Greenhouse gases kind of do two things. They both trap and repel heat, like insulation. In the short term, it traps heat. Eventually the heat dissipates (not the correct or scientific term) and starts repelling more heat than it is trapping, which translates into cooling.

We could, very rapidly, have rapid (relatively) heating followed by rapid cooling, likely in the form if an ice age.

The earth goes through these cycles naturally, but much more slowly. Man made climate change is a shorter way of saying "We are speeding up the natural cycles of earth heating and cooling, which means organisms will not adapt as easily and could lead to catastrophic events."

I am not a scientist, so please someone correct me if I'm wrong on any of the above.