Climate change is a huge, knotty, incredibly difficult problem. The more you dig in and understand the science and politics of it, the more hopelessly vast and complex it can seem. What's more, the public has not even begun to grapple with it; public discussions, especially in the U.S., remain polarized, shallow, and stupid.
Given this situation, it's natural for climate hawks to yearn for a Grand Gesture, something that clearly announces our intention to Solve the Problem. They want a policy response as powerful as the threat, one that can power past all the fog and ignorance and greed.
In short, they want Archimedes' lever. "Give me a lever long enough and a fulcrum on which to place it," said the great scientist, "and I shall move the world." The pursuit of carbon targets by environmentalists and carbon taxes by economists both, I think, reflect this yearning.
First, carbon targets. Over the years, I've become much more skeptical about targets as a goal of climate policy. The key insight here is that targets do not, in and of themselves, have any motive force. A target is not an emissions-reduction policy, any more than a weight-loss goal is a way to lose weight. A carbon target is a promise, a commitment to develop emissions-reduction policies.