As it happens, I recently wrote a term-paper on this exact issue for my philosophy of science class. I've decided to make it available here for those who are interested in this debate!
To summarise, I don't see anything inherently wrong with scientists engaging in political advocacy, as long as they are explicit in their intentions... and the scope of their expertise. (E.g. I often see physical scientists make strong pronouncements about economic matters and that makes me uncomfortable.) One climate scientist whom I feel always struck a good balance on these issues and quote in my term-paper is the late Stephen Schneider. To channel Schneider: Our response to climate change must be underpinned by scientific facts, but it should ultimately also be reflective of society's value judgements — including those of our scientists.
Some brief points/caveats:
- This was a “pass/fail” essay, aimed at gaining admission to sit the exam, and I hope that you’ll evaluate the material accordingly. That said, the emphasis on citations and quotations probably means that it provides a good overview of the issues.
- Apart from scientists working on the physical basis for climate change, I also tried to pay special attention to the role of economists. The Stern Review, which many people regard as the archetypal blend of economics and advocacy, therefore comes in for special attention.