In a few short weeks, world leaders will assemble in Copenhagen for the much anticipated United Nations Climate Change Conference. Their goal: to draft an agreement that will limit global warming, chiefly by reducing greenhouse gas emissions. As the 12-day meeting gets closer, the chorus from jaded pundits and politicians gets louder: “It can’t be done.”

Nonsense. The naysayers have two reasonable concerns. One: Countries will never agree on limits because they are out to protect their own interests, which differ. Two: Even if they reach an agreement, it will never hold because it will raise energy prices, which people will resist. Fortunately, both worries can be resolved.

The path to overcoming the diplomatic hurdle is daunting but clear. Leaders from China, Japan, the European Union and elsewhere have stated plainly that the U.S. must prove it will clean up its own backyard before they will agree to international limits. In June the U.S. House of Representatives passed the American Clean Energy and Security Act, also known as the Waxman-Markey bill. Originally a dictate to reduce fossil-fuel use, the bill was weakened as it was hammered out, so much so that some leading supporters claimed it no longer did enough. A bill that the Senate took up in September, introduced by John Kerry and Barbara Boxer, aimed to fix many of the problems.

But the important point is that Congress is finally acting. In his influential blog, policy expert Joseph Romm wrote: “The original Clean Air Act didn’t do enough. And the 1987 Montréal protocol … would not have saved the ozone layer. But [each of these measures] began a process and established a framework that ... could be strengthened over time.” Commitment in Congress and President Barack Obama’s personal attendance in Copenhagen may be enough to prompt nations to seek a meaningful agreement.

As politicians and diplomats begin to clear the first hurdle, scientists and engineers have been dismantling the second: the claim that an aggressive goal can never be achieved economically because developed countries will never cut back on their lavish existence and developing nations will never slow their rise in living standards. In fact, reducing emissions does not mean cutting lifestyles. It does not mean punitive strategies. Rather it means replacing fossil fuels with clean, sustainable energy sources.

This notion is not naive ideology; it is hard-headed pragmatism. As Mark Z. Jacobson and Mark A. Delucchi show in their article “A Path to Sustainable Energy by 2030,” starting on page 58, wind, water and solar resources could supply 100 percent of the world’s energy by 2030. Step by step, the authors prove that more than enough sustainable energy exists, that the needed technologies are available now, and that they can produce power at the same or lower cost than traditional fossil and nuclear plants.

Wind power is already as cheap as coal power. Other renewables are not, but incremental improvements are steadily making them competitive. The key is to subsidize renewable sources, for a limited time, in a way that brings down their per-watt cost and hastens the day when they will be competitive on their own. Not all subsidies do that; in the U.S., a requirement that each state obtain a certain fraction of its energy from renewable sources, or a nationally mandated price for renewable power, could encourage builders to put up wind turbines in windless valleys and solar panels in sunless climes. A better approach would be a national renewable portfolio standard and state-by-state incentives to encourage renewables where they would be most productive, such as wind in North Dakota and solar in Arizona. An alternative is direct cash grants to boost installation of renewables, which the Department of Energy and other agencies have begun to make through the federal stimulus plan.

At the same time, the price of fossil fuels must be raised to account for their environmental damage. And existing subsidies for fossil energy should be eliminated. Some fossil-energy companies are shifting to renewables, but on the whole, the coal, natural gas and oil industries will not give up the government largesse meekly, so politicians will have to resist intense lobbying from them.

Now that the world has a plan to transform the global energy system economically, leaders in Copenhagen can commit to cutting emissions without diminishing their citizens’ standard of living. The missing piece is leadership, which the U.S. can provide if Congress acts definitively.

Note: This article was originally printed with the title, "Keys to Copenhagen."