The contest is joined. On one side there is the near-unanimous conclusion of thousands of active climate scientists throughout the world: the global climate is changing and human technology is the primary cause. From the other side we are told that "climate change" is at worst a "hoax" or at least a normal and natural phenomenon not significantly affected by human activity. This position is endorsed by right-wing media, almost all congressional Republicans, and a few bought-off "scientists" ("biostitutes") lavishly funded by fossil fuel industries.
So how do you deal with a "denier" willing to engage you in a debate?
If the "denier" tells you that "God would not allow the climate to change" or that "Jesus will fix all that when he comes back in the next few years," and then quotes the Bible as "evidence," save your breath and his time. His is a hopeless case.
But if your adversaries are citing what they believe is "scientific fact" or otherwise exhibit some indication of a capacity to yield in the face of scientific evidence, they just might listen to reason and consider evidence -- but don't count on it.
You might proceed by citing scientific studies, to which your opponent will likely respond with anecdotes, out-of context quotes, and citations of dissenting "biostitutes" (Cf. "The Tobacco Institute"). But this promises to be an endless harangue. As one wit put it, "for every Ph.D there is an equal and opposite Ph.D." Except, of course, in this case, with regard to the weight of empirical evidence, the "experts" in question, while "opposite," are not equal.
Three Questions for the Denier:
Instead of citing an endless list of scientific studies, I propose a different approach. Pose just three questions.
"Putting aside for the moment the issue of the reality of climate change, will you acknowledge that a recent survey of 10,000 active climate scientists found that 98% affirmed the existence of anthropogenic climate change?"
"Will you acknowledge the existence of a recently released report of the International Panel on Climate Change (IPCC), an agency with 195 member countries, which concludes with 95% confidence that the climate is changing, due to human activity."
"How, then, do you deal with these acknowledged facts?"
Now he might reply that the press has lied: that there never was such a survey, and that there is no such thing as the IPCC. But such a reply will only confirm that your adversary is a certified citizen of Fantasyland, and that it is time for a polite but prompt exit.
But if your opponent answers the first two questions affirmatively, it seems that there are only four conceivable responses to these compelling facts:
1. "Global climate change" is a hoax, perpetrated by a world-wide conspiracy of thousands of scientists.
2. Those scientists have been "bought off" by funding agencies -- primarily governments -- who have a secret agenda (variously described).
3. These scientists, along with their inferences from thousands of peer-reviewed accounts of field and laboratory studies, are all simply wrong.