"Climate change denialism is a natural state of mind"
We as apelike animals just did not optimize for thinking globally. We optimized for the local here and now and the immediate future.
"Fire burns, fire warm, fire good. More firewood we need!".
And I think why we're having such a hard time with "climate denialism" is because we're not really presenting arguments against the underlying argument.
First, we're talking about trend lines on the order of less than +1C per human lifetime. Recently, there was some buzz here in Toronto about some day or other having been the hottest that-calendar-day in a very long time, and near the record since measurements started 200 years ago. But if you look at that scatter plot, what you see is that yes, the trend line goes up by perhaps 2C over that period, but the year-to-year variation is on the order of 20C. And the difference between the average daily highs in the hottest and coldest months here is about 27C, to which you can add about another 8C for intra-day variation from high to low. Month by month, the recorded extremes of heat range 12-20C above the averages, and record lows plunge 13-27C below averages for daily minimums. All in all, a temperature range of over 73C has been observed here.
Regardless of the consequences scientists expect as a result, a couple of degrees of warming since the Industrial Revolution (with some more effectively priced in for the future) is mere noise against that backdrop. Humans are simply not sensitive to that rate of change; nor can they be expected to realize the effects intuitively given that they're adapted to dealing with such great natural variation in temperature. So they have to know the science to get there. The result is not intuitive. If it were, there would have been no need to do the science in the first place.
Almost no humans are equipped to replicate the science themselves - there are huge barriers in every category: awareness, willingness, time, resources and knowledge (of scientific methods, of research methods, perhaps even of how to use more sophisticated equipment than just thermometers). So they have to trust the authorities that present the science to them.
Trust in authority is not natural for humans - it has to be socialized into them. This is especially the case for humans born and raised in a democracy, and especially when the authority in question is implying a need for lifestyle changes that seem like they would cause lower overall quality of life. If that trust were natural, North American schools could actually focus on education.
Climate change is a coordination problem. In a coordination problem, treating non-cooperators as opponents - especially by implying that they've been brainwashed by some other party, thus denying them agency - is an incredibly shortsighted and counterproductive move. Especially when it comes with such openly tribalistic framing (i.e. citing as evidence some partisan bias in lobbying by specific businesses).
In short: people don't believe you because you don't show them things they can see for themselves, and you frame yourself as someone who wants them to sacrifice themselves for a greater good that you don't make legible to them. Warning about the threat of impending doom is not presenting a legible "greater good". If that worked, everyone who lives in Christian-majority countries would be an evangelical.
* Value moral superiority and "being right" over results.
* Broadly think that people who categorically disagree with them are stupid and just need to be educated about the truth.
* Believe that the mere existence of climate change implies that we have to do everything they say to combat it.
* As a group are largely incapable of knowing when they're being put on and baited.
So say hypothetically you "deny climate change." But of course you don't outright deny it, you say that there's no evidence. The discussion shifts away from what the proper response to climate change is to whether it even exists. In public discussions you can dismiss any argument with "well it doesn't even exist." They will then proceed to spinlock boiling the oceans with the energy expenditure trying to prove it exists— "surely this next piece of evidence will be undeniable and I'll have them cornered!"
But that couldn't possibly work, right?
Also, there is no downside believing the earth is round.
But believing burning things causes global warming is a) way more abstract b) inconvenient, as it makes you question your luxory. For some it is apparently easier not believing it and maintain a (pseudo) clear consciousness.