Do US greenhouse emissions breach international trade law?

Nobel Prize-winning economist Joseph Stiglitz has an article out today which suggests that the US is in breach of its World Trade Organisation (WTO) obligations by unfairly subsidising energy:

Fortunately, we have an international trade framework that can be used to force states that inflict harm on others to behave in a better fashion. Except in certain limited situations (like agriculture), the WTO does not allow subsidies—obviously, if some country subsidizes its firms, the playing field is not level. A subsidy means that a firm does not pay the full costs of production. Not paying the cost of damage to the environment is a subsidy, just as not paying the full costs of workers would be. In most of the developed countries of the world today, firms are paying the cost of pollution to the global environment, in the form of taxes imposed on coal, oil, and gas. But American firms are being subsidized—and massively so.

There is a simple remedy: other countries should prohibit the importation of American goods produced using energy intensive technologies, or, at the very least, impose a high tax on them, to offset the subsidy that those goods currently are receiving… Japan, Europe, and the other signatories of Kyoto should immediately bring a WTO case charging unfair subsidization.

While it is clear that the US’s greenhouse emissions impose unfair environmental costs on other countries, it’s much harder to say what level of energy taxes would or would not represent a subsidy. As Stiglitz says "A subsidy means that a firm does not pay the full costs of production. Not paying the cost of damage to the environment is a subsidy." But the problem is, how do you work out what the cost of damage to the global environment is – now and in the future – from each unit of energy? And how do you tell that Europe or Japan's taxes are not themselves a subsidy?

I’d welcome thoughts from any international lawyers but it seems to me that a WTO case against the US – while I’d love to see it! – would have more political than legal merit.