To create a metaphor: If I worry about some growth on my body, I ask a doctor, not an IT guy who never studied medicine even once. Further, if I go to 20 doctors, and 19 say "you are sick, you need to buy medicine" and one says "Aw, it's nothing," I'll trust the 19.
This is how stupid it is to think global warming isn't manmade. You've gone to 20 doctors and only one says it's nothing, and you believe the one.
Plus, let's look at outcomes.
1. What happens if we act, and it isn't real: Slightly cleaner air, slightly cleaner water, less dependency on foreign nations for fuel, and maybe some overspending. Overall, not catastrophic. Economic green jobs may help, or maybe it hurts the economy. That's a danger, to be sure. Worst case: moderate to sever economic calamity.
2. We don't act, it isn't real: Life as usual.
3. It's real, and we act: We avert or partially avert whatever catastrophe would have occurred, thousands or millions are saved, fewer cities flooded, more clean water for the world, etc.
4. We don't act, it's real: major cities flood, wars occur over clean water or land that can support agriculture, millions, even billions could die. Economic collapse happens anyways. Clearly the worst case. Again, worst case here: Mass death, severe economic disruption, wars, flooding of major cities, loss of resources, thousands of species die, etc.
Acting now, just to be safe, is clearly the better bet based on cost/benefit. Also the more well supported bet.