Really? That's a good one. Ok, these scammers have been promoting the idea that CO2 is the problem. Fine. Explain this:
http://www.geocraft.com/WVFossils/last_400k_yrs.html
This shows that global temperature has been cycling on its own (long before mankind was a potential factor). Oddly enough, it also shows that CO2 increases lag behind increases in temperature. Kind of hard to twist that into a causal relationship in any way.
Earth's climate has been changing ever since there has been an Earth's climate.
Or just toss this on them and wait while the crickets play a tune:
1. Define the “correct” temperature range for the planet.
2. Define the “correct” humidity range for the planet.
3. Define the “correct” mean sea level for the planet’
4. Define the “correct” amount of precipitation for the planet.
5. Define the “correct” makeup of the atmosphere.
6. Define the “correct” amount of sea ice at the N/S poles.
7. Define/explain past glaciation and subsequent warming without any input from humans.
Go ahead, call me a denier. I wear the label proudly. :>)