From a purely scientific point of view, if there were no atmosphere, then the natural temperature of the earth would vary dramatically, but hover around an average of about -18 degrees C.
It is a fact that the atmospehre traps heat, and thus warms the average temperature of the earth to life sustaining levels. There should not be any dispute on this point.
I think the dispute comes from additional warming caused by human activity. Some people say that human activity has nothing to do with increasing temperatures. However, the temperature record shows that the average temperature of the earth has increased since the mid 1800's when reliable records were first kept. This temperature record is not in dispute by any honest scientist.
What has not been conclusively shown is how this increase in temperatures will affect our climate in the future. Most honest scientists will say they don't know for sure. Most climate scientists argue that because we don't know, we should not gamble with the future of the planet on something that., if we get it wrong, would totally wipe us out.
People arguing the other side say that we don't know if increasing temperatures will cause problems, and we shouldn't destroy our economy with draconian measures that massivley increase the cost of energy and lower the standard of living everywhere.
In summary, Global warming is a scientific fact. The consequences of that temperature change is where the debate lies.