Though a federal cap-and-trade system for carbon would largely be a foray into the unknown, we can examine how the idea is working in the states, many of which are far ahead of Washington.

• 尽管联邦的碳排放总量管制与排放交易系统可能会是一个冒险，但我们可以考察一下这一理念在各州是如何发挥作用的，有很多州的作法已经遥遥领先于华盛顿。