For any climate related discussion – please be civil and minimize repeating yourself. It’s okay to agree to disagree.
Fellow Dutchman Collin Maessen interviewed me via skype about our recent paper “Scientists’ views about attribution of global warming“:
Collin wrote a blogpost about it as well which is well worth a read, giving a bit of context from other opinion and literature surveys.
The interview starts off with the general findings regarding the level of consensus, then focusing on how this compares with previous studies, how the media coverage is slanted towards contrarian views, and he gives me a chance to talk about my favorite part, how aerosol cooling masks the greenhouse warming and how this makes the phrasing of the IPCC AR4 attribution statement, by focusing only on the greenhouse warming part, prone to being misinterpreted. These aspects were also discussed in my blogpost from last month.
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.
Graphs with the ‘straight counts’ for every question (i.e. the number of responses for each answer option) are available in a background report. These results are 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”).
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 less then 5% 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.
19. Are the survey questions public?
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.
- A survey among more than 1800 climate scientists confirms that there is widespread agreement that global warming is predominantly caused by human greenhouse gases.
- This consensus strengthens with increased expertise, as defined by the number of self-reported articles in the peer-reviewed literature.
- The main attribution statement in IPCC AR4 may lead to an underestimate of the greenhouse gas contribution to warming, because it implicitly includes the lesser known masking effect of cooling aerosols.
- Self-reported media exposure is higher for those who are skeptical of a significant human influence on climate.
In 2012, while temporarily based at the Netherlands Environmental Assessment Agency (PBL), my colleagues and I conducted a detailed survey about climate science. More than 1800 international scientists studying various aspects of climate change, including e.g. climate physics, climate impacts and mitigation, responded to the questionnaire. The main results of the survey have now been published in Environmental Science and Technology (doi: 10.1021/es501998e).
Level of consensus regarding attribution
The answers to the survey showed a wide variety of opinions, but it was clear that a large majority of climate scientists agree that anthropogenic greenhouse gases are the dominant cause of global warming. Consistent with other research, we found that the consensus is strongest for scientists with more relevant expertise and for scientists with more peer-reviewed publications. 90% of respondents with more than 10 climate-related peer-reviewed publications (about half of all respondents), agreed that anthropogenic greenhouse gases (GHG) are the dominant driver of recent global warming. This is based on two different questions, of which one was phrased in similar terms as the quintessential attribution statement in IPCC AR4 (stating that more than half of the observed warming since the 1950s is very likely caused by GHG).
Literature analyses (e.g. Cook et al., 2013; Oreskes et al., 2004) generally find a stronger consensus than opinion surveys such as ours. This is related to the stronger consensus among highly published – and arguably the most expert – climate scientists. The strength of literature surveys lies in the fact that they sample the prime locus of scientific evidence and thus they provide the most direct measure of the consilience of evidence. On the other hand, opinion surveys such as ours can achieve much more specificity about what exactly is agreed upon and where the disagreement lies. As such, these two methods for quantifying scientific consensus are complementary. Our questions possibly set a higher bar for what’s considered the consensus position than some other studies. Furthermore, contrarian viewpoints were likely overrepresented in our study compared with others.
No matter how you slice it, scientists overwhelmingly agree that recent global warming is to a great extent human caused.
For all climate-related discussions that don’t fit under a recent thread.
After a bit of a “hiatus”, ClimateDialogue (CD) has re-opened again with a discussion on climate sensitivity. On the one hand this site is unique in bringing together ‘mainstreamers’ and ‘contrarians’ (both in the organization and in the discussions), hopefully leading to both enhanced clarity on what the (dis)agreements are really about and to decreased polarization. On the other hand it’s controversial because a ‘false balance’ is embedded in its structure (by purposefully inviting contrarian scientists to the discussion, rather than e.g. randomly inviting experts).
Whether the positives or negatives dominate is in the eye of the beholder (opinions about that vary wildly), but also depends very strongly on the participation of the mainstream (both as invited experts and as contributing to the public discussion). See also my initial reflections at the time of the first launch. Discussions on ClimateDialogue will be facilitated and moderated by Bart Strengers (NL Environmental Assessment Agency, PBL) and Marcel Crok (freelance journalist), where the former has a mainstream view of climate science and the latter a contrarian view. I am still involved in the background, as is KNMI (NL Meteorological Institute). ClimateDialogue is funded by the Dutch Ministry of Infrastructure and Environment.
In the current ‘dialogue’ James Annan, John Fasullo and Nic Lewis are discussing their views about climate sensitivity (the equilibrium warming after a doubling of CO2 concentrations, ECS). In the latest IPCC report (AR5) the different and partly independent lines of evidence are combined to conclude that ECS is likely in the range 1.5°C to 4.5°C with high confidence. The figure below shows the ranges and best estimates of ECS in AR5 based on different types of studies, namely:
– the observed or instrumental surface, ocean and/or atmospheric temperature trends since pre-industrial time
– short-term perturbations of the energy balance such as those caused by volcanic eruptions, included under “instrumental” in the figure
– climatological constraints by comparing patterns of mean climate and variability in models to observations
– ECS as emergent property of global climate models
– temperature fluctuations as reconstructed from palaeoclimate archives
– studies that combine two or more lines of evidence
John Christy, Richard McNider and Roy Spencer trying to overturn mainstream science by rewriting history and re-baselining graphsFebruary 22, 2014
Who are the Flat Earthers?
Before the advent of modern climatology, common wisdom had it that we tiny humans couldn’t possibly influence climate. Modern science shows we can. Yet John Christy and Richard McNider claim the exact opposite in a recent WSJ op-ed, in which they claim that their outdated views on climate somehow make them modern-day Galileo’s (or in their words: Why they are the ones declaring that the earth is round while the vast majority of the climate scientists persist in thinking the earth is flat). They couldn’t be more wrong.
Back then, scientific evidence slowly overturned the religious-cultural notion that the Earth was the centre of the universe. This resulted in a scientific consensus that the Earth revolves around the sun. More recently scientific evidence has started overturning the notion that humans can’t possibly influence something as gigantic as the Earth’s climate. This too has resulted in a scientific consensus (though a public consensus is still lagging behind). In both cases, the pre-scientific notion was mostly culture-based, as opposed to being evidence-based.
As Jeff Nesbit tweeted: “Being the last scientist to accept established climate science doesn’t make you Galileo.” Quite the opposite indeed.
The Galileo-complex also suggests a rather simplistic view of how science progresses. Rather than a lone skeptic overthrowing a scientific (rather than a cultural) consensus, scientific progress is a usually a gradual process. 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. “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, though as Carl Sagan said: “they laughed at Galileo, but they also laughed at Bozo the clown”.
Besides their entirely misplaced Galileo-framing, Christy and McNider also make a range of unsupported and/or incorrect statements. One argument deals with the so-called tropical tropospheric hot spot. This refers to the expected stronger warming of the tropical troposphere as compared to the surface. This “hot spot” is independent of the cause of the warming. But what do Christy and McNider write in the WSJ:
(the warming of the deep atmosphere is) the fundamental sign of carbon-dioxide-caused climate change, which is supposedly behind these natural phenomena
“Yes, the hot spot is expected via the traditional view that the lapse rate feedback operates on both short and long time scales. (…) it [the hot spot] is broader than just the enhanced greenhouse effect because any thermal forcing should elicit a response such as the “expected” hot spot.”
So why is he claiming something in the WSJ that he knows to be untrue?
It almost goes without saying that any climate model-observation mismatch can have multiple (non-exclusive) causes (as succinctly summarized at RC):
- The observations are in error
- The models are in error
- The comparison is flawed
But rather than doing a careful analysis of various potential explanations, McNider and Christy, as well as their colleague Roy Spencer, prefer to draw far reaching conclusions based on a particularly flawed comparison: They shift the modelled temperature anomaly upwards to increase the discrepancy with observations by around 50%. Using this tactic, Roy Spencer showed the following figure on his blog recently:
So what did he do? Jos Hagelaars tried to reproduce the different steps involved. A comparison of annual data, using a 1986-2005 baseline, would look as follows:
Spencer used a 5 year running mean instead of annual values, which would (should) look as follows:
The next step is re-baselining the figure to maximize the visual appearance of a discrepancy: Let’s baseline everything to the 1979-1983 average (way too short of a period and chosen very tactically it seems):
Which looks surprisingly similar to Spencer’s trickery-graph. But critiquing Roy Spencer comes at a risk: He may call you a “global warming Nazi”. Those nasty CO2 molecules, that’ll teach them!
Many thanks to Jos Hagelaars for the data analysis and figures.
Guest post by Hans Custers. Nederlandse versie hier.
A very, ehhrmm… interesting piece on
Variable Variability, Victor Venema’s blog: Interesting what the interesting Judith Curry finds interesting. And I don’t mean interesting in a rhetoric, suggestive way; I mean it is a well-written and well-reasoned article, worth reading.
Victor writes about the meme regularly used by the anti climate science campaign, often supported by some straw man arguments, that the science of human impacts on climate would not be falsifiable. He shows it’s nonsense, by giving some examples of how it could be falsified. Or, more likely, already would have been falsified, if the science would be wrong. Victor’s post inspired me to think of more options to falsify generally accepted viewpoints in climate science. If there are any ‘climate change skeptics’ who want to contribute to real science, they might see this as a challenge. Maybe they can come up with a research proposal, based on one of the options for falsification. Like proper scientists would do.
First, a few more things about falsifiability in general. Bart wrote a concise post about the subject four years ago, explaining that a bird in the sky does not disprove gravity. What looks like a refutation at first, might on second thoughts be based on partial or total misunderstanding of the hypothesis. Natural climate forcings and variations do not exclude human impacts. Therefore, the existence of these natural factors in itself, cannot falsify anthropogenic climate change. A real skeptic is cautious about both scientific evidence and refutations. ‘Climate change skeptics’ like to mention the single black swan, that disproves the hypothesis that all swans are white. Of course that is true, unless that single black swan appears to be found near some oil spill.
Some of the falsifications that I mention later on might be somewhat cheap, or far-fetched. It is not very easy to find options to falsify the science of human impacts on climate. Not because climate scientists don’t respect philosophical principles of science, but simply because there’s such a huge amount of evidence. There are not a lot of findings that would disprove all the evidence at once. A scientific revolution of this magnitude only happens very rarely. Whoever thinks differently, doesn’t understand how science works. Read the rest of this entry »
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.
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?
In his recent testimony, Andrew Dessler reviewed what he thinks “are the most important conclusions the climate scientific community has reached in over two centuries of work”. I think that’s a very good choice to focus on, as the basics of what we know is most important, “at least as to the thrust and direction of policy” (Herman Daly). This focus served as a good antidote to the other witness, Judith Curry, who emphasizes (and often exaggerates) uncertainty to the point of conflating it with ignorance.
Dessler mentioned the following “important points that we know with high confidence”:
1. The climate is warming.
Let’s take this opportunity to show the updated figure by Cowtan and Way, extending their infilling method to the entire instrumental period (pause? which pause?):
2. Most of the recent warming is extremely likely due to emissions of carbon dioxide and other greenhouse gases by human activities.
This conclusion is based on several lines of evidence:
– Anthropogenic increase in greenhouse gases
– Physics of greenhouse effect
– Observed warming roughly matches what is expected
– No alternative explanation for recent warming
– Fingerprints of enhanced greenhouse effect (e.g. stratospheric
warming cooling, which was predicted before it was observed)
Thus, we have a standard model of climate science that is capable of explaining just about everything. Naturally, there are some things that aren’t necessarily explained by the model, just as there’re a few heavy smokers who don’t get lung cancer. But none of these are fundamental challenges to the standard model.
He goes on to explain that the so-called “hiatius” is not a fundamental challenge to our understanding of climate, though it is “an opportunity to refine and improve our understanding of [the interaction of ocean circulation, short-term climate variability, and long-term global warming].”
What about alternative theories? Any theory that wants to compete with the standard model has to explain all of the observations that the standard model can. Is there any model that can even come close to doing that?
And making successful predictions would help convince scientists that the alternative theory should be taken seriously. How many successful predictions have alternative theories made?
3. Future warming could be large
On this point I always emphasize that the amount of future warming depends both on a combination of factors:
– the climate forcing (i.e. our emissions and other changes to the earth’ radiation budget)
– the climate sensitivity (the climate system’s response to those forcings)
– the climate response time (how fast will the system equilibrates).
Internal (unforced) variability also plays a role, but this usually averages out over long enough timescales.
4. The impacts of this are profound.
In the climate debate, we can argue about what we know or what we don’t know. Arguing about what we don’t know can give the impression that we don’t know much, even though some impacts are virtually certain.
The virtually certain impacts include:
• increasing temperatures
• more frequent extreme heat events
• changes in the distribution of rainfall
• rising seas
• the oceans becoming more acidic
Time is not our friend in this problem.
The scientific community has been working on understanding the climate system for nearly 200 years. In that time, a robust understanding of it has emerged. We know the climate is warming. We know that humans are now in the driver’s seat of the climate system. We know that, over the next century, if nothing is done to rein in emissions, temperatures will likely increase enough to profoundly change the planet. I wish this weren’t true, but it is what the science tells us.
A key distinction in the two senate hearings was that Andrew Dessler focused on what we know, whereas Judith Curry focused on what we don’t know (though “AndThenTheresPhysics” made a good point that Curry goes far beyond that, by e.g. proclaiming confidence in certain benign outcomes (e.g. regarding sensitivity) while claiming ignorance in areas where we have a half-decent, if incomplete, understanding, e.g. regarding the hiatus). I have argued before that emphasizing (let alone exaggerating) uncertainties is not the road to increase people’s understanding of the issue, where what we do know is much more important to convey (if your goal is to increase the public understanding of scientific knowledge). Alongside that I argue that much more attention is needed to explain the nature of science, which is needed to e.g. place scientific uncertainties in a proper context.
Herman Daly said it as follows, in a quote I’ve used regularly over the past few years:
If you jump out of an airplane you need a crude parachute more than an accurate altimeter.
Arguing whether the altimeter might be off by a few inches is interesting from a scientific/technological perspective, but for the people in the plane it’s mostly a distraction.