









Moreover, an expensive effort to reduce Western emissions sets a worthless example. Only emissions cuts that provide measurable economic benefit to the developing nations will be adopted by them. If the 80% U.S. emissions cut winds up hurting the U.S. economy, it guarantees China will never follow our example.

Cheap green energy is not going to be easy. Coal is dirt cheap, and China has been installing a new gigawatt coal plant each week—enough to supply five completely new cities the size of New York every year.

Technological change can help a great deal. For now carbon capture and sequestration (CCS) from coal combustion is unproven, but so is cheap solar. I expect we can make CCS work. Perhaps the West can subsidize CCS in China or pay to make its plants CCS ready. A dollar spent in China can reduce CO2 much more than a dollar spent in the U.S.

There is another alternative: luck. Here's how it could help. Scientists are aware of a phenomenon that would counter the greenhouse effect: warmth evaporates water; water creates clouds; clouds reflect sunlight. A small cloud increase would significantly reduce predicted warming. The IPCC gives such cloud feedback only a 10% chance. My estimate is 30%. Clouds may already be kicking in, responsible for the negligible global warming of the past 12 years. Maybe, but we don't know. That's why we need luck.

Perhaps we could geoengineer a solution: Squirt a few million tons of sulfur dioxide into the stratosphere to reflect sunlight, emulating the 1991 Mt. Pinatubo eruption. We'll certainly get pretty sunsets. Or we could foam up the oceans to increase reflectivity. Many people find such ideas scarier than warming because of the threat of unintended consequences.


But the bottom line is that 80% cuts in U.S. emissions will have only a tiny benefit. The bulk of our effort is best directed at helping the emerging economies conserve energy and move rapidly toward efficient solar, wind and nuclear power. Developing cheap carbon capture and sequestration is also a priority. Above all, we need to recognize that make-the-West-bear-the-burden Copenhagen proposals are meaningless.

Mr. Muller is professor of physics at the University of California, Berkeley, and author of "Physics for Future Presidents" (Norton, 2008). References and a spreadsheet with the numbers for the chart are at