There are serious skeptics. Like you say nobody really disagrees with 1) humans release more c02 and 2) more c02 = warming via greenhouse.

But that isn't the entire global warming theory.

There are many feedback effects that occur. Like warming -> melting ice -> water vapor from condensation -> water vapor is a greenhouse gas -> more heat

But there are negative feedback effects too. Like warming -> melting ice -> less reflection of sun -> less heat

So many people are pushing theories with high feedback multiples. Some people are pushing theories with high negative feedback.

Depending on how you model the feedbacks--Climate change can either be a small issue with a degrees or two warming or a mass extinction event with 5 degree C warming.

The problem is it is very hard to test the models.