Although it is true that global climate change is the result of eccentricity, obliquity and variation in the tilt of the earth's poles, the thing is, so-called greenhouse gases are of concern, because we know that gases in the atmosphere cause energy to be "trapped". The earth tilting to absorb more radiation in a concentrated area from the sun (the cosine law), as well as an orbit which places the earth closer to the sun leads to a rise in temperature. However, to deny that gases in the atmosphere have any effect on earth's climate is silly.

Gases like all matter, react differently to radioactive energy. For example, if a gas appears dark, we know it is absorbing a lot of the light spectra. If you ever harvested salt using seawater and black construction paper on a hot day, you know that dark colors tend to absorb light, and transmit it as heat (as there is always energy "loss" emission as various other em waves) Gases, like all matter can also reflect or transmit em waves as well. Ozone is probably one well accepted gas that has the effect of reflecting much of the harmful radiation in the atmosphere, as well as absorbing the earth's reflected rays. Greenhouse gases work similarly; they absorb even more of the earth's reflected rays. Because of the laws of thermodynamics energy in must be equal to energy out, as energy and matter are neither created nor destroyed. So, the CO2 molecules must re-emit the radiation in some form, usually lower energy, and at the low energy end, below the light spectra (which is the spectra the sun primarily emits) is infared, or heat. In essence, the greenhouse gases tend to trap energy in the atmosphere longer, leading to a rise in temperature. (The energy does eventually escape) The most damning proof of this is to simply take a spectral image of the earth's emitted spectra, and compare it to that of a perfect black body of equivalent size. You will notice a spike at 10^3 micrometers, followed by a deep trough. This could not be caused by simple orbital patterns. Something is trapping the spectra above 10^3 micrometers.