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There is a some consensus among economists, environmentalists, and politicians that some form of “cap and trade’ program is the appropriate regulatory mechanism to achieve the greenhouse gas emissions reductions necessary to avoid disastrous global climate disruptions. “Cap and trade” programs necessarily incorporate tradable emissions rights – essentially tradable rights to pollute. As such, they run into principled objection by some environmentalists who oppose the notion of creating economic rights in the global commons – essentially the “right to pollute.” This principled objection derives doctrinal support from the public trust doctrine – the ancient notion rooted in common law and Roman law that certain public resources such as flowing water, shorelands, and the air are not susceptible of private ownership, but are instead held by the sovereign “in trust” for the benefit of the public.

This article will consider practical and principled arguments for and against emissions trading as a global warming solution as well as the application of public trust principles to greenhouse gas cap and trade. I conclude that, at least under the mature version of the public trust doctrine that prevails in U.S. law, a cap and trade system is not irreconcilable with a cap-and-trade greenhouse gas emissions control program, but that the legacy interests underlying public trust doctrine preclude cap-and-trade programs that exceed sustainable levels of greenhouse gas emissions. Under this approach, all cap-and-trade systems for regulating greenhouse gas emissions proposed to date would exceed public trust limits based on sustainability (including the First Implementation Period of the Kyoto Protocol).