One day I was sitting in an Atlanta traffic jam wondering about all the engine exhaust heat warming the localised environment. Later that night, things will have cooled down. Where did the heat go? On a global scale, I understand our sun heats the earth via radiation, not conduction as the intervening vacuum of space cannot transfer heat. So if the sun has been beaming gazillions of watts of energy into the earth over the last few billion years, and energy is conserved and the earth does not radiate out daily a material proportion of the radiation it has absorbed, where has all that energy gone?
The hotter the Earth gets the more it re-radiates energy back into space.You can see a tiny fraction of this energy by looking at the new moon. The feint glow is just a small part of the re-radiated spectrum.
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.
A body has to emit as much thermal energy as it absobs to remain in thermal equilibrium. The Earth has been doing the same since its formation, i.e, it absorbs solar shorwave radiation by its atmosphere, solid earth and water body, and releases it in the form of longwave radiation. The amount that is released has to be equal to the amount that is absorbed to maintain same temperature. However, due to much emission of greenhouse gases in to the atmosphere (or according to sceptics of such concept due to solar activity, volcanic emission), some amount that the Earth has to release is absorbed by the gases in the atmosphere and are causing the Earth to warm by few degrees every year.