[Update: 22 January 2009] Some of the information in the post below is inaccurate. Namely, it implies that some level of continuous emissions is compatible with climate stabilization. In fact, stabilizing climate required humanity to have zero net emissions in the long term. For more about this, see this post.
The graphic above is probably far too nerdy to have popular appeal – and it is possible the numerical figure will need revision – but it does strike me as a concise expression of what needs to be tackled.