Well, it's an unknown, isn't it? No surprise. We never could predict jobs before, in a fossil fuel economy. No reason to think we'll be all that much better at it in a green economy.

There are some reasons for long-term optimism, though - reasons Mr. Levi didn't get around to addressing in his short article for Slate.

Let's see if we can repair the damage:

- Less pollution means better health. This is probably a very big deal for the economy. Better health means fewer sick days. When you feel good, you work better and smarter. You're more ingenious. More optimistic. All of these things play into economic strengths.

- Energy portability. In the short term, electricity is less portable than petroleum products, because of limitations of battery technology. Shifting over to battery-powered cars and weed-whackers will arguably be a drag on economic strength. But in the long term, the picture changes. The energy density of a tank of gasoline is fixed by the laws of chemistry. The energy density of a battery (or ultracapacitor) is not fixed, and is increasing every year. We don't even have a theoretical upper limit in mind yet. What this means is that portability of energy will eventually be *better* than it is today, maybe a lot better, and that should pump up economic strength.

- Economies of scale. Right now solar and wind are more expensive sources of electricity than coal. But every year the prices have been converging as economies of scale in the green power industry improve. Perhaps it's too soon to insist that green power will eventually be cheaper than coal without incentives and disincentives to distort prices. Not enough data to prove the case. But it's a real possibility.

In other words, we may end up with a green economy that produces energy more cheaply, makes it more portable, and leaves us in much better health, all very powerful forces for economic robustness. I'm not saying we'll get there quickly. But I think there is reason for considerable optimism.