Anthropogenic climate change is a theory in which the basic mechanism is well understood, but in which the magnitude of the climate change is highly uncertain owing to feedback processes. We know that the climate changes naturally on decadal to century time scales, but we do not have explanations for a number of observed historical and paleo climate variations, including the warming from 1910-1940 and the mid-20th century cooling. The conflict regarding the theory of anthropogenic climate change is over the level of our ignorance regarding what is unknown about natural climate variability.
Great read on climate change, heavily recommended.
