The long term differential between incoming and outgoing energy is what is causing global warming. It isn't 'a few degrees (Centigrade) every year'. It is currently in the order of about 1.5 degrees per century. That's because of the following reason:

Initially, the carbon dioxide makes Earth emit less radiation than it's receiving. That heats up the air until the oceans are sucking heat out of the air at the same rate as it's being heated by radiation. That process is slowly warming the oceans so they're suck heat out of the air slower so Earth will get hotter. As a result of the higher temperature, the ground will emit radiation at as fast a rate as it's absorbing radiation from the sun and re-absorption from the carbon dioxide. The reason the average temperature is rising is because the amount of Carbon dioxide was increasing.

Notwithstanding some diehard skeptics (who refuse to entertain the facts) the effect of greenhouse gasses in raising the global temperature is now so well verified as to be beyond doubt. Yes, variation in insolation rates and vocanic emissions also plays a relatively minor role, but cannot possibly explain all the evidence of global warming.