Posts Tagged ‘Consensus’

FAQ for the article “Scientists’ Views about Attribution of Global Warming”

August 11, 2014

published in Environmental Science and Technology (open access), DOI: 10.1021/es501998e, Supporting Information here.

A formal version of the FAQ is also available at the website of the PBL Netherlands Environmental Assessment Agency. A blog post with a brief description of the main conclusions is here.



1. What are the objectives of this survey?

The PBL aimed to characterize the spectrum of scientific opinion about physical climate science issues. The research was focused on issues that are a frequent topic of public debate, and explored questions such as:

  • On which issues is there widespread agreement amongst scientists?
  • On which issues do scientists hold varied opinions?
  • How does the spectrum of scientific opinion compare to IPCC assessments?
  • How do scientists view skeptical arguments and viewpoints?

2. What is the relevance of an opinion survey or of measurement of consensus in trying to assess the science?

Science is based on the critical evaluation of available evidence in the context of existing knowledge. It is not “just an opinion.” With this survey, we tried to identify how scientists assess the different viewpoints that exist in public discussions of climate science. If the evidence for a certain viewpoint has become sufficiently strong and stable over time, the scientists’ aggregated opinion could be expected to reflect that.

3. Are the survey results publicly available?

The full survey results are not publicly available, because the PBL intends to use the data for further analyses.
The ‘straight counts’ for every question (i.e. the number of responses for each answer option) will be made publicly available in the near future. This is not segregated in different sub-groups.



4. How does this study compare to the often-quoted 97% consensus?

Our results are consistent with similar studies, which all find high levels of consensus among scientists, especially among scientists who publish more often in the peer-reviewed climate literature.

Cook et al. (2013) found that 97% of papers that characterized the cause of recent warming indicated that it is due to human activities. (John Cook, the lead author of that analysis, is co-author on this current article.) Similarly, a randomized literature review found zero papers that called human-induced climate change into question (Oreskes, 2004).

Other studies surveyed scientists themselves. For instance, Doran and Kendall-Zimmermann (2009) found lower levels of consensus for a wider group of earth scientists (82% consensus) as compared to actively publishing climatologists (97% consensus) on the question of whether or not human activity is a “significant contributor” to climate change. Our results are also in line with those of e.g. Bray and von Storch (2008) and Lichter (2007).

In our study, among respondents with more than 10 peer-reviewed publications (half of total respondents), 90% agree that greenhouse gases are the largest – or tied for largest – contributor to recent warming. The level of agreement is ~85% for all respondents.

While these findings are consistent with other surveys, several factors could explain the slight differences we found:

  • Surveys like ours focus on opinions of individual scientists, whereas in a literature analyses the statements in individual abstracts are tallied. Literature analyses have generally found higher levels of consensus than opinion surveys, since the consensus is stronger amongst more heavily published scientists.
  • This study sets a more specific and arguably higher standard for what constitutes the consensus position than other studies. For instance, Doran and Kendall-Zimmermann (2009) asked about human activity being a “significant contributor” to global warming, and Anderegg et al. (2010) investigated signatories of public statements, while we asked specifically about the degree to which greenhouse gases are contributing to climate change in comparison with other potential factors.
  • Contrarian viewpoints are somewhat overrepresented in our survey and they may have overestimated their self-declared level of expertise (see question 9).

5. How is the consensus or agreement position defined?

The consensus position was defined in two ways:

  • Greenhouse gases contributed more than 50% to global warming since the mid-20th (Question 1). This is analogous to what was written in IPCC AR4.
  • Greenhouse gases have caused strong or moderate warming since pre-industrial times (Question 3). “Moderate” warming was only interpreted as the consensus position if no other factor was deemed to have caused “strong” warming. This response means that greenhouse gases were considered the strongest –or tied for strongest- contributor to global warming.

The former definition exactly mirrors the main attribution statement in IPCC AR4 and served as a ‘calibration’ for the latter.

6. What does “relative response” mean on the y-axis of many Figures?

This gives the percentage of the respondents (often within a certain sub-group) for the specific answer option. We opted to show the relative response rather than the absolute response to enable comparing the responses of different sub-groups (with differing group sizes as denoted by N=…) within one graph.

7. What are “undetermined” answers?

Those are the sum of responses “I don’t know”, “unknown” and “other”.

8. Why do IPCC AR4 authors show a higher consensus than the other respondents?

AR4 authors are generally domain experts, whereas the survey respondents at large comprise a very broad group of scholars, including for example scientists studying climate impacts or mitigation. Hence we consider this to be an extension of the observation -in this study and in e.g. Anderegg et al. (2010) and Doran and Kendall-Zimmermann (2009) – that the more expert scientists report stronger agreement with the IPCC position. Moreover, on the question of how likely the greenhouse contribution exceeded 50%, many respondents provided a stronger statement than was made in AR4. Using a smaller sample of scientists, Bray (2010) found no difference in level of consensus between IPCC authors and non-authors.

9. How reliable are the responses regarding the respondent’s area of expertise and number of peer-reviewed publications?

Respondents were tagged with expertise fields, though these were in many cases limited and not meant to be exhaustive. These tags were mainly used to ensure that the group of respondents was representative of the group that the survey was sent to. A subset of respondents was also tagged with a Google Scholar metric. Those who were tagged as “unconvinced” reported more expertise fields than the total group of respondents and also a higher number of publications compared to their Google Scholar metrics, if available (see Supplemental Information).

10. Since most scientists agree with the mainstream and therefore most media coverage is mainstream, what is the problem with “false balance”?

Scientists with dissenting opinions report receiving more media attention than those with mainstream opinions. This results in a skewed picture of the spectrum of scientific opinion. Whether that is problematic is in the eye of the beholder, but it may partly explain why public understanding lags behind scientific discourse (e.g. the “consensus gap”).


Survey Respondents

11. How many responses did you get to the survey?

Out of 6550 people contacted, 1868 filled out the survey (either in part or in full).

12. How did you compile the list of people to be surveyed?

Respondents were selected based on

  • keyword search in peer-reviewed publications (“global climate change” and “global warming”)
  • recent climate literature (various sources)
  • highly cited climate scientists (as listed by Jim Prall)
  • public criticisms of mainstream climate science (as listed by Jim Prall)

13. Are all of the survey invitees climate scientists?

The vast majority of invitees are scientists who published peer-reviewed articles about some aspect of climate change (this could be climate science, climate impacts, mitigation, etc.). Not all of them necessarily see themselves as climate scientists.

14. Why did you invite non-scientist skeptics to take part in the survey?

They were included in the survey to ensure that the main criticisms of climate science would be included. They constitute approximately 3% of the survey respondents. Viewpoints that run counter to the prevailing consensus are therefore somewhat magnified in our results.

15. How representative are the survey responses of the “scientific opinion”?

It’s difficult to ascertain the extent to which our sample is representative, especially because the target group is heterogeneous and hard to define. We have chosen to survey the wider scientific field that works on climate change issues. Due to the criteria we used and the number of people invited we are confident that our results are indeed representative of this wider scientific field studying various aspects of global warming. We checked that those who responded to the survey were representative of the larger group of invitees by using various pieces of meta-information.

16. Did you take into account varying levels of expertise of respondents?

Respondent were asked to list their area(s) of expertise and their number of peer-reviewed publications. These and other attributes were used to interpret differences in responses.

17. How did you prevent respondents from manipulating the survey results, e.g. by answering multiple times?

An automatically generated, user specific token ensured that respondents could only respond once.

18. How did you ensure respondent anonymity?

Survey responses were analyzed by reference to a random identification number.


Survey Questions

19. Are the survey questions public?

Yes, survey questions and answer options are available on the PBL website and as Supporting Information (part 2) to the article.

20. How did you decide on the questions to ask?

The survey questions are related to physical science issues which are a frequent topic of public debate about climate change.

21. Was the survey reviewed before it was sent to respondents?

Yes, before executing the survey it has been extensively tested and commented on by various climate scientists, social scientists and science communicators with varying opinions, to ensure that questions were both clear and unbiased. Respondents were not steered to certain answers.


Reference: Bart Verheggen, Bart Strengers, John Cook, Rob van Dorland, Kees Vringer, Jeroen Peters, Hans Visser, and Leo Meyer, Scientists’ Views about Attribution of Global Warming, Environmental Science and Technology, 2014. DOI: 10.1021/es501998e. Supporting Information available here.

Please keep discussions on this thread limited to what is mentioned in this FAQ and to other questions you may have about the survey or the article. Discussion of the survey results should be directed at the more generic blog post.

The role of scientific consensus in moving the public debate forward

February 7, 2014

Mike Hulme had an interesting essay at The Conversation, the main message of which was

In the end, the only question that matters [for the public debate about climate change] is, what are we going to do about it?

Hulme correctly argues that the basic science is clear enough so that for society the important issues to discuss are not science related, but policy related. I argued much the same here. He writes:

What matters is not whether the climate is changing (it is); nor whether human actions are to blame (they are, at the very least partly and, quite likely, largely); nor whether future climate change brings additional risks to human or non-human interests (it does).

Let’s leave the minor quibble aside that AR5 puts the anthropogenic contribution at ‘extremely likely’ having caused more than half of the recent global warming.

The part where I disagree with Hulme is where he argues that showing the existence of a scientific consensus on the above (it is warming; it’s due to us; it’s bad news) somehow stands in the way of  getting society to discuss that most important question. I think the opposite is true. It is the continuous doubt about the science, sowed by those who oppose a serious discussion about what to do, that is a stumbleblock. Showing that a consensus amongst experts exists would enable society to more swiftly move on to the important conversation on what to do about it. I agree with Hulme that on this deeply ethical question there is, and ought to be, a multitude of opinions.

As Stephan Lewandowsky and John Cook write in a response to Mike Hulme:

The public’s perception of that scientific consensus is necessary to stimulate political debate about solutions.

Another element that’s missing from this discussion is that scientific and ideological arguments  should be clearly distinguished from each other (“is” vs “ought”).

Unfortunately, ideological arguments are often dressed in a sciency-looking cloak. From that perspective, I appreciate the honesty in Lindzen stating blunty “we’ll all be dead by then”, the obvious implication being: so why care. That’s indeed what a lot comes down to: How do you value the future compared the present?

IPCC history and mandate

October 1, 2010

The purpose of the IPCC was to assess the state of knowledge on the various aspects of climate change including science, environmental and socio-economic impacts and response strategies.

I.e. it was meant to report on and asses the scientific knowledge. This includes the question of how much evidence and (as a result) how much agreement amongst experts (consensus) there is for human induced climate change.

Some science historians point to other important aspects of IPCC’s history. The National Academy of Sciences wrote in 1979:

A plethora of studies from diverse sources indicates a consensus that climate changes will result from man’s combustion of fossil fuels and changes in land use.

Oreskes shows that the IPCC was set up in response to the emerging consensus in the 70’s/80’s that global warming due to GHG emissions would likely become a problem.

Spencer Weart writes:

The concern [about impending climate change] gave rise to the IPCC.

And also points to the Reagan administration being in favor of the clumsy IPCC approach, hoping that it would downplay the scientists’ fears.

When pointing to scientists who disagree with the IPCC consensus, it is important to note that people can agree in basically two directions. The survey by Brown, Pielke Sr and Annan for example shows this to be an approximate bell curve: Most (45-50%) of the respondents (scientists) more or less agree with the main thrust, and sizeable minorities (15-20%) think that IPCC overstated or understated its case. I discussed this survey and the broader question of why the consensus matters before. And I probably will pick this up again soon.

Scientifically, the more uncertain areas are the most interesting. However, if I look at the political decision making in terms of emission reductions and knowledge of the big picture (and the length of time that we’ve known about this big picture, albeit in gradually more certain terms), I can’t but conclude that the politics is hopelessly lagging behind the scientific knowledge in taking this problem seriously. (see e.g. my Dutch post “tijd voor de politiek om wetenschap serieuzer te nemen”.) Of course I’m aware that there’s more that informs politics than just the science, but still, there seems to be an uncomfortably big disconnect there. Stark warnings from science are ignored at our peril.

At this point in time, the uncertainties are pretty much irrelevant for policymaking, because any realistic change in the uncertain details is not going to affect the main trust of what we know, and thus the policy response that people may favour. For the long term, of course we need to finetune our knowledge, so research is still needed. (Hey, I’m a scientist, so I kind of have to say that, right?)

As Herman Daly said:

“Focusing on them [the big picture of what we know] creates a world of relative certainty, at least as to the thrust and direction of policy.” On the other hand, focusing on the more uncertain rates and valuations creates “a world of such enormous uncertainty and complexity as to paralyze policy”.

Perhaps that’s indeed what’s happening now, also cf. Judith’s uncertainty monster and complexity monster.

“To make the point more simply, if you jump out of an airplane you need a crude parachute more than an accurate altimeter.”

Funny how Daly and Curry both address the huge issues of uncertainty and complexity and arrive at diametrically opposed strategies of dealing with them. In terms of public communication, I’m with Daly.

From the principles governing IPCC work:

The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation.

IPCC reports should be neutral with respect to policy, although they may need to deal objectively with scientific, technical and socio-economic factors relevant to the application of particular policies.

Funnily enough, a commenter at Judith’s, Paul in Sweden, took this as proof of a “blatantly biased agenda”. Which is a little strange in light of the history of climate science and of the IPCC as mentioned above. Its mandate is a consequence of the scientific evidence for human induced climate change having become increasingly strong and societally relevant. It doesn’t state what the conclusion ought to be –it has to follow the science-, but of course it states what it’s supposed to assess.

Judith Curry also seems to suggest that the IPCC reports are working towards a predetermined conclusion, when she claims that they are akin to a legal brief (meant to persuade). If true, that should be reflected by large differences between the scientific evidence and the IPCC reports, and between scientists’ opinions and the IPCC reports.  I have not seen evidence of either.

Tom Curtis made some very thoughtful comments on the consensus thread at Judith’s, e.g.

In other words, the IPCC was tasked with reporting the consensus view of the science, were such a consensus existed; and to explain and report the differing opinions where no such consensus existed. Whether they have done that is not best judged by whether they have explained and included the opinions of every crackpot fringe group with an axe to grind on global warming; nor even those of every climatologist, no matter how small a number might support their views. Rather, they are to be judged by the agreement between the IPCC reports and the known consensus and divergences of scientific opinion.

Fortunately, we have available several anonymous surveys of the scientists opinions, which show conclusively that the IPCC reports fairly represent the consensus of relevant scientists on those topics on which it reports. (…)

The purpose of IPCC is to provided as succinctly as possible the best possible scientific advice for policy deciders to operate on. If they were required to consider all and every idea on climate change that circulates on the blogosphere; then the resulting document would be to large, and to ill organised to be usefull as a guide to policy.

Nevermind that the politicans still wouldn’t have a clue as to what is more likely true. The science has to be assessed and weighted; that is what makes the IPCC process useful. There already is another outlet for every crackpot idea out there (NIPCC report); it doesn’t need to be done by the IPCC as well.

To quote the Dutch newspaper “Volkskrant” again:

its work has political implications, but that that doesn’t mean that it’s engaged in doing politics.

And since I discussed history as well, see also my first blog post where I described the IPCC process. I don’t think anyone has read it yet, so I’d be much obliged. And while doing self-promotion, I kind a like this oldie featuring Fred Singer.

Web-iquette for climate discussions

October 15, 2009

When checking some odd stuff regarding taking care of our 3 year old, I came across this web-iquette for discussing your internet search results with the doctor. Many of the same principles apply to a layperson browsing the web about climate change, which inevitably raises questions about what is true and what isn’t. The list bears some resemblance to my effort at providing hints for how to distinguish sense from nonsense in the climate change ‘debate’ (or other complex scientific issues such as health).

Now I’m a bit of an internet-doctor myself, so it’s useful mirror to look into. The list is as follows (replace “doctor” by “scientist” for more climate relevance):

1. Do bring it up. “Most doctors don’t see your research as an attempt to second-guess them.” (as long as you don’t accuse them of being a bunch of frauds, of course)

2. Keep the tone conversational, not confrontational. It sounds so sensible, yet so many people offend this very basic rule of human interaction. Me too, more often than I want to. Tell us that the science is just garbage, and we’ll stop listening to you very quickly. Or we’ll tell you to bugger off, if we’re in a bad mood and forget about this rule ourselves. Usually with a reference to a link or two.

3. When you get a diagnosis, ask the doctor to spell it.
A-n-t-h-r-o-p-o-g-e-n-i-c  G-l-o-b-a-l  W-a-r-m-i-n-g.

4. Check who’s behind the site. ”Falling for information from untrustworthy sources is the biggest mistake parents laypeople make. “Anyone can set up a site called the National Association for Such-and-Such,” pediatrician Alanna Levine notes. Check the “about us” section to see if the information was written or vetted by doctors scientists. Look for sites from government agencies (look for the .gov at the end of the link), universities (look for the .edu ending), or medical scientific organizations such as the American College of Obstetrics and Gynecology IPCC or the American Academy of Pediatrics NOAA, KNMI, etc.”

5. Don’t confuse personal experience with science. Anyone can write anything on the internet. Beware, it’s a dangerous place. And, don’t confuse weather with climate. Please. It’s making us soooooo tired.

6. Don’t assume your doctor has blown it based on what you read. Proclaiming that the science is all garbage just because you know how to use google does not impress anyone. Please refrain from doing so, it just makes you look silly. And if you’re really convinced of yourself, why don’t you submit your ideas to Science or Nature?

7. Let your doctor help you evaluate what you find. “There’s something to be said for a professional (such as a scientist) who has a base of knowledge and can help you sort through your own research,” says pediatrician Joseph Kahn. A doctor scientist can help you determine if the site is trustworthy (…) and if the information you found is outdated.” Expertise matters. You wouldn’t let your accountant do the plumbing and your plumber file your taxes, right?

8. Realize no doctor can read every single study the moment it comes out.Medicine Science is constantly evolving”. Though that doesn’t usually mean that everything we know today will be invalid tomorrow. The big picture only changes very slowly.  Eat and drink in moderation and varied, and get enough exercise. More greenhouse gases in the air cause more warming. It’s been pretty stable advice/science for a while. Details may shift (is coffee good or bad for you? How important is aerosol nucleation for climate?), but the big picture less so.

9. Definitely ask about what’s confusing or troubling you. Asking, yes. Bringing up doubts, fine. Rational discussion, absolutely. Accusations of fraud, claiming that we just want to steal your SUV, nah.

Btw, it’s Blog Action Day! (with climate change as its theme.) Check out some of the links if you’re interested in more climate related blogs.

Who to believe?

February 8, 2009


Amidst all the different information about climate change, how is a layperson going to know who is right? There are a few clues one could follow, that don’t require any specialized knowledge.


-          Seeing the forest for the trees. Nitpicking on small details, and then claiming or insinuating that this challenges the foundation of a whole scientific field. Over-interpreting the significance of a specific finding. This is by far the most prevalent style of argument that one has to watch out for. It usually goes something like “This particular study proves that global warming isn’t due to greenhouse gases”. People tend to forget that observing a bird in the sky doesn’t disprove gravity.

-     Consensus matters. If you get a second opinion on your health condition, and it confirms what your specialist said in the first place, your trust in the diagnosis probably increases. Now imagine that you collect the interpretations of medical professionals all over the world, and by and large they their conclusions converge to the same broad picture. This happens to be how the IPCC comes to its conclusions. If the professionals do their work seriously, than the existence of a consensus amongst them is absolutely relevant (though of course it is not absolute proof). The only way in which you can ignore a consensus as being irrelevant, is if you can somehow show that the professionals are all lying or incompetent. (See the next clue for what that brings you into.) Oreskes has an excellent presentation on this.

-          Beware of conspiracy theories. The consensus wouldn’t matter if somehow all those scientists had bought into the same conspiracy of wanting to take away your SUV. (Don’t laugh, there are many people who seem to think this way.)

-          Timescales. Climate is defined as the average weather over 30 years. (Year-to-year variability usually averages out over 30 years). Weather and climate are very frequently confused in the popular debate.

-          Spatial scales. Combined with the previous one, an example would be: “It was a very cold winter in the US this year (so global warming is a farce)”. This is also used the other way around: “It was a very warm winter in the US this year (so global warming is getting worse)”. Both are equally silly conclusions.

-          Logic. Some of the most heard ‘skeptical’ arguments don’t stand up to basic logic, and no knowledge of climate is needed to see that. Example: “Climate has always changed, so it is not caused by humans.” It wasn’t in the past, but that’s no evidence that it isn’t now. Try that line of argument in a court of law against a pyromaniac, by saying that forest fires have always happened naturally; it won’t fly.

-          Confusion of cause and effect. A consensus emerged as a result of the strength of the accumulated evidence, not the other way around. Activists may try to persuade you to trade in your SUV for a Prius because they’re worried about climate change; not the other way around. A tricky one is temperature and CO2: They influence each other both ways. (see also here

-          Think in terms of likelihood. How likely is the previous proposition of a mass conspiracy amongst stubborn scientists? How likely is it that scientists have for decades overlooked this little finding that supposedly shakes the foundations of a relatively well established science?

-          Think in terms of risk. What if we take measures against global warming and it turns out less bad than expected? What if we don’t take measures against global warming and it turns out worse than expected? (False positive and false negative, respectively. See also this comment at RealClimate) 

-          Check for consistency. If someone sais (rightfully) that one particular event (e.g. Kathrina) is not proof of man-made climate change, but then claims that the current cold winter is proof against, you should raise your eyebrows.

-          Expertise. In gauging the credibility of a source, their expertise is important to consider. When it concerns your health, you usually trust a doctor’s opinion more than that of a software engineer. It is not unreasonable to trust a climate scientist more than a doctor when it concerns climate. Of course this is not proof, but there is a difference in likelihood of them knowing what they’re talking about.

-          Motive. The credibility of a source also depends on their motives, both on economic and ideological grounds, for telling you a certain side of the story. What vested interests, if any, are there to the different sides of the story? In a worldview where government interference is deeply hated, could it be tempting to distort evidence that could possibly lead to calls for government action? In a worldview where societal problems are preferably tackled by government, does it make any sense to make up a phony problem to call for government action? I don’t know anyone who wants government action on a phony problem just for the sake of it. Don’t underestimate the power of ideology, but always include a sanity check. See also here and here.


This list is not exhaustive, so if you have other pointers to add to make sense of the public debate on climate change, please share them in a comment. Similar issues of weeding through sources have been discussed in a number of thoughtful posts here, e.g. this one on cherry picking. Other good discussions here, here and here.


Ideally, you would critically assess the evidence for each position to form a well founded opinion on anthropogenic global warming (AGW). In the absence of time to do so, you need to take some shortcuts to assess the flood of information about the topic. None of the clues discussed here constitute proof for or against AGW, but applying multiple clues simultaneously to gauge the credibility of a source can be helpful to ‘distinguish the chaff from the grain’ (het kaf van het koren te scheiden, as we say in Dutch).


How do we know there’s a consensus, and why does it matter?

June 14, 2008

Investigating the consensus

In a study of over 900 scientific articles that had the keywords “climate change”, Oreskes found none (!) that disagreed with the consensus position (see here). That doesn’t mean that there weren’t any articles promoting a different view than the consensus position. (Articles that didn’t have the keyword “climate change” in their listing were not included in Oreskes’ review after all.) I’ve read quite a few papers myself that oppose the consensus view. But they can hardly be very many, relatively speaking. Otherwise Oreskes’ review would not have resulted in zero “skeptical” articles.

Almost all relevant scientific organizations and National Academies of Science endorse the consensus view that recent climate change is for a large part due to human activity. Even the most recent statement from the organization of petroleum engineers (!) acknowledges that recent climate change is linkes to human activity. A large scale review of the scientific literature, undertaken and endorsed by many, many scientists working in the field, is more trustworthy than any individual’s book or article or website. (Yes, that includes this one. You’ll get the most complete and balanced picture of climate change by reading the IPCC reports)

A recent survey of scientists having authored a recent journal article on climate change found that the majority concurred with the IPCC position. A sizeable minority was of the opinion that the IPCC reports overstate the importance of and/or certainty regarding CO2 compared to other forcings (both natural and anthropogenic!). Only very few respondents were of the “skeptical” opinion that warming is predominantly natural. Nobody denied that the globe is warming. A large minority found the IPCC too cautious, understating the human influence on climate and/or the seriousness of the problem. Note that the authors do not claim that their survey is representative; less then 10% of the 1800 scientists contacted replied. Moreover, the positions that respondents had to choose between were sometimes a little ambiguous.

In an excellent presentation, Naomi Oreskes provides an overview through history to show that this consensus is actually much older than the IPCC process; the IPCC was set-up in response to the growing consensus that emerged in the late seventies. And already before World War II global warming was recognized by individual scientists. This begs the question why it takes society so long to recognize the problem, and do something about it? The second half of her presentation (similar powerpoint presentation here), Oreskes explains one of the reasons why: A successful campaign to create doubt in people’s minds about the reality of climate change, along similar lines (and starring some of the same people) as the campaign to downplay the risks of smoking. See also here and here.


Proof versus probability

Does having a broad consensus automatically make something true? No. Is there absolute proof that the climate is changing, that it is predominantly caused by human activity, and that the consequences will be severe? No, there is not (at least not in the absolute, mathematical sense). Do we need proof? If yes, then we have to wait until the disaster strikes, and even then we cannot possibly prove the abovementioned claims. If a plane technician tells you that there is a 75% chance that the plane you are about to board will crash, would you board the plane? Would your action (presumably of not boarding) change if an economist points you to some screws in the wing of the plane that are perfectly in place, telling you that he therefore concludes that he regards it as totally save to board the plane? What if 99 engineers tell you it is unsafe and one tells you it is safe?

Everybody makes decisions each day based on an assessment of the probability of something happening, and the consequences (positive and negative) of when it happens. Governments make policy based on such probabilities. Many of these probabilities are much weaker constrained by scientific knowledge than climate change is.

How many types of insurance do you have? You have these not because accidents are so likely to happen, but often because the effects if they happen are severe. With unmitigated climate change, not only are the effects potentially severe, but the chance of severe consequences is rather high. Isn’t that worth insuring ourselves against? Isn’t that worth serious attempts to decrease the likelihood of those consequences materializing? If you want proof, please go and find yourself another planet to play Russian roulette with. Go and board that plane. But don’t force me and my children to join you. I better be safe than sorry.


Scientific Progress

Scientific progress is a usually a gradual process. “Skeptics” and their supporters often bring up Galileo as an example of that the scientific consensus can also be wrong, and has been wrong in the past. True enough, but for every Galileo there probably are thousands of “fossil fools”. Actually, the theory that increasing the concentration of greenhouse gases could influence the climate was perceived as wildly strange and improbable at the time of its first proposition (late nineteenth century). But rather than that one person suddenly overturned current wisdom, it is a matter of accumulating evidence by many scientists over a long time period that gradually changes and sharpens the scientific picture of what is happening. That is typically how scientific progress works these days: cumulative, piece by piece. The likelihood that a tiny minority of scientists, or some new piece of evidence, radically alters this picture is very small indeed. New evidence has to be reconciled with the existing mountain of evidence; it doesn’t simply replace it. Observing a bird in the air doesn’t disprove gravity. Small changes here and there in our understanding of specifics, that happens all the time. That’s how the mountain of evidence has been built in the first place, and that is how it continues to be shaped. That’s science at work.


What if the consensus is wrong?

The only way in which the consensus view could be “wrong” is that the effect of greenhouse gases on climate is smaller than currently thought. No warming effect from greenhouse gases at all is physically impossible (the earth would be 30 degrees colder than it currently is if there were no greenhouse effect). If warming will be less severe, it means that we have some more time to take serious action; not that we can forget about doing anything about it. It reminds me of a guy’s t-shirt I once saw with the text: “Sex is like pizza. If it’s good, it’s really good. If it’s bad, it’s still pretty good”. Well, perhaps we could say that “Climate change is like Brussels sprouts. If it’s bad, it’s really bad. If it’s good, it’s still pretty bad”.

If the contribution from greenhouse gases to climate change is smaller and the contribution from natural sources is larger than currently thought, we would have to curtail their emissions even more to stabilize the climate. If a situation is undesirable (global warming), you should change the things you can (greenhouse gas emissions), and accept the things you cannot change (the sun). Not reducing our greenhouse gas emissions is simply not an option.

Fossil fuels are finite, and it is only a matter of time that the oil production will start to decline. We will have to develop alternative energy sources. There is still plenty of coal, but the air pollution that it causes is reason enough to curtail its use. Already now hundreds of thousands of people die prematurely from air pollution. The CO2 that is emitted form fossil fuel burning causes the oceans to become more acidic, with potentially severe consequences for ocean life and fisheries. There are also geo-political reasons to want to become less reliant on fossil fuels, and oil in particular. Reducing our greenhouse gas emissions by changing our energy system and agricultural practices does not only cost money; it also creates (economic) opportunities. There will be winners and losers. The ones foreseeing that they’re losing are often the ones claiming the loudest that climate change is not occurring, or is natural, or even beneficial. All in all, developing alternative energy sources makes a lot of sense for many other reasons than climate change.


What if the consensus is right?

That one’s easy. We better be happy that we started reducing our greenhouse emissions soon and strongly. Act and vote accordingly. And don’t move to the Netherlands, as I did.


Final remarks

Which situation would you be rather in: Having reduced our greenhouse gas emissions when later it appears that global warming is not as severe as expected, or not having reduced our greenhouse gases when later it appears that global warming is even more severe than expected?

Scientific consensus on climate change

June 1, 2008

Is there a scientific consensus on climate change?

The short answer is yes, in broad terms there is. That of course doesn’t mean that all scientists agree on everything having to do with climate change. First of, scientists are in general argumentative and stubborn: they will usually find something, be it a minor point, about which they disagree with someone else’s opinion or interpretation. But in light of that nature, the broad consensus about the recent climate chang is very strong indeed. This consensus is laid out in the IPCC reports (Intergovernmental Panel on Climate Change), by means of a thorough review of all recent scientific literature on the relevant topics. Minority opinions are also discussed in these reports, but are -as are all results- put in the context of the bigger picture of the literature as a whole. Indeed, this is how it should be done, for outsiders to get a proper view of what the current state of the science as a whole is.


What is the scientific consensus on climate change?

Short answer: Recent climate change is for a large part due to human activity.

Longer version:

  • Surface temperature readings jiggle up and down, but over the course of the past century, their overall trend is up. Rising global temperature is also confirmed by the global retreat of glaciers, melting of ice sheets, poleward migration of species and increased ocean heat content. Issues with individual measurement stations do not impact this rising trend.
  • The levels of carbon dioxide and other greenhouse gases in the atmosphere have also been rising. The concentration of CO2 in seawater has increased as well. The total observed increase is consistent with the amount of fuel humanity has burned and the amount of land it has cleared since the industrial revolution began. Isotope analysis confirms that the additional CO2 comes predominantly from fossil fuels.
  • Basic theory (known since at least 150 years) links these two trends. Because of greenhouse gases, a smaller fraction of the thermal radiation emitted by the surface escapes into space. The planet becomes a net absorber of energy. The surface heats up and emits more thermal radiation, until the total amount of escaping thermal radiation again balances the energy deposited by sunlight. The radiative properties of CO2 have been measured over 150 years ago (Tyndall), and the warming of the earth resulting from our emissions was predicted shortly thereafter (Arrhenius). His prediction was quantitatively in the same ballpark as was later confirmed. Since then, research has -by and large- backed these results up into a mountain of evidence. Current global warming is a prediction come true. That doesn’t happen all that often in science…
  • Natural factors, such as variations in the sun’s output, have been too small to account for the observed temperature increase.

See here for another discussion of what the consensus is all about. A bit more technical description is here. For more detail, see here.

Criticism on the consensus view

Most criticism concerns the first and the last points of the consensus view as stated above. The second and third points are hardly ever criticized, at least not by people minimally versed in physics. Indirectly however, they are often implied to be false.

  • There has been a more or less constant attack on the observed temperature trend, even when it has now significantly risen above the level expected from natural variability of the weather. Fact is, the trend is so overwhelming and there are so many observations, even of different physical quantities, pointing in the same direction, that it really starts to be laughable to contest that the globe is warming. Known problems such as the urban heat island effect are corrected for in all temperature series, and the procedures to arrive at a global temperature are very carefully thought out and publicly documented. Of course climate scientists are all for good quality control of the observations, which admittedly still have a lot of issues to be dealt with (see eg here and here the latest “controversies” in how to deal with such issues). But when such criticisms are driven by an obvious desire to discredit the entire temperature trend (and, by extension, any mitigation policies) without proper justification for such far reaching claims, then I take the criticism with a grain of salt.
  • Sometimes it is implicitly questioned that the current rise in CO2 is due to human activity, but in such a way that the claim is hard to recognize. E.g. by stating (correctly) that during the great ice ages the temperature rose before CO2 did, “skeptics” often conclude that the current CO2 rise is a consequence of warming rather than vice versa. But this last claim is patently false: we know for a fact that we put the CO2 in the air. (The cause-effect relation between CO2 and temperature goes in both directions; they can act as a feedback on each other. More CO2 causes warming; warming causes more CO2 emissions. It’s like the chicken and the egg.) As another example, the bogus documentary “The great global warming swindle” implicitly claims that the increased CO2 comes from the oceans (and thus, by extension, not from fossil fuel burning). This is so obviously wrong, it’s not even funny. (For a critique of this documentary, see e.g. here)
  • The third point is implicitly pushed aside when trying to promote or exaggerate the sun’s role in climate change of the past century. As if by making the sun responsible for global warming the known physics of greenhouse gases magically stops functioning. Just like there is no knob to switch off gravity on the earth, there is no knob either to switch off the radiative effects of greenhouse gases. It’s there, deal with it. If it wasn’t there, the earth would be a whopping 30 degrees C colder than it is, and life as we know it would not exist. But what if the radiative effects of greenhouse gases are smaller than what is currently thought? While there is some playing room for the climate sensitivity (the equilibrium temperature response to a doubling in CO2 equivalents) to vary, this playing room is rather limited by several constraints from e.g. paleo-climate and climate response to large volcanic eruptions (see e.g. here and here). You would still have to explain away a whole body of observations in order to come up with a substantially larger contribution of the sun to current climate change than laid out in the IPCC reports.
  • The sun’s energy output is huge, and variations in the sun have caused climate changes in the past. The sun’s energy output has not been as high as in the past century for thousands of years. True, but it only explains a small portion of the current warming, mainly in the early 20th century. Over the last 50 years the sun’s output has been roughly constant (decreasing weakly), so even with exotic and unsubstantiated magnifying mechanisms (e.g. cosmic rays, UV-dynamics feedbacks) the sun can not be held responsible for the recent global warming since 1975. Often you will hear the logical fallacy that “since the sun has been responsible for climate changes in the past, it must now also be responsible”. On an equal level are emotional arguments such as “How can we possibly influence something so big and complex such as the earth’s climate, when contrasting us little humans to that giant ball of energy in the sky?” Such arguments may sound plausible when listening to a radio-show; in a real scientific debate they are not taken seriously.

Scientific debate and the media

May 21, 2008

(Voor Nederlandse versie klik hier)

Scientific conferences and journals provide the stage where scientific debate among scientists is generally held (i.e. not the newspaper or the television or the internet). The proportion of so-called “skeptical” arguments in the scientific venues is very tiny compared to the overwhelming majority of arguments broadly in line with the consensus view that human activity is altering the climate. Don’t scientists debate anything in the field of climate science anymore then? Of course they do. But those debates generally cover very specific topics, the outcome of which will not dramatically change the consensus view, if at all (see e.g. this realclimate post). Any new finding still has to obey the laws of physics, and has to be consistent with the massive body of observations and evidence already available. Observing a bird in the air doesn’t disprove gravity. The statement that “the debate is over” refers to the “debate” whether current climate change is predominantly due to human activity and whether it poses a problem that needs addressing. That “debate” is over indeed, at least in the scientific arena. There are plenty of interesting tidbits left to debate, but they will not likely change the big picture.

Role of the media

The popular media often paint a false picture of the scientific debate by giving the tiny minority viewpoints equal footing as the consensus viewpoints. That’s like discussing creationism as being a scientific theory of equal merit as evolution. Or to have someone make the case that smoking is not bad for your health at all, when smoking laws are being discussed. Perhaps it feels fair to provide those minority viewpoints with an equal sized platform to communicate their viewpoints, but it gives a false picture of the current scientific thinking on the subject. In doing so, the media do a disservice to their audience. And in some cases they even endanger their audience, as in the example about smoking, and also in the case of climate change. “Teaching the controversy” only makes sense when there really is a scientific controversy. Creating a controversy outside of the science arena and then presenting it as a scientific controversy is deceptive. See for a nice review of this “false objectivity of balance” Stephen Schneider’s website.


Public debate

There are numerous debates and panel discussions organized everywhere about real and apparent controversies. For example, debates between believers in creationism and defenders of the scientific theory of evolution have been common. Debates about climate change also abound. How useful are these debates? They often result in the audience being confused: Many of them have no clue as to who was right or wrong. Many of them will leave the debate with the impression that the science is not settled at all (or they will confuse pseudo-science and real science). This consequence of the debate is often very useful for the defenders of the (scientific) minority view: Uncertainty and doubt about the scientific consensus provides the minority view with more traction.



If the objective of certain “skeptics” is to delay serious mitigation (emission reduction) measures, sowing doubt about the reality of anthropogenic climate change is a very effective strategy. Public debates and so-called “balanced” reporting in the media serve this purpose very well. They have successfully framed the public debate in concepts such as proof and uncertainty. Whereas for a policy basis, the concept of risk is much more useful. For a long time the tobacco industry successfully delayed actions against smoking with the claim that adverse health effects were not proven. That statement may or may not still be true, dependent on your criteria as to what constitutes proof. But the reasoning lost its effect when people started to realize that the probability of there not being any effect was becoming very small with all the information available, and that the health risks were very substantial indeed. We need a similar realization about climate change. Absolute certainty is not required as a basis for action; rational risk assessment is.


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