Venus is a situation which we would call a runaway greenhouse . The global warming crowd is not concerned with Earth being a greenhouse. It is a greenhouse, and must be so to remain hospitable. The concern is the Earth will become a runaway greenhouse like Venus. While this mixture remains constant over long periods of time, dynamic changes occur constantly.
Microbes fix nitrogen and remove it from the air.
Different gases have different abilities to retain heat from the sun. Carbon dioxide and methane, for instance, retain heat better than nitrogen and oxygen. That is a simple statement of the theory, which was first stated a century ago. The fact is that the Earth is quite large and many factors contribute to heat retention. No one has yet devised a model that we can all agree upon.