Jeffrey Sachs (Jeffrey Sachs is a professor of economics and director of the Earth Institute at Columbia University)

The United Nations Climate Change Treaty, signed in 1992, committed the world to “avoiding dangerous anthropogenic interference in the climate system.” Yet, since that time, greenhouse-gas emissions have continued to soar.

The United States has proved to be the biggest laggard, refusing to sign the 1997 Kyoto Protocol or to adopt any effective domestic emissions controls. As we head into the global summit in Copenhagen in December to negotiate a successor to the Kyoto Protocol, the U.S. is once again the focus of concern. Even now, American politics remain strongly divided over climate change – though President Barack Obama has new opportunities to break the logjam.

A year after the 1992 treaty, Bill Clinton tried to pass an energy tax that would have helped the U.S. to begin reducing its dependence on fossil fuels. The proposal not only failed, but triggered a political backlash.

When the Kyoto Protocol was adopted in 1997, Mr. Clinton did not even send it to the Senate for ratification, knowing that it would be rejected.

President George W. Bush repudiated Kyoto in 2001 and did essentially nothing on climate change during his presidency.

There are several reasons for U.S. inaction – including ideology and scientific ignorance – but a lot comes down to one word: coal. No fewer than 25 states produce coal, which not only generates income, jobs and tax revenue, but provides a disproportionately large share of their energy.

Per capita carbon emissions in U.S. coal states tend to be much higher than the national average. Since addressing climate change is first and foremost directed at reduced emissions from coal – the most carbon-intensive of all fuels – America’s coal states are especially fearful about the economic implications of any controls (though the oil and automobile industries are not far behind).

The U.S. political system poses special problems as well. To ratify a treaty requires the support of 67 of the Senate’s 100 members, a nearly impossible hurdle. The Republican Party, with its 40 Senate seats, is simply filled with too many ideologues – and, indeed, too many senators intent on derailing any Obama initiative – to offer enough votes to reach the 67-vote threshold.

Moreover, the Democratic Party includes senators from coal and oil states who are unlikely to support decisive action.

The idea this time around is to avoid the need for 67 votes, at least at the start, by focusing on domestic legislation rather than a treaty. Under the U.S. Constitution, domestic legislation (as opposed to international treaties) requires a simple majority in Congress and the Senate to be sent to the President for signature. Getting 50 votes for a climate-change bill (with a tie vote broken by the vice-president) is almost certain.

But opponents of legislation can threaten to filibuster (speak for an indefinite period and thereby paralyze Senate business), which can be ended only if 60 senators support bringing the legislation to a vote.

Otherwise, proposed legislation can be killed, even if it has the support of a simple majority. That will certainly be true of domestic climate-change legislation. Securing 60 votes is a steep hill to climb.

Political analysts know that the votes will depend on individual senators’ ideologies, states’ voting patterns and states’ dependence on coal relative to other energy sources. Based on these factors, one analysis counts 50 likely Democratic “Yes” votes and 34 Republican “No” votes, leaving 16 votes still in play. Ten of the swing votes are Democrats, mainly from coal states; the other six are Republicans who conceivably could vote with the President and the Democratic majority.

Until recently, many believed that China and India would be the real holdouts in the global climate-change negotiations. Yet, China has announced a set of major initiatives – in solar, wind, nuclear and carbon-capture technologies – to reduce its economy’s greenhouse-gas intensity.

India, long feared to be a spoiler, has said that it is ready to adopt a significant national action plan to move toward a trajectory of sustainable energy. These actions put the U.S. under growing pressure to act. With developing countries displaying their readiness to reach a global deal, could the U.S. Senate really prove to be the world’s last great holdout?

Mr. Obama has tools at his command to bring the U.S. into the global mainstream on climate change. First, he is negotiating side deals with holdout senators to cushion the economic impact on coal states and to increase U.S. investments in the research and development, and eventually adoption, of clean-coal technologies.

Second, he can command the Environmental Protection Agency to impose administrative controls on coal plants and automobile producers, even if the Congress does not pass new legislation. The administrative route might turn out to be even more important than the legislative route.

The politics of the U.S. Senate should not obscure the larger point: America has acted irresponsibly since signing the climate treaty in 1992. It is the world’s largest and most powerful country, and the one most responsible for climate change to this point; it has behaved without any sense of duty – to its own citizens, to the world and to future generations.

Even coal-state senators should be ashamed. Sure, their states need some extra help, but narrow interests should not be permitted to endanger our planet’s future. It is time for the U.S. to rejoin the global family.