A weak link
But often, global warming will be the weakest, not the strongest, argument for policy X.
But often, global warming will be the weakest, not the strongest, argument for policy X. Here's why: to get people to change their minds about policy X based on concerns about global warming, you have to persuade them that:
1. Global warming is real- a claim that are still controversial in some circles, and
2. Global warming is induced by carbon dioxide emissions (ditto) and
3. Global warming is so dangerous that it should affect your jurisdiction's policies (ditto), and
4. Policy X will reduce carbon dioxide emissions, and finally that
5. Even though global warming is a really serious crisis, policy X can reduce pollution enough to limit global warming.
All of these claims will be hard to prove, for the simple reason that all of them involve the kind of scientific issues that most ordinary citizens know little about. For example, you may want to argue that more walkable communities reduce pollution by reducing driving- but even if your opponents are willing to admit the seriousness of global warming (points 1-3 above), they may argue that faster, smoother traffic flow also reduces pollution by reducing congestion. How can the average scientifically illiterate citizen (or city councilperson) know who is right?
And even in a relatively pro-environmentalist state or city, point 5 may be a hard sell. For example, suppose you want your city to rezone land for higher density. You argue that more compact development means more walking, which mean less driving, which means less pollution, which means less global warming.
Opponents of your proposal may respond: if global warming is such a huge global problem, how is making our city slightly more walkable likely to matter? Won't your proposal give us the worst of both worlds, by giving us all the negative side effects of density without reducing pollution enough to affect a worldwide problem?
Of course, this doesn't mean you should never raise global warming as an argument for policies you favor; such an argument will resonate with the most environmentally-minded voters and policymakers. But global warming should never be your only argument.