There will be a real temptation to do so. Anything that makes U.S. emissions look smaller is helpful to U.S. diplomatic efforts. Encouraging a robust offset system can also help push developing countries toward bigger emissions cuts than they would otherwise make, a goal shared by the U.S. government.
But there are also real risks. International offsets are essentially created by regulators who decide that the projects that those offsets support are legitimate -- in particular, that they would not have occurred without offset support. But the legitimacy of particular offset schemes is often controversial. For example, scholars and policymakers have fought over whether wind turbine projects in China would have occurred even without offset support. Others have pointed out that some industrialists appear to have built chemicals plants primarily so that they could collect offset payments in exchange for destroying those plants' climate-warming byproducts -- which wouldn't have existed in the first place if the plants hadn't been built. By choosing to take credit for Californian offsets, Washington would be vouching for a system regulated by a state entity over which it has limited control.
And that is only the beginning of what could become a considerably more complex problem. As California's use of international offsets develops, the state will essentially enter into a system of contracts with foreign entities. The integrity of that system will rest on the continuation of the rules governing the state's cap-and-trade system. Yet somewhere down the road, if the United States gets more serious about climate change, it may decide to revisit cap-and-trade (or a related system) at the national level. A national cap-and-trade law would probably seek to preempt state level efforts -- much like national fuel economy regulations for cars and trucks have preempted Californian regulations in the past. Washington would then face the prospect of potentially invalidating (or materially interfering with) many of the Californian offset contracts, possibly with severe consequences for U.S. relations with the various foreign countries involved.
Of course, no national cap-and-trade scheme is in the immediate offing. But even if a U.S. cap-and-trade system isn't in the cards for a decade or more, steps taken soon in California will lock the United States in to a future of long-term headaches. In the event of a national effort to combat climate change, Congress could also try to find ways to grandfather in the policies of the Californian scheme -- but even that would mean accommodating the foreign-policy decisions Sacramento is making today.
Avoiding this sort of debacle is precisely the reason that the U.S. Constitution made the conduct of foreign policy an exclusive province of the federal government. The line between what states can and cannot do internationally has always been fuzzy, and legal precedent is thin. But as California's cap-and-trade program becomes increasingly linked up with foreign countries -- and as similar efforts rise elsewhere in the United States -- the case for caution will become stronger, lest the system be set up for a mess down the road. Achieving the right balance will require much more coordination between California and the federal government.
None of this should take away from the positive example that California's program has set for the rest of the United States, and the rest of the world. But the task of fully exploiting that success, and avoiding important dangers ahead, has just begun.