My response to climate change skeptics is to point out that CO2 is firmly established as a "greenhouse gas", so increasing its levels in the atmosphere will inc

My response to climate change skeptics is to point out that CO2 is firmly established as a "greenhouse gas", so increasing its levels in the atmosphere will increase temperatures, regardless of any other measurements, theories, etc. So the question a skeptic should answer is what would be preventing this increase if climate change was a hoax?