This article was first published by the Heinrich Böll Stiftung Transatlantic Climate Policy Group.

After years of stalemate in the international climate negotiations, the inauguration of a new U.S. president presents an opportunity for a genuine breakthrough.  Both John McCain and Barack Obama support mandatory limits on U.S. greenhouse gas emissions, and both favor renewed international engagement.  But unrealistic expectations about how quickly the United States will move – and how far – could severely damage prospects for any sort of agreement next year in Copenhagen.

An effective post-2012 climate agreement is impossible without the United States, the world’s largest economy and largest historic emitter.  Europe was able to persuade other developed countries to push ahead with initial commitments under the Kyoto Protocol despite the U.S. withdrawal.  But there appears very little appetite among those countries to take on new, stronger commitments without the United States, and even less prospect of commitments by the major developing countries.

Fortunately, there is at long last real momentum for stronger efforts to reduce U.S. emissions.  While skeptics remain, the political establishment has largely accepted the scientific consensus that human-induced warming is underway and must be addressed.  Many states are taking mandatory steps to reduce emissions; 24 states have entered into regional initiatives to establish cap-and-trade systems.  Many corporate leaders are calling for mandatory federal action, and Congress is seriously debating the establishment of an economy-wide cap-and-trade system more than twice the size of Europe’s Emissions Trading Scheme.