James Hansen is a great climate scientist. He was the first to warn about the climate crisis; I take what he says about coal, in particular, very seriously.

Unfortunately, while I defer to him on all matters climate, today’s op-ed article suggests that he really hasn’t made any effort to understand the economics of emissions control. And that’s not a small matter, because he’s now engaged in a misguided crusade against cap and trade, which is — let’s face it — the only form of action against greenhouse gas emissions we have any chance of taking before catastrophe becomes inevitable.

What the basic economic analysis says is that an emissions tax of the form Hansen wants and a system of tradable emission permits, aka cap and trade, are essentially equivalent in their effects. The picture looks like this:

A tax puts a price on emissions, leading to less pollution. Cap and trade puts a quantitative limit on emissions, but from the point of view of any individual, emitting requires that you buy more permits (or forgo the sale of permits, if you have an excess), so the incentives are the same as if you faced a tax. Contrary to what Hansen seems to believe, the incentives for individual action to reduce emissions are the same under the two systems.

This is true even if some emitters are “grandfathered” with free allocations of permits, as will surely be the case. They still have an incentive to cut their emissions, so that they can sell their excess permits to others.

The only difference is the nature of uncertainty over the aggregate outcome. If you use a tax, you know what the price of emissions will be, but you don’t know the quantity of emissions; if you use a cap, you know the quantity but not the price. Yes, this means that if some people do more than expected to reduce emissions, they’ll just free up permits for others — which worries Hansen. But it also means that if some people do less to reduce emissions than expected, someone else will have to make up the shortfall. It’s symmetric; there’s no reason to emphasize only one side of the story.

And as far as I can see, the question about uncertainty is secondary; the fact is that cap and trade works. Hansen admits that the sulfur dioxide cap has reduced pollution, but argues that it didn’t do enough; well, it did as much as it was designed to do. If Hansen thinks it should have done more, he should be campaigning for a lower cap, not trashing the whole program.

Oh, and the argument that if you create a market, you’re opening the door for Wall Street evildoers, is bizarre. Emissions permits aren’t subprime mortgages, let alone complex derivatives based on subprime; they’re straightforward rights to do a specific thing. It will truly be a tragedy if people generalize from the financial crisis to block crucially needed environmental policy.

Things like this often happen when economists deal with physical scientists; the hard-science guys tend to assume that we’re witch doctors with nothing to tell them, so they can’t be bothered to listen at all to what the economists have to say, and the result is that they end up reinventing old errors in the belief that they’re deep insights. Most of the time not much harm is done. But this time is different.

For here’s the way it is: we have a real chance of getting a serious cap and trade program in place within a year or two. We have no chance of getting a carbon tax for the foreseeable future. It’s just destructive to denounce the program we can actually get — a program that won’t be perfect, won’t be enough, but can be made increasingly effective over time — in favor of something that can’t possibly happen in time to avoid disaster.