Yet if Copenhagen ends in failure, the Kyoto Protocol itself may well fall apart. Why would European companies (and those elsewhere) pay for the right to emit greenhouse gases in, say, 2011, when they will have no binding restrictions on their emissions in 2013? And if there is no subsequent agreement, there can be no enforceable penalty for countries that miss their targets.

Since conservatives can ensure there is no U.S.-ratified treaty, Obama must pursue a different strategy, a high-leverage approach focusing on the world’s major emitters. Only two dozen countries account for about 85 percent of global emissions, and none of the remaining countries alone accounts for even 1 percent. Obama needs to move toward replacing the UNFCCC process with one that focuses on bilateral and multilateral negotiations with the major emitters, the most important of which is China. Together, our two countries account for nearly half of CO2 emissions from burning fossil fuels.

Talks with China over climate action will probably be the most difficult and most important negotiations in U.S. and world history. I have spoken to a number of experts on Chinese energy and climate policy, who say the leaders of the country understand the nature of the threat that climate change poses to them — including the loss of inland glaciers that provide water for the rivers on which hundreds of millions rely. They say that strong U.S. domestic action, coupled with strong U.S. international leadership, could move China to act. Others tell me China will not agree to emissions reductions anytime soon, since it sees itself as a developing nation with much higher priorities.

In fact, China is in a special category by itself. It has announced plans to spend more than half a trillion dollars on an economic stimulus and infrastructure plan. It is a hyper-developing country, with vast amounts of capital in key advanced technologies, including wind and solar.

At the same time, China has become the leader in carbon dioxide emissions. It is now at about 7 billion metric tons of CO2 a year, rising several percent a year. What would happen if it spent most of its stimulus money on clean energy and related infrastructure? Suppose it used some of it to build up an industry in concentrated solar thermal power and other clean technologies? Half a trillion dollars should be enough to keep Chinese emissions frozen for more than a decade, and possibly two decades, while dramatically reducing the projected air pollution. It could also ensure China’s leadership in the key clean-tech industries of the century.

From a strictly economic point of view, China has the know-how and skilled labor to meet a 20-year cap on emissions, starting sometime in the next few years. China is already the world’s top manufacturer of both wind turbines and biogas fermenters. It is projected to become the top manufacturer of solar photovoltaics by 2010. And it has aggressively pursued leadership in electric drives for cars.

Last month, the Chinese premier opened a two-day climate conference in Beijing by calling on the rich nations to spend 1 percent of their GDP on clean-technology projects in the developing world. That would be more than $280 billion a year, with nearly half from the United States. Whether such action by the rich countries ever becomes politically feasible, one thing is certain: Americans won’t embrace giving much money to China, which already gets hundreds of billions of dollars from Americans each year, thanks to our large and growing trade deficit.

What’s more, I fear China has been building coal plants at such a rapid rate, two a week for many years, while knowing that those emissions and those plants are not sustainable, at least in part under the dubious assumption that the West will at some point pay to shut the plants down. After all, the West got suckered into giving China some $6 billion to destroy greenhouse gas refrigerants that probably cost Chinese companies $100 million to capture and destroy.

Americans should not have to pay a country as rich as China to shut down its coal plants. But we can step up as leaders to show the world how to come together to tackle the climate problem. We have done it before. In the 1970s and 1980s, it was our domestic leadership that was crucial to saving the ozone layer.

In 1974, climate scientists warned us that chlorofluorocarbons (CFCs) were destroying the earth’s ozone layer, threatening to bring about a sharp increase in skin cancer. Within five years, the United States voluntarily banned their use in spray cans, and CFC production began to decline. But other uses for CFCs, as refrigerants and solvents, began driving up the demand again by the early 1980s.

In 1985, scientists discovered a hole in the ozone shield over Antarctica. As the National Academy of Sciences wrote, this was “the first unmistakable sign of human-induced change in the global environment period … Many scientists greeted the news with disbelief. Existing theory simply had not predicted it.”

Chlorine concentrations had been increasing over Antarctica for decades, up from the natural level of 0.6 parts per billion. Yet as Richard Benedick, President Ronald Reagan’s chief ozone negotiator, explained in a 2005 Senate hearing, “No effect on the ozone layer was evident until the concentration exceeded two parts per billion, which apparently triggered the totally unexpected collapse.” His ominous lesson for today: “Chlorine concentrations had tripled with no impact whatsoever on ozone until they crossed an unanticipated threshold.” The earth’s climate system is approaching many such thresholds faster than expected, which is why climate scientists are desperate that humanity act now.”

The stunning revelation of an ozone hole drove the world to negotiate the Montreal Protocol. The 1987 agreement called for a 50 percent cut in CFC production by 1999. Significantly, the protocol’s targets and timetables slowed the rate of growth of concentrations only slightly and would have still led to millions of extra skin cancer cases by midcentury. Further, the protocol allowed developing countries to delay implementing the control measures for about 10 years. It also required rich countries to give developing ones access to alternative chemicals and technologies, together with financial aid.

Nevertheless, President Reagan endorsed the protocol, and the Senate ratified it. By the end of 1988, 29 countries and the European Economic Community — but not China or India — had ratified it. The treaty came into effect the next year. But it took many more years of negotiations, continuous strengthening of the scientific consensus, and significant concessions to developing countries before amendments to the treaty were strong enough and had enough support from both rich and poor countries to ensure that CFC concentrations in the air would be reduced.

The analogy of the ozone layer and the Montreal Protocol to global warming and the UNFCCC process from Kyoto to Copenhagen is far from perfect — greenhouse gases are more integral to modern life than CFCs ever were. American politics has changed in two decades, and conservatives would no doubt unanimously oppose the Montreal Protocol today, especially without ratification by China and India. Yet this small first step by the rich nations jump-started a multiyear process that saved the ozone layer and prevented millions of cases of skin cancer.

The health and well-being of future generations rests on the United States and China ending their mutual suicide pact. China won’t act until we do, and we won’t act if they don’t. President Obama can lead this nation in breaking half of that self-destructive cycle with a strong domestic climate bill. He has repeatedly laid out the targets: returning to 1990 emissions levels by 2020 and then reducing them another 80 percent by 2050. And that’s on top of a major energy bill and green recovery plan that will jump-start the transition to a clean energy economy.

But domestic legislation alone will not make Obama a successful president, let alone a great one. Future historians will inevitably judge all 21st-century presidents as failures if the world doesn’t stop catastrophic global warming. If Obama wants to be a great president, he will not merely have to put this country on a sustainable path; he will have to help bring China and the whole world onto that path too. And that is almost certainly the single hardest task he faces as president.

Obama said many times during the campaign that he would meet with our worst enemies in the name of world peace. Climate change is a far graver and for more preventable threat to the health and well-being of future generations of Americans than any current national security threat.

During the transition period, Obama should appoint a high-level envoy — paging Al Gore — to engage in direct shuttle diplomacy with China and other key emitters. He should meet with Chinese leaders himself in the first half of 2009. His presidency — and the fate of humanity — depend on it.