For example, the greenhouse effect is a piece of well-established science. Climate change is even less scientific. To what extent humans contribute to the problem is even less scientific. Analyzing the costs/benefits of climate change is an even less scientific question. And what sorts of policy prescriptions might be effective is the least scientific question of all.
Climate activists fail to acknowledge any of that nuance, instead lumping all of those things together and labeling anyone who has a nuanced opinion on one of those points as a "denier".
It's total intellectual corruption.