It's fairly straightforward to measure how much carbon dioxide a given country is emitting within its own borders. Just count the factories and power plants and cars and so forth and tally up all that pollution. But what about outsourced emissions? After all, the United States and Europe consume a whole bunch of goods manufactured overseas, and those emissions usually get chalked up to developing countries like China. So who bears the responsibility here?

It's a dicey question, though the first step is to get a handle on how much carbon pollution actually gets outsourced. And the answer seems to be: quite a bit. A new study by Steven Davis and Ken Caldeira of the Carnegie Institution for Science finds that the United States outsources about 11 percent of its emissions abroad, while Japan outsources nearly 18 percent and European nations outsource anywhere from 20 percent to 50 percent of their emissions—most of it to developing countries. On the flip side, nearly one-quarter of China's emissions, for instance, go into making goods for other countries. Here's a map showing annual net flows (in millions of tons of CO2):

A couple points could be made here. One is that the EU's success in reducing greenhouse-gas emissions looks somewhat less impressive in this context—if European countries are reducing pollution domestically but outsourcing more of it overseas, that's not progress. Mind you, it's not clear that this is true of all EU nations, though one study by the Stockholm Environment Institute found that in Britain's case, at least, outsourced emissions were offsetting a good chunk of carbon reductions.

There's also the question of whether the United States and Europe should pay for these outsourced emissions at all. It is their junk being produced abroad, after all. One possible way to do this would be to slap a simple carbon tax on imports. China, though, hates this idea (since it also benefits from this outsourcing, after all) and would prefer that, instead, wealthier countries help finance low-carbon projects in the developing world directly. These are the sorts of questions that always tie up global climate talks in knots.

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