Doctor Who: I don't deny climate change. What I deny is the influence man has on it.
How do we know climate change is real and that much of it is due to human activities? That's a great question. As a science teacher, I would encourage you to examine the evidence, of which there is plenty (NASA: "Climate Change: How Do We Know?").
"The current warming trend is of particular significance because most of
it is extremely likely (greater than 95 percent probability) to be the
result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia" - IPCC Fifth Assessment Report, Summary for Policymakers, (Emphasis added).