(b) Baez has written elsewhere that he's risk averse with respect to charity. Thus, he may reject expected value theory based utilitarianism.
You can work towards a positive Singularity based on purely selfish motives. If he doesn't want to die and if he believes that the chance that a negative Singularity might kill him is higher than that climate change will kill him then he should try to mitigate that risk whether he rejects any form of utilitarianism or not.
John Baez's This Week's Finds (Week 311) [Part 1; added for convenience following Nancy Lebovitz's comment]
John Baez's This Week's Finds (Week 312)
John Baez's This Week's Finds (Week 313)
I really like Eliezer's response to John Baez's last question in Week 313 about environmentalism vs. AI risks. I think it satisfactorily deflects much of the concern that I had when I wrote The Importance of Self-Doubt.
Eliezer says
This is true as stated but ignores an important issue which is there is feedback between more mundane current events and the eventual potential extinction of the humane race. For example, the United States' involvement in Libya has a (small) influence on existential risk (I don't have an opinion as to what sort). Any impact on human society impact due to global warming has some influence on existential risk.
Eliezer's points about comparative advantage and of existential risk in principle dominating all other considerations are valid, important, and well-made, but passing from principle to practice is very murky in the complex human world that we live in.
Note also the points that I make in Friendly AI Research and Taskification.