CLIMATE CHANGE is global. Unless enough big-emitting nations stop pumping carbon into the atmosphere, no single country’s efforts will matter much. That is why, despite the many unmet deadlines, petty squabbles and dashed hopes, it is still important for world leaders to gather and work toward a climate deal, as they have done many times in the past two decades and as they have been doing in Doha, Qatar, since last Monday.

World governments were supposed to have made a big step toward solving the problem through the 1997 Kyoto Protocol, but that agreement has proved inadequate for a variety of reasons, including the fact that the United States pulled out of the treaty. Leaders were then supposed to have achieved the dream of a legally binding, consensus-driven international climate treaty at a U.N. conference in Copenhagen three years ago, but they failed. Three major U.N. climate confabs later, the goal is to have such an agreement negotiated by 2015 and in force by 2020. The science, meanwhile, counsels both faster action and larger emissions reductions than countries have pledged.

Not surprisingly, the toughest question to resolve in such negotiations is which countries will be obliged to cut their projected emissions deeply. It pits developed countries, particularly the United States, against big developing ones such as China and India. U.N. talks in the 1990s enshrined the notion that developing nations need not enforce serious emissions restrictions, because they were poorer and responsible for less of the greenhouse gas in the atmosphere. But with China now emitting nearly twice as much carbon dioxide as is the United States, demanding lots from the West and little from China is nonsensical on the science and impossible politically, since Western voters would balk.

U.S. representatives have been right to insist that developing nations do their part as their economies expand. The United States is on track to meet its commitment to reduce emissions by 17 percent of 2005 levels by 2020; Americans must have a sense that China’s activities won’t render further effort futile.

Somewhat paradoxically, though, securing larger commitments from big developing nations requires Western, particularly American, leadership. President Obama has failed to put climate policy high on his agenda, let alone to push a policy framework for cutting carbon after 2020. U.S. negotiators must convince other nations that Washington will follow through on a deal that involves shared action to cut global emissions. In the process, they must also ensure that the action is genuinely shared. That will require more work at home. And it will require a flexible approach to negotiations abroad, instead of putting all hopes in one big U.N.-brokered agreement.

Climate change is now on the agenda of all sorts of international meetings. These can lead to small-scale agreements that build trust — or even to bigger pacts among key players to which others eventually sign on.