I'm sorry, but I'm going to have to duck that question, since I'm not an economist. The Stern report is 750 pages long, and I haven't been able to obtain a copy yet.
I've read the report's executive summaries. Basically he says that it will probably be more costly to do nothing, than if in fact we tackle the issue and the threat of climate change.
There is already a debate among economists. I saw some of it in the Financial Times last week in an article by Martin Wolf. The conclusion of many of these economists is that it's folly to ignore the issue and not do anything, simply because we can't do the economics perfectly.