How to respond to a global warming skeptic?

Have you ever discussed global warming with someone who’s skeptic about it? Is hard isn’t it? It doesn’t really matter how many studies, graphs or papers you show him/her, they will have no effect. So, what can we do when facts don’t seem to matter? Here’s a proposal.

Some people in the scientific community see this problem as a no-brainer. They just assume that it can be resolved by facts: “if there is a GW skeptic show him some graphs and figures. If he’s not convinced, then he’s just an idiot or he is lying to you.”. But that is not the case: more data won’t do anything, since many skeptics are acting as Greek Skeptics: they are not only dubious about the facts presented, they are dubious about the structure of knowledge itself[1].

Greek Skeptics developed many  arguments to prove that, since knowledge is not possible, we need to suspend judgement about everything we know. One of my favourite arguments is Agrippa’s Trilemma (rebranded later as Munchaussen Trilemma). This argument claims that we can’t know anything at all since everything we claim to know needs a justification. Therefore, if I claim P, the skeptic would ask: how do you know P? To which I must respond with Q. Then he will ask: how do you know Q? To which I must respond with R. Then the skeptic will keep going. Finally, I will have only three options:

  1. Justify ad infinitum: P because Q because R because S, and so on.
  2. Stop at an unjustified premise: P because Q because R.
  3. Reason in circles: P because Q because R because P. [2]

Now, this is exactly the kind of argument the GW skeptic uses. Imagine the following dialogue between a global warming believer (GWB) and a global warming skeptic (GWS)

GWB: C02 driven global warming is happening.

GWS: How do you know?

GWB: because I read it on the IPCC report.

GWS: How do you know that is true?

GWB: Because it shows a consensus of the leading scientists in the field.

GWS: How do you know that consensus is real and not fabricated?

GWB: Because there are many scientific practices, journals and institutions behind it.

GWS: Hou do you know those institutions aren’t corrupt.


As we can see, the dialogue can keep on going forever. The skeptic can always ask for a new justification and the believer will fall into one of the three outcomes predicted by Agrippa. The GW skeptic will go home with the idea that he defeated the believer and will reinforce his skepticism.

 So, what can the believer do? The traditional epistemological answers to this problem have been two: foundationalism (option b of the trilemma) and coherentism (option c of the trilemma). I won’t try any of these solutions since I believe that this is not an epistemological but, rather, a practical and argumentative problem[3]. The right answer, then, is to use a presumption.

What is a presumption? That is indeed a good question. “Presumption” is a term borrowed from the legal field, so it is clear what they mean in that field, not so much outside of it. The Cambridge dictionary defines it as “the act of believing that something is true without having any proof”. In the legal field it is totally necessary to believe certain things without any proof. For instance: everyone is presumed to be innocent. That means that nobody needs to prove his/her innocence in any way, is the accuser the one who has to prove guilt.

Outside the legal field, argumentation theory has also used the concept of presumptions (see Walton 1996). The reason to do it is to resolve a fatal flaw in assertions. An assertion is any statement I present whose truth I believe. If I say: “the door is open”, “god exists” or “global warming is happening”, those are assertions as long as I believe them to be true.

The flaw of assertions is the following: whenever I use an assertion I have the burden of proof to prove it. Therefore, if I say, “global warming is happening”, I’m saying something like: “I’m justified in believing that global warming is happening.” Therefore, my interlocutor has the right to ask: “how do you know that?”. And the only way in which I can answer is by using a new assertion that will give me, again, the burden of proof. So, the interlocutor will ask again: “how do you know that”, and so on.

The conclusion is simple: if it is true that any party making an assertion takes the burden of proof, then the interlocutor can always ask: “how do you know that?”. We get Aggrippa’s trilemma all over again.  

But here comes a presumption to save the day. Presumptions shift the burden of proof. So, if a presumption is in place, the one who has to provide a proof is not the one who makes an assertion, but the one who doubts it. The relevant question here is the following: is there a presumption in favour of someone asserting that global warming is real? I say it is, at least in most cases: There’s an authority presumption in place.

People usually get confused over fallacies and legitimate ways of reasoning. One of these cases is the use of arguments from authority. Arguments from authority are perfectly valid, as long as the authority cited is actually an authority on the field. If not, it is a fallacy called “ad verecundiam”.

Compare these cases:

  • I believe in global warming because the IPCC says so.
  • I believe in global warming because my mother says so.

While (1) is a perfectly valid argument from authority, (2) is a fallacy ad verecundiam, since my mother is no authority on climate science[4].

In conclusion, my claim is the following: when I use an assertion like (1) I’m not only using a valid argument, but I’m also using a presumption: since the IPCC is an authority in climate science and I have no expertise to doubt its findings, we can presume that what they say is true.

That doesn’t mean that the conclusion is undoubtfully true, nor that (1) cannot be defeated. It only says that, as long as the interlocutor is not also an authority on the field, we need to believe what the authority says. Then, since the burden of proof has been shifted is not the one who makes assertion (1) the one who must prove it, is the counterpart the one who must provide grounds for criticism.

Given so, the dialogue between the skeptic and the believer would look like this:

GWB: C02 driven global warming is happening.

GWS: How do you know?

GWB: because I read it on the IPCC report.

GWS: How do you know that is true?

GWB: They are an authority on the field, what grounds do you have to doubt them? Are you a climate scientist?



Klein, Peter (2008). Contemporary Responses to Agrippa’s Trilemma. In John Greco (ed.), “The Oxford Handbook of Skepticism”. Oxford University Press.

Walton, D. (1996) “Argumentation Schemes for Presumptive Reasoning” (Studies in Argumentation Theory). London: Routledge

[1] The SEP has a nice introduction to Greek scepticism:

[2] See Klein 2008 for contemporary answers to this problem.

[3] The bigger picture is the following intuition: “you can’t defeat a skeptic with theoretical arguments, only with practical ones”.

[4] (1) could also be fallacious if the one asserting is a climate scientist. In that case she should read the original papers, not blindly trust a source.