America’s environmentalists were torn about whether to support the Waxman-Markey climate bill, which passed the House on June 26, and for good reason. On the one hand, passage of the American Clean Energy and Security Act was a historic achievement. After twenty years of denial, deception and delay, Washington had at last ordered reductions in the greenhouse gas emissions that drive global warming. On the other hand, the bill’s specifics fell far short of what science says is necessary to (perhaps) prevent catastrophic climate change.

The Intergovernmental Panel on Climate Change (IPCC)has said that global emissions must fall by 25 to 40 percent from 1990 levels by 2020. The cap-and-trade provisions of Waxman-Markey will cut US emissions by only 1 percent by 2020, a shortcoming backers disguise with creative accounting. They claim the bill will cut emissions by 17 percent–which it might, if one measures against the higher baseline of 2005–and includes credits for halting deforestation overseas, though of course the earth’s atmosphere would not be tricked by such maneuvers. Adding insult to injury, most of the bill’s pollution permits will be given away rather than sold, thus subsidizing today’s polluters and delaying the transition to low-carbon energy sources. Finally, the bill cancels the president’s current authority under the Clean Air Act to regulate greenhouse emissions, a clear step backward.

Supporters argued that such dilutions were necessary to gain enough votes to pass the bill (the vote was still close–219 to 212) and that Congressional backing of emissions cuts is essential to establish US credibility at the crucial global climate negotiations in Copenhagen in December. Doubtless these same arguments will be repeated when Waxman-Markey goes to the Senate, where the legislation is likely to be weakened further, if it passes at all.

But why assume that US credibility in Copenhagen rests on Congress passing a climate bill, no matter how weak? The United States has other ways to send the world a message–and Barack Obama began exploring them even before he became president.

In July 2008, shortly after securing his party’s presidential nomination, Obama sent representatives to Beijing for two days of high-level, off-the-record talks on climate change, held in a luxury hotel overlooking the Great Wall. Leading the Chinese side was Xie Zhenhua, China’s top climate negotiator. The US delegation included Republicans (though John McCain’s campaign declined to participate) and Democrats, notably John Holdren, now President Obama’s science adviser. The talks went so well that a second back-channel meeting was held in October, where unofficial agreement was reached on three points. Chief among them: China and the United States would “work together for a successful outcome” to the climate negotiations in Copenhagen, according to William Chandler of the Carnegie Endowment for International Peace, who helped organize the discussions.

If that pledge becomes policy, it could rank as the most important breakthrough in the history of climate change diplomacy. Together, the United States and China produce 40 percent of annual global greenhouse gas emissions. That gives them veto power over the rest of the world’s progress; no matter how much the European Union, which has pledged 20 percent reductions by 2020, and other governments might do, they cannot reverse global warming if the two carbon superpowers don’t do their part. Conversely, if China and the United States were to announce credible, ambitious plans to limit emissions, it would build momentum for reaching a strong agreement among all nations in Copenhagen, a meeting widely seen as humanity’s last chance to avoid catastrophic climate change.

The turning point in the back-channel discussions came on their very first day, said Chandler. For more than ten years, Washington had refused to cut emissions unless China did too. But Beijing resisted, pointing out that it was US and other rich countries’ emissions during the past 200 years of industrialization that had caused global warming, and besides, China’s per capita emissions were one-fifth of America’s. The familiar impasse surfaced at the Great Wall gathering when one American asked what the Chinese were prepared to do if the next president promised to cut US emissions. “Xie started answering,” recalled Chandler, “and it was like he pushed the button on a tape recorder. Out came the same boilerplate we’d heard so many times before: the US bore historical responsibility for the problem, China was still a developing nation and had the right to use more energy and so forth. But after forty-five seconds, Xie stopped talking. It was as if he turned off the tape. And then he said, ‘But we have to move beyond all that now.’ That’s when we knew things had changed and a real breakthrough was possible.”

No doubt China anticipated a shift in US policy with Bush’s exit, but having recently returned from two weeks of reporting in China, I suspect another reason for the new flexibility: the Chinese leaders have at last recognized how hard climate change will hit their country. One government study has warned that higher temperatures and volatile rainfall could cause production of rice, wheat and corn–the staples of the Chinese diet–to fall 37 percent by 2040 unless effective adaptation measures are taken. Such a decline would gravely endanger China’s ability to feed itself and thus the Communist Party’s hold on power.

Obstacles to a formal US-China climate agreement remain. Publicly China is demanding that we cut US emissions by 40 percent by 2020 and provide tens of billions of dollars to developing nations to help them adapt to the impacts of climate change. The Obama administration rebuffs these demands as “not serious” while continuing to insist that China accept reduction targets too. Beijing still rejects this.

But the back-channel talks suggest a way around the impasse. Recall that there was agreement during the talks. In addition to a successful outcome in Copenhagen, the US and Chinese delegations agreed, according to Chandler, “to rapidly deploy existing technologies to boost energy efficiency”–the quickest path to large emissions cuts. A leading Chinese think tank has concluded that better efficiency could reduce China’s emissions by one-third by 2050; the United States could cut its energy use 30 percent if all states emulated California’s efficiency. Such reductions could buy time for a second back-channel agreement to bear fruit: a joint US-China program to develop low-carbon technologies for vehicles and coal plants, which would enable China to continue burning coal, the source of three-quarters of its energy consumption.

President Obama could sign such an agreement regardless of what Congress does (besides, there is considerable support on Capitol Hill for energy efficiency and green-tech R&D), and in the short to medium term the results could match the IPCC’s recommendations. To be sure, binding emissions targets are important. But if Washington and Beijing can’t agree on them yet, at least the two carbon superpowers could launch a green efficiency revolution that can achieve many of the same ends. In that case, Copenhagen might have “a successful outcome” after all.