global warming can be easily explained by this natural climate change, and that there is no need to invoke man-made global warming theory.

He summarised some of his main views on global warming in this interview with U.S. TV station KUSI San Diego’s David Davis in February 2012 (5 minutes):

3. Prof. Henrik Svensmark

Svensmark is the head of the Center for Sun-Climate Studies at the National Space Institute in the Technical University of Denmark. He believes that climate is strongly influenced by changes in solar activity, and that global warming over the 20th century is due to increasing solar activity.

In the 1990s, he proposed a theory to explain how slight changes in solar activity could cause large changes in global temperature. During periods of low activity, the solar wind is unable to repel as many galactic cosmic rays from the Earth’s atmosphere. Svensmark argues that this leads to an increase in the amount of cloud coverage, and that the extra clouds cool the planet by reflecting sunlight back into space. Similarly, during periods of high solar activity, less clouds form, leading to global warming.

He believes that this mechanism could potentially explain recent global warming trends, without invoking man-made global warming theory. Svensmark’s theory has been controversial, and is still the subject of active debate in the scientific literature. But, he also has support from a number of researchers in the climate community.

As an example of how active the debate over his theory is in the scientific literature, Sloan & Wolfendale, 2013 (Open access), which criticises the theory, was published in the journal Environmental Research Letters on 7th November 2013, but coincidentally, on the next day, a new paper by Svensmark – Svensmark et al., 2013 (Abstract; access to ArXiV preprint) – was published in Physics Letters A. has produced and directed the following 6 minute clip summarising his theory:

If the video above doesn’t play, try this link

Unfortunately, the presenters of the above clip included an inappropriate graph (at 1 min 10 sec) which falsely implies that there is a blindingly obvious correlation between temperature and solar activity. This is misleading, because the graphs stop in 1985, and if more data were included the apparent correlation wouldn’t be as straightforward.

We suspect this graph is based on Friis-Christensen & Lassen, 1991 (Abstract; Google Scholar access, which while it is an intriguing paper, is now more than two decades old.

Nonetheless, aside from that, the clip does accurately summarise Svensmarck’s theory.

4. Why do people think there is a scientific consensus?

It seems that instead of there being a 97% scientific consensus amongst climate scientists on man-made global warming, there are actually a lot of different views. This shouldn’t actually be surprising. Science is a dynamic subject in which scientists are supposed to be constantly reassessing and re-evaluating the evidence and their paradigms.

If the science on global warming were truly “settled”, then there would be no need for any further research, and none of the researchers we mentioned in this essay would be bothered with studying it anymore. This is not the case.

So, where did this notion that the scientific community is in unanimous agreement on global warming come from? We think there are several factors:

Peer pressure


Several climate scientists have spoken of how they have felt peer pressure to promote the idea that there is a scientific consensus on global warming. Cartoon by Henry Martin.

One important factor seems to be internal peer pressure amongst the scientific community to present a unified front on the global warming issue.

The Intergovernmental Panel on Climate Change (IPCC) seems to have played a major role in creating this pressure. The United Nations rules require the IPCC to seek unanimous consensus on each of their claims. This is because the purpose of the IPCC reports is to offer scientific advice to policymakers, and policymakers don’t want to base their decisions on contentious claims.

As we discuss in our “What does the IPCC say?” essay, the IPCC reports do not always represent the views of the IPCC authors, but the claims made by the reports are presented as if they are unanimous.

This desire to present a unified front seems to have spread from amongst the IPCC organisation into the larger scientific community. In November 2009, Prof. Judith Curry wrote an open letter describing what she termed “climate tribalism” amongst the climate science community. She described how groups of climate researchers will “circle the wagons and point the guns outward” to protect the members of each “climate tribe” from criticism by anyone outside the tribe. This “Us vs. Them” mentality seems to have contributed to the desire to create a public perception of unity amongst the climate science community.

Prof. Eduardo Zorita has also spoken of the pressure on climate scientists to create the perception of a unified scientific consensus:

I [do not] think anthropogenic climate change is a hoax. On the contrary, it is a question which we have to be very well aware of. But I am also aware that in this thick atmosphere—and I am not speaking of greenhouse gases now—editors, reviewers and authors of alternative studies, analysis, interpretations, even based on the same data we have at our disposal, have been bullied and subtly blackmailed. In this atmosphere, Ph D students are often tempted to tweak their data so as to fit the “politically correct picture”. Some, or many issues, about climate change are still not well known. Policy makers should be aware of the attempts to hide these uncertainties under a unified picture. I had the “pleasure” to experience all this in my area of research. Prof. Eduardo Zorita, December 2009

There is also some anecdotal evidence suggesting that some peer reviewers and editors have been actively trying to prevent critics of man-made global warming theory from publishing their research, e.g., here, here, here, here and here. However, this evidence should be treated cautiously, because the conventional closed peer review system is mostly a confidential one, so it is difficult to know how representative these anecdotal instances are.

The role of the media

There seems to be a popular perception that if a scientist has an opinion about a scientific topic, then they have carefully analysed the data and rigorously assessed the evidence for and against that opinion. However, in the last few decades, science has become an increasingly specialised subject. Often, scientists will only have the time to study their own chosen field of expertise in detail. On other scientific matters, they will then just rely on the media, just like everyone else.

If the media implies that there is a scientific consensus on some topic, then many scientists who have not looked at the topic for themselves will just accept that consensus as being valid, without checking for themselves. Of course, this can in itself lead to a scientific consensus, if enough scientists accept the media presentations as being accurate! But, that is only a superficial consensus, i.e., not one which has been subjected to scientific scrutiny.

For instance, Prof. Nir Shaviv assumed for a long time that man-made global warming theory was valid. It was only when he began actually investigating the data for himself in the early 2000s that he became sceptical about it:

A few years ago if you were to ask me, I would tell you ‘It’s CO2.’ Why? Just because, like everyone else in the public, I listened to what the media had to say. – Prof. Nir Shaviv, “The Great Global Warming Swindle” (2007 documentary)

With this in mind, it is worrying that some journalists seem to feel pressure to avoid discussing weaknesses and uncertainties of man-made global warming theory, for fear that it will be seized upon by critics of man-made global warming, e.g., see this post by Mike Lemonick (there is a response to Lemonick’s post by Prof. Roger Pielke Jr. here).

In any case, it is important to mention the influence of former US Vice President, Al Gore’s “An Inconvenient Truth” 2006 presentation. This presentation involved a best-selling book, an Oscar-winning film, and several speaking tours around the world. In it, Gore argues that humans are causing dangerous man-made global warming, and that we need to urgently reduce our carbon footprint if we are to avoid an environmental catastrophe. In his presentation, Al Gore implies that he is describing the “scientific consensus”, and so this leads to the impression that most scientists agree with what he is saying.

Al Gore’s film has influenced many people, including prominent science writers, such as Prof. Michael Shermer, and as a result, Al Gore was jointly awarded the Nobel Peace Prize (with the IPCC) in 2007. So, it seems to have played a substantial role in creating the perception of a scientific consensus on global warming.

Assuming man-made global warming critics are being non-scientific

Because the perception of a scientific consensus on global warming is so widespread, this has led some people to believe that any criticism of man-made global warming theory is by definition “anti-science”, and part of an apparent “war on science”. This notion seems to have discouraged many people from actually listening to the criticism, and to dismiss it out-of-hand as being non-scientific “denialism”, e.g., see Diethelm & McKee, 2009 (Open access).

A problem with this theory is that much of the criticism of man-made global warming theory is definitely not “anti-science”, and seems to be based on careful consideration of the scientific data. Even if you disagree with the views of the researchers we discussed in this essay who are critical of man-made global warming theory, we hope you agree that they are reputable scientists and that their criticisms are science-based.

To rationalise the apparent paradox that man-made global warming criticism is allegedly “anti-science”, but the critics themselves are often scientists, some people have proposed that the critics are deliberately spreading “misinformation” as part of a concerted conspiracy.

One such conspiracy theory is that the critics are ideologically driven, e.g., see Oreskes & Conway’s “Merchants of Doubt” book. Another is that they are in the pay of the fossil fuel industry, e.g., see Ross Gelbspan’s “Boiling Point” or Hoggan and Littlemore’s “Climate Cover-up” books.

When these conspiracy theories are analysed objectively, they typically turn out to be mostly conjecture and usually are based on the selection of cherry-picked quotes and figures taken out-of-context, e.g., see here or here for some attempts to place these conspiracy theories in actual context.

In any case, the popular perception that criticism of man-made global warming theory is “non-scientific” seems to have encouraged many people to dismiss the criticism out-of-hand. Perhaps this has contributed to the scientific consensus notion.

Confusion over terminology

Another factor seems to be confusion over exactly what different scientists mean by “global warming” and “climate change”. Neither of these terms explicitly refers to human activity. The climate has been changing since long before humans existed, and will continue to do so in the future. For this reason, it should not be too surprising if the world goes through periods of natural global warming or global cooling.

However, many people seem to assume that “global warming” and “climate change” are just synonyms for “man-made global warming”.

One reason for this could be because when Prof. Wally Broecker coined the term “global warming” in the 1970s, in his Broecker, 1975 (Abstract; .pdf available from Columbia University) paper, entitled “Climatic Change: Are We on the Brink of a Pronounced Global Warming?”, he was referring to man-made global warming from carbon dioxide.

Another reason might be that many people find it easier to say “global warming”, rather than the more accurate, but cumbersome, “man-made global warming” or “anthropogenic global warming”.

Whatever the reasons, “global warming” is not the same as “man-made global warming”. It is simply a term to describe an increase in globally-averaged temperatures. If we identify a global warming or global cooling trend, that doesn’t in itself tell us what the cause of the trend was!

Consider the following theories:

  • Global warming is all man-made
  • Global warming is partly man-made, and partly natural
  • Global warming is an entirely natural process

All of the scientists we mentioned in this essay have said that they believe there has been some “global warming” since the 19th century. So, if you believe that “global warming” is just a short-hand version of “man-made global warming”, then you might think that they were all in agreement about man-made global warming.

That would be a mistake. Some scientists argue that there has been a natural global warming trend. Other scientists argue that there has been some man-made global warming, but that most of the global warming is natural in origin, and that man-made global warming will only gradually become significant over the next century or so.

Saying “there has been some global warming” is not the same thing as saying “we’re dangerously warming the planet and we have to do something urgently”…

Overconfidence in computer models


The results of a computer model often have a certain mystique and inspire awe amongst people who are unfamiliar with them. Photo of a IBM Blue Gene/P supercomputer installation in the Argonne National Laboratory, taken from Wikimedia Commons.

Finally, the current climate models were mostly developed by researchers who explicitly assumed that carbon dioxide concentrations are the main driver of climate. As a result, their computer predictions of future climate trends show dramatic global warming roughly proportional to projected carbon dioxide concentrations in the future.

In recent years, computer models have gained a mystique amongst many in the scientific community, as well as in the general public. There is a popular assumption that the climate models represent the “scientific consensus”. As the French geopolitician, Pierre Gallois, put it [Updated 13th Aug 2014 to include Pierre Gallois citation:]:

“If you put tomfoolery into a computer nothing comes out but tomfoolery. But this tomfoolery, having passed through a very expensive machine, is somehow ennobled and none dare criticize it.” – Pierre Gallois, “Science et vie”, reprinted in Reader’s Digest (pre-1973?)

The climate modelling community don’t share this mystique. Nonetheless, there seems to be a desire amongst climate modellers to promote the idea that the models are useful, meaningful, and somewhat reliable. This desire seems to have led to a tendency to downplay the known limitations and uncertainties of the models.

For instance, Shackley et al., 1999 surveyed climate modellers about one type of fudge factor, called a “flux adjustment”, which was used by many of the climate models in the 1990s.

If the modellers didn’t put these arbitrary adjustments into their models, the model simulations had a tendency to give unrealistic results after a few simulated “decades”. But, the adjustments themselves had no physical justification. Critics of the models had suggested that this indicated the models were still too simplistic for accurately describing the current climate, let alone being used for predicting future climate trends.

For this reason, Shackley et al. found that many climate modellers didn’t want to talk openly about their adjustments, in case critics of man-made global warming (who they referred to as “climate contrarians”) would use them to question the reliability of the models:

… many of our respondents expressed a preference for keeping discussion of the issue of flux adjustments within the climate modeling community, apparently fearing that climate contrarians would exploit the issue in the public domain. – Shackley et al., 1999 (Abstract; Google Scholar access)


Each of these factors has probably contributed to the formation of the popular notion of “the scientific consensus on man-made global warming”. There are probably many other factors too. But, regardless, it is an incorrect notion. Amongst researchers who have actually considered climate change, there is actually a significant range of opinions about man-made global warming theory.

5. Conclusions

There is a wide spectrum of opinions on man-made global warming amongst the scientific community. The science is not settled.

Moreover, the various interviews and quotes we presented in this essay were all made before we published our new findings on climate change. We believe that many scientists who were previously worried about man-made global warming theory will change their minds when they consider the results of our papers.

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