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:
- Don't be put off by the moderate length. I believe the paper itself to be a fairly quick and easy read. There's no complicated maths or anything like that.
- This was a "pass/fail" essay, ultimately aimed at gaining admission to sit the exam. Please evaluate the material accordingly.
- That said, the emphasis on citations and quotations probably means that it provides a good overview of the issues.