Global warming can be a natural event caused by volcanic activity or meteorite strikes when the planet was young, as a result of this there has been mass extinctions of both animal and plant life throughout the various epochs. The Earth has recovered from these events but has taken millions of years to do so, That I learnt in first year Geology at Uni.
Now we see the same conditions arising again but the Earths volcanic activity is a fraction of what it was when it was younger and I certainly haven't heard of any large scale meteorite strikes recently so something must be causing it and it seems that these temps/pollution levels have risen since the start of the industrial era and the widespread removal of forests.
You can listen to the arguments for and against by scientists and politicians, but all you have to do look at what is happening around us, the conditions speak for themselves.
cheers