I think Andy in particular and slapshot are missing something about the model being used. The simple picture is that of a bath with a tap putting water in, and a plughole letting water out again. If the two rates are in balance then the water level remains about the same. On the other hand, someone throwing a bucket of water into the mix and the level rises -- or, simply, adding a new tap, with a slower rate. (or even drilling a new hole). In both cases the delicate balance is disturbed.
The human contribution to Climate Change is that small tap. It's clear and obvious that our contribution will make a difference, even if on the face of it our emissions are dwarfed by those of, saying, rotting plant life in lakes and rivers, or volcanic emissions. Those were always there, and formed a background. The human signal on top of that is strong, clear, and evidently dangerous.
When you add to the mix of CO2 emissions, the extra contributions of methane (principally from livestock), and CFCs, which are entirely artificial, there should be no room for doubt that human activity is going to impact the planet. The remaining question is no longer "if" human activity is making a difference, but what that difference is going to be.