High levels are pretty bad. They drive the greenhouse effect, so yes the warming of the planet is good for coming out of an ice age, not so good if we are not in one.
I am mad this is a question. The simplest way to answer this would be to simply look at our neighboring planet Venus. Although it is not the closest to the sun. It is hotter and less likely to be able to sustain life because of the green house affect in its atmosphere. That is all
Green house gasses is what's causing global warming.
A long time ago, the entire Earth froze over due to a lack of greenhouse gasses. A bunch of micro-organisms breathed in methane and produced oxygen thus eliminating the Earth's major greenhouse gas at that time. If we had no greenhouse gasses, the Earth would freeze over to a point worse than an ice age.
Greenhouse gases are the reason we're alive, however if there are too many than it can have harmful effects.