Telegraph: The Tory environmentalist Zac Goldsmith has a line he sometimes uses in speeches: “Saving the world might not be cost-effective, but we should probably give it a go anyway.” It’s a good gag — and fair up to a point — but in fact, we won’t save the world unless we start thinking hard about whether it’s worth it. What with the economic crisis and the drama of the G20, it’s been easy to forget about the environment recently. In fact, the WWF claims that this is precisely what world …