I had a lengthy post on this topic on another thread, so I won't copy it all here. What I will mention is that until about three years ago I was quite a skeptic of the causal relation between human effects and warming. The paper I cite in the above link I found to be an excellent study to answer exactly that question. The study found that both natural and human effects played a role, and I don't find their conclusions particularly extremist. However, many who read them cannot escape the evidence for human contribution to warming, and this leads to more dramatic statements in the media.
Obviously there's a variety of things that can influence the global climate in possibly non-linear ways. However, CO2 is the dominant greenhouse gas, and ice samples have shown it to be fairly correlated with temperature. Together with experiments that show greenhouse gases effects on temperature, we can assume some causality.
