The United States has abandoned comprehensive greenhouse-gas curbs, but California is pressing ahead. Mary Nichols is leading the fight against emissions.

Ten capital cities and the Californian government have recently signed up to a greenhouse gas inventory system that will help ciies and regions make deep cuts in their emissions.

For all its economic capacity, population size, and resource base, California remains only one among the 50 United States and, essentially, is a subnational actor attempting to play a role in the climate change policy arena on par with the nation-states of the world. This raises a series of questions about the substance and breadth of the state's new policy and what has motivated it.

Measuring the costs and benefits of projects intended to offset the emission of greenhouse gases is one of many thorny issues that the state of California must tackle as it begins drafting a cap-and-trade system of carbon credits.