The skeptic vs denier debate won’t go away. I fear the issue is far too nuanced for a broad popular consensus. But that should not prevent a consensus among science communicators, who should have a technical understanding of terminology.

A recent editorial in Forbes illustrates the problem. The author, Brian Brettschneider, makes a recommendation for when to use which term, which sounds superficially reasonable but I think he misses the essence of the issue. His solution is this – if you have an advanced degree in climate science and you have doubts about the mainstream view, then you are given the benefit of the doubt and should be referred to as a skeptic. If you do not have a formal degree in climate science, then you have no business going against the consensus of mainstream scientific opinion and you should not be given the benefit of the doubt, and are hence a denier.

This is not a bad rule of thumb as an initial assumption, but does not work as a technical distinction.

First let me say that I agree with the underlying premise. It is not a logical fallacy (argument from authority) to defer to a strong consensus of legitimate expert opinion if you yourself lack appropriate expertise. Deference should be the default position, and your best bet is to understand what that consensus is, how strong is it, and what evidence supports it. Further, if there appears to be any controversy then – who is it, exactly, who does not accept the mainstream consensus, what is their expertise, what are their criticisms, and what is the mainstream response? More importantly – how big is the minority opinion within the expert community.

This is where a bit of judgment comes in, and there is simply no way of avoiding it. There is no simple algorithm to tell you what to believe, but there are some useful rules. Obviously, the stronger the consensus, the more it is reasonable to defer to it. There is always going to be a 1-2% minority opinion on almost any scientific conclusion, that is not sufficient reason to doubt the consensus. But you also need to find out what, exactly the consensus is, and what is just a working hypothesis. Any complex theory will have multiple parts, and it’s not all a package deal.

For example, let’s take evolutionary theory. There is almost unanimous consensus (>98%) among experts that evolution happened, that all living things on Earth are related through a nestled hierarchy of common descent. Further, the evidence for that conclusion is overwhelming and cannot be reasonably denied. Further still, there is no alternative scientific hypothesis that can account for that mountain of evidence (note the word “scientific” in that sentence). But the same is not true of all aspects of evolutionary theory. That natural selection is a main driving force of evolutionary change is also well established, but there is still legitimate debate about the role and magnitude of other factors, such as genetic drift. When we drill down to details about which species evolved into which other species and when, drawing a precise tree of evolutionary relationships, then there is considerable debate and much that is unknown.

The same is true of climate change, which is often the focus of the skeptics vs denier debate. That the Earth is warming at a faster rate than has historically been seen is fairly solid, with about 97% of climate scientists (yes, that is the real number) agreeing that this is almost certainly true. That this forcing of the climate is largely anthropogenic is also fairly certain. But the more detailed we get, the less certain we get also. Exactly how much warming will happen in the future, with what climate sensitivity, and with what effects becomes increasingly murky as we try to extrapolate further into the future. Also, what will be the effect of specific policies to mitigate warming is also open for debate.

With this as background, let me propose an alternate definition of skeptic vs denier. Actually, I already did:

– Deniers do not fairly assess the scientific evidence, but will cherry pick the evidence that seems to support their position.

– They will make unreasonable or impossible demands for evidence, move the goalpost when evidence is presented, and refuse entire categories of legitimate scientific evidence.

– They will attempt to magnify scientific disagreements over lower level details as if they call into question higher level conclusions. (For example, biologists might disagree over the details of evolutionary history, without calling into question evolution itself.)

– They primarily focus on sowing doubt and confusion over the science they deny, rather than offering a coherent alternate theory or explanation.

– They will exploit ambiguity (and even create ambiguity) in terminology or employ shifting definitions in order to create confusion or apparent contradiction.

– They will attack scientists personally, and engage in a witch hunt in order to impugn their reputations and apparent motives.

– They will cast doubt on whether or not a scientific consensus exists, attempt to claim that the tide is turning in their favor, or claim that a secret consensus of denial exists but is suppressed. They may also cite outlier opinions as if they were mainstream.

– When all else fails they will invoke a conspiracy theory to explain why mainstream views differ from their own.

In short – being a denier is about your behavior, not your position or even necessarily your credentials. A climate scientist with impressive degrees can be a denier if they act like one, and a lay person can be a skeptic if they act like one. By contrast, how does a legitimate skeptic behave:

– A skeptic will try to understand the scientific consensus and defer, as a default, to superior expertise.

– A skeptic will deviate from the mainstream view only cautiously, reluctantly, and with very good specific reasons grounded in logic and evidence. In short, a good skeptic is humble.

– A skeptic is open to any conclusion, going wherever the evidence and logic leads. Specifically, they will follow the evidence, and not start with a conclusion and then backfill the evidence.

– A good skeptic will not rationalize away contradicting evidence or problems with internal logical consistency, but will modify their opinions accordingly.

– Above all a good skeptic is intellectually honest.

So you are a denier if you behave like a denier, and a skeptic only if you behave like a real skeptic. This is all about process, not any particular position.

This also means that if you call someone a denier you should be prepared to back up that designation with specific examples of how they are behaving like a denier. It is also fair to refer to a position or even movement with the term denier or denial. It’s fair to refer to “global warming denial” as a phenomenon, especially since there is a solid-enough consensus on the basic conclusion that the Earth is warming and humans are causing it that it does create a reasonable starting position that anyone who disagrees is engaging in denial until proven otherwise.

And a lot of this does have to do with the burden of proof. Anyone making a scientific claim carries the burden of proving that claim. However, once a claim has met that burden, to the satisfaction of a vast majority of experts, the burden of proof then shifts to those who would refute the consensus. Now that it has been clearly demonstrated that DNA is the molecule of inheritance, for example, anyone proposing an alternate view has the burden of proof.

Again, I don’t suspect this debate will go away, if for no other reason than deniers reject the label. Of course they do – if they are deniers by definition they don’t understand the difference between denial and skepticism (or they do but are being deliberately intellectually dishonest). But science communicators and journalists should know the difference.