Last week, 175 world leaders inked an accord to slow climate change by cutting carbon emissions worldwide. The agreement commits the U.S. to reduce carbon emissions 26 percent by 2025. Other nations have made similar commitments.

But even as the ink dries, a debate rages about how to achieve the reductions. Some believe it should be done top-down, by regulation, directing the industries that emit most of the carbon pollution to make specific cuts. Others believe that the government should refrain from industry-by-industry directives and instead tax carbon uniformly across the entire economy. Proponents of a carbon tax (including the American Sustainable Business Council) argue that it a tax is simpler, easier to administer, harder to cheat and less disruptive of healthy market competition than any other approach.

Conservative politicians continue to doubt climate change, but the evidence grows. Behind the scenes, some conservatives are quietly discussing how to back away from denial and what to propose in its place. Some conservatives are beginning to consider a "revenue-neutral, border-adjusted carbon tax." They see this option as the most market-friendly, limited-government approach to climate change.

Let's focus on the two words "border-adjustment." These two words are essential for political support of a carbon tax. Border adjustment means that that the U.S. would apply a tax on products and services from countries that don't put a price on carbon.

Without border adjustment, if the U.S. imposes a carbon tax and the countries that we trade with don't, then U.S. companies (and workers) suffer a disadvantage. U.S.-made goods and services will be more expensive than the same goods produced elsewhere. Jobs and profits will move to countries that refuse to address climate change. The threat is greatest for industries that use a lot of energy such as the metals, cement and paper industries.

The bad news is that border adjustment may not be legal. Existing trade agreements like NAFTA and the General Agreement on Tariffs and Trade prohibit most tariffs on international trade, and the World Trade Organization enforces these restrictions. New agreements in formation like the Trans-Pacific Partnership carry these limits even further.

Economists and policy experts disagree on whether a border-adjusted carbon tax can withstand legal challenge. It's a murky area, without clear precedent. Tabitha Benney, a professor at the University of Utah, writes, "Under current international law ... border adjustments are illegal." She points out that international bodies like the WTO have the legal muscle to force the U.S. to roll back a carbon tax if it does not comply with existing international trade agreements.

This is more than an abstract threat. The WTO has already flexed its muscle to undermine climate progress. In February it struck down a plan by India to install 100 gigawatts of solar capacity, because India's plan required that some of the solar cells and panels be made locally. (The ruling is under appeal, with China taking India's side against the U.S.)

Others claim a border adjust for carbon can survive WTO scrutiny. Jennifer Hillman is a former member of the WTO Appellate Body (a group within WTO that judges claims of unfair trade practices) and argues that a carbon tax can be designed in a way that complies with existing trade agreements. "The key is to structure [it] as a straightforward extension of domestic climate policy to imports. If so designed, there should be few questions about the measure's consistency with the WTO rules." The American Action Forum, a self-described "center-right" think tank, expands on Hillman's argument, showing that the question of border adjustment cuts across the usual partisan lines in the climate debate.