You know the one I'm talking about. "97% of scientists believe in climate change" or some other variant. I can't tell you how many people have cited this number to me. The funny thing is, the speaker is always highly precise that it's 97%, but what exactly that 97% refers to varies wildly. It's of the form "97% of X say Y", where X ranges from "scientists" to "climate scientists" to "climate scientists who actively publish research", and Y can be anything from "humans are causing climate change" to "we're in a climate crisis" to whatever issue they are talking about at the moment. And when it's from a left-leaning source, the following line is usually "but it's actually even higher because..." and from a right-leaning source it's about it being "debunked".
So, what does this 97% actually refer to? Although there are a variety of surveys of climate scientists, I think the one that popularized the 97% statistic is from "Quantifying the consensus on anthropogenic global warming in the scientific literature" by Cook et al. Here's what that study says (my bolding):
We analyze the evolution of the scientific consensus on anthropogenic global warming (AGW) in the peer-reviewed scientific literature, examining 11 944 climate abstracts from 1991–2011 matching the topics ‘global climate change’ or ‘global warming’. We find that 66.4% of abstracts expressed no position on AGW, 32.6% endorsed AGW, 0.7% rejected AGW and 0.3% were uncertain about the cause of global warming. Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position that humans are causing global warming.
And here's the table from the report:
So, once and for all, the "Y" value is that humans are causing global warming. Nothing more, nothing less. This number has nothing to say about the impacts of this warming, what downsides there might be, how to combat this, or what tradeoffs are worthwhile for societies to make.
I want to show an example of why it annoys me. The first place I looked to get the context on this statistic was Skeptical Science, one of my favorite places for detailed information on climate science. However, I was disappointed by their page on this stat, which is surprising because I think it's a great site. It makes the exact mistake I'm talking about, so now it will serve as my foil instead of my evidence. Here's how they start their page on the 97% statistic:
The "science" and "myth" are saying two different, non-contradictory, things. The site describes the notion that "There is no convincing scientific evidence that human release of carbon dioxide will, in the foreseeable future, cause catastrophic heating of the Earth's atmosphere" as a myth, suggesting that there is evidence of future catastrophic heating. I don't know anything about the Petition Project, but I do know that none of the surveys found that 97% of anyone agreed that something "catastrophic" is happening.
Has it been "debunked"? I've seen that phrase many times regarding this number. One of the better sources for it is Steven Koonin, who in his book Unsettled: What Climate Science Tells Us, What It Doesn't, and Why It Matters, says: "The study that produced that number has been convincingly debunked" and references "Comment on 'Quantifying the consensus on anthropogenic global warming in the scientific literature'". I read that study and found it somewhat interesting, but it didn't convince me that the true value is meaningfully different. (As always, I linked to it so feel free to read and draw your own conclusions.) It's sort of funny that Koonin called it "debunked". From reading his book, he is absolutely on the side of the 97% and I imagine would argue vehemently with that remaining 3%.
Note: I looked at several other surveys that asked similar questions. The results weren't different enough to go into here, but for those who want to explore this in more detail, take a look at Oreskes 2004, Doran 2009 or Anderegg 2010. An updated report by the same author found that "The consensus that humans are causing recent global warming is shared by 90%–100% of publishing climate scientists according to six independent studies by co-authors of this paper." I'll quote the last line because it's exactly what I was thinking, "From a broader perspective, it doesn't matter if the consensus number is 90% or 100%. The level of scientific agreement on AGW is overwhelmingly high because the supporting evidence is overwhelmingly strong."