It looks like a lot of things may have important effects - no wonder it is hard to agree on the prospects for global warming:

1.) changes in the distance from the sun

2.) changes in the axial inclination

3.) solar activity

4.) greenhouse effect

5.) geological activity like volcanoes that put carbon dioxide and other gases into the atmosphere

6.) continental drift (on a long time scale)

7.) dust and clouds in the atmosphere

8.) Changes in ocean currents

9.) human activity

In addition, weather is what is termed a "chaotic system", meaning that very small changes can have large consequences in the following events. Thus, it is very hard to predict, as we all know.