Summary: Here’s a briefing about climate change, prepared for Politifact at their request. Unused, of course, since the reporter was just fishing for smears (here’s an analysis of what they published). However, it’s a useful introduction to this complex subject. What do we know about the consensus of climate scientists, and why does it matter? These are unedited emails, and so roughly written and unproofed. {2nd of 2 posts today.}
Initial Inquiry by Linda Qiu of Politifact, and my response
I’m a reporter with PolitiFact, the fact-checking website of the Tampa Bay Times. I’m currently looking into something Rick Santorum said: 57% of scientists “don’t buy into the idea that CO2 is the knob that’s turning the climate.” His campaign hasn’t gotten back to me on his source but one of your posts also has the figure, but it’s not quite what Santorum said. So I was hoping for your take on Santorum’s reading on your analysis — how accurate is it?
I have a lot of data about this. Here’s a quick data dump. I’m in the middle of something about this very subject, so don’t have time to compose. Tell me what more you’d like. I can provide links and cites for all of this. I work late, so deadlines are not a problem.
There have been many surveys seeking to determine the consensus of scientists and the subgroup of climate scientists (neither group having a clear definition) about the headline attribution statements of the IPCC. That is, how much of the warming since 1950 is attributed to us. The IPCC states its findings in two parts: the finding, and the IPCC’s confidence in that finding. The latter is off little relevance to science, but obviously of great importance when taking public policy action. For example, it’s nice to know that all scientists believe “X”, but what if they have little confidence in that belief?
The standard measure of confidence is 95% (defining what this means is both complex and controversial, especially now with the replication crisis). The IPCC defines 90%+ as “very likely” and 95%+ as “extremely likely”.
The first round of surveys concerned only the first part: how much of the warming is from us. Almost all scientists agree that there has been warming during the past 2 centuries; only the amount and cause is debated. They found strong agreement with the IPCC finding (what that finding is I’ll discuss below).
In March – April 2012 the PBL Netherlands Climate Assessment Agency, with several other scientists, conducted a survey of approximately 6,550 scientists studying climate change. This was imo excellent — the best survey on the subject done so far, by far. It was published as “Scientists’ Views about Attribution of Global Warming” by Bart Verheggen et al in the 19 Aug 2014 issue of Environmental Science and Technology (peer-reviewed). In April 2015 they published a more detailed report. The first questions asked about the finding of the IPCC’s Fourth Assessment Report (AR4) in their Summary for Policy-makers:
“Most of the observed increase is global average temperature since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” {The subject is important since proposed measures to reduce climate change {Obama’s Clean Power Plan} focus almost exclusively on reducing emissions of greenhouse gases.
The PBL survey found that only 64% of climate scientists responding agreed that over half of the warming since 1950 was from anthropogenic greenhouse gases (GHG) — a majority, but less than I (and others) expected).
For the second part, only 65% of those had a confidence level of 95%+ — so that only 64% * 65% = 43% agree with the finding at the 95% level. If you lower the bar — 89.3% agree at the 90% level (not significant by the usual standards) — so 64% * 89% = 57% agree with the finding as stated in AR4 (more than half + very likely). It’s a majority, but not an impressive one compared to the scale of public policy action advocates recommended.
Restated in absolute numbers, 797 respondents (43%) of the 6,550 people asked and 1,868 who responded (47% excluding the “don’t know” group) agreed at the 95% level.
The authors of the survey responded to the results by looking at the smaller numbers who self-reported publications — deeming high publication numbers to indicate “expertise”. I believe that’s a poor measure of expertise, and almost a “no true Scotsman” response. But that’s a separate discussion.
What about the AR5 (2013)? They changed the headline finding from about anthropogenic greenhouse gases to all anthropogenic forcings. They reiterated their finding about GHG on page 884, in Chapter 10 of WGI:
“We conclude, consistent with Hegerl et al. (2007b) {i.e., chapter 9 of AR4}, that more than half of the observed increase in GMST {global mean surface temperature} from 1951 to 2010 is very likely due to the observed anthropogenic increase in GHG {greenhouse gas} concentrations.”
This shift in subject of the headline finding confused almost everybody – journalists, amateurs, scientists — hence the widespread comment that the IPCC had increased its confidence in human-caused climate change. They hadn’t.
I hope this helps. Additional information available on request.
Follow-up questions by Linda Qiu, and my response
I really appreciate you replying so quickly Larry! Two follow-ups.
1. Can you comment on Sen. Santorum’s reading of your analysis? He says: “The most recent survey of climate scientists said about 57 percent don’t agree with the idea that 95% of the change in the climate is caused by CO2 … There was a survey done of 1,800 scientists, and 57 percent said they don’t buy off on the idea that CO2 is the knob that’s turning the climate.”
2. Have you seen the survey author’s response to your post? Would love to hear your thoughts on that as well.
I’m glad to help.
(1) The incredible immense complexity of climate change, both as a science and vital public policy issue, defies the soundbites the define modern American politics. Almost anything about climate change that is specific, understandable, brief and exciting is probably inaccurate. {response about Santorum omitted as off-topic here.}
(2) I made several responses on that thread, before giving up. Science is often contentious. Politicized science freezes positions so that discussions often become like clashing blocks of wood.
Bart downplayed the surprising “headline” findings from question 1a and 1b, instead focusing on the results sorted by the self-reported number of publications — as a measure of expertise. I believe that’s not a useful measure of expertise, and undercut one of the most valuable aspects of the PBL survey — the breadth of the climate scientists in the sample. Some objections. —
- The breadth of the survey included people from different fields who will have different rates of publication.
- Quantity is not quality. Do a few major articles show less expertise than many publications in minor journals (even pay-to-play journals)?
- More useful measures of expertise include education, job history, impact factors, and H-indexes.
Also, the survey probably included both academics and practitioners. Do the later have less expertise because they tend to publish less? There might be a difference of opinion between the two groups, tentatively suggested by “Meteorologists’ views about global warming: A survey of American Meteorological Society professional members“, Neil Stenhouse et al, Bulletin of the American Meteorological Society, July 2014. They found lower agreement with the IPCC headline findings among meteorologists than found in studies of climate scientists.
I believe Bart points to a fruitful subject for further research, but the PBL survey provides a weak basis for conclusions about scientists’ beliefs vs their expertise.
(3) Sidenote: The comment I found most interesting was by Tom Curtis (attorney) at Skeptical Science, pointing out that AR4 and AR5 had the identical finding about greenhouse gases (in the Summary for policy-makers of AR4, buried on page 884 of AR5). I believe the authors didn’t know this. Judging from the quotes I’ve found, almost nobody did. I’m writing about this now.
This is one of the most fascinating aspects of climate change: some aspects of the material are poorly understood. Another example: how many journalists pointed out that the IPCC finding about greenhouse gases — the foundation of Clean Power Plan — was at the 90%+ (“very likely”) level, below the usually required 95% level. I doubt the FDA would approve a new flavor of aspirin (I’m dating myself here) with a 90% confidence level. Details here.
For More Information
See a report confirming the findings describe in this post: “97 consensus? No! Global warming math, myths, and social proofs” by the Friends of Science, 17 February 2014. Thoroughly documented; 48 pages long.
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- Climate scientists speak to us. What is their consensus opinion?
- In the center ring: scientists debate the process of climate science.
- Thomas Kuhn & Twitter tell us what we need to know about climate science.
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