Premise #1: Global Warming is bad.
Premise #2: Global Warming is caused by an increase in "greenhouse gases" primarialy carbon dioxide
So far I am OK.
Question: What percent of the 155 billion tons of carbon dioxide pushed into the atmosphere each year comes from human activity?
Answer: Just over 4%.
It may be the case that we are looking at an effect which is a longer version of night and day or winter and summer - something more or less natural.
If we hit a "tipping point" because of human activities this must have been a delicately balanced system. While that may be the case it is something less than obvious.