At some point last year, China became the largest producer of greenhouse gases in the world. Here's part of the explanation for the stunning increase: A soon-to-be-published study in Geophysical Research Letters finds that fully one-third of China's emissions come from making goods for other countries, with the United States (9 percent) and Europe (6 percent) topping the list. By itself, that fact won't shock anyone, but it makes global climate politics surprisingly tricky.

After all, China has resisted attempts to impose binding emissions targets on the country by arguing, in part, that developed countries should take responsibility for at least some of this pollution. After all, it's their junk being manufactured. And there's some merit to this argument. Take Britain, which, under existing Kyoto Protocol rules, appears to have reduced its emissions 18 percent since 1990. That looks laudable, until one includes imported goods and services, in which case Britain's total carbon footprint has actually grown by nearly 20 percent, according to a study by the Stockholm Environment Institute. While there are no doubt real reductions being made at home, a good chunk of the country's emissions are being outsourced abroad.

This is one reason why, in the absence of an international treaty, a domestic cap-and-trade system in a country like the United States would be so problematic—if a slew of manufacturers and polluting industries simply decamp to China and start exporting back to the United States, then U.S. domestic emissions may slink downward, but the country's still effectively generating the same amount of pollution (or even more, given that U.S. factories tend to be cleaner than their counterparts abroad; plus there's shipping to consider). In theory, the United States could drop a tariff on all imports from countries without carbon regulations, but it's unclear whether that would fall afoul of WTO rules.

The Guardian quotes Oxford economist Dieter Helm arguing that, because of all this, "focusing on consumption rather than production of emissions is the only intellectually and ethically sound solution." Sounds intriguing. Except that calculating a consumptive footprint is tricky—it's much easier to simply measure emissions where they're produced, and use that to shape policy. But then how do you get developed countries to reduce their actual carbon footprint, rather than merely capping greenhouse gases at home and then outsourcing pollution? And to what extent should the United States and Europe pay for those outsourced emissions? Neither of those questions are going away anytime soon.

--Bradford Plumer