China is now the largest emitter of CO 2 on the planet, as it powers a large industrial base primarily through the use of coal-fired power plants. However, many of those goods are immediately shipped overseas, often to the US and EU, which generate and use power far more efficiently. A new paper, which will be published in the Proceedings of the National Academies of Science, now takes a look at the impact of outsourcing these carbon emissions by tracking CO 2 based on a product's point of use. For some Western European economies, the result is enormous: anywhere from 20 to 50 percent of their emissions come in the form of imported goods.

The calculation was performed by Stanford's Steven Davis and Ken Caldeira, who built a database of national energy production and tracked international trade of both raw materials (including fossil fuels) and finished goods. The most recent year for which all that data was available was 2004, which means the figures don't cover some of the changes that have accompanied the recent economic downturn. The basic calculation involves taking the CO 2 emissions for various nations and regions, subtracting those associated with exported goods, and then adding back emissions associated with imports. The result, termed consumption emissions, was then analyzed on a per-capita and per-GDP basis.

Overall, nearly a quarter of emissions globally went towards the production of goods that were pumped into the export economy (that's 6.2 gigatons of carbon dioxide). China alone accounted for 1.4 of those Gt. A lot of those wound up in the US and Western Europe, where each citizen was responsible for literally tons (anywhere from two to 10 tons) of CO 2 emitted elsewhere.

In consumption terms, the US still leads China, consuming roughly 6,500 megatons of CO 2 annually; China is in second, at just under 4,000 Mt. Japan and Western Europe fill out most of the remainder of the top ten, although Russia, India, and the Middle East also appear on the list. For Russia and the Middle East, the primary factor that gets them onto the list is their export of fossil fuels.

Considered in GDP terms, the former Soviet Union is the single worst performer, thanks to a combination of energy exports and a very carbon-intensive domestic energy economy. Several of the individual nations in that region also make the list, as do Iran, China, Vietnam, and India.

On a per-capita basis, the situation is largely reversed: countries with mature economies (and, often with efficient energy economies) dominate the list, with Luxembourg edging out the US for top honors. Singapore, Australia, and Canada round out the top five, with Hong Kong joining various European nations in the rest of the top 10.

At the other end of the spectrum, the nations with the lowest consumption emissions were, not surprisingly, largely in the developing economies of Africa and Asia. The same held true for per-capita consumption emissions. But, in terms of GDP, the developed economies of Western Europe ran away with all but one spot in the top 10, with Japan being the lone exception.

So far, the authors suggest that the trend of effectively outsourcing carbon dioxide emissions has been driven by economic concerns, rather than any sort of emissions regulations. But the clear subtext is that any greenhouse gas control system that doesn't account for global trade may exacerbate this trend. As the authors conclude, "the prosperity of developed countries was not only founded on two centuries of fossil fuel emissions, but also in some cases is now being maintained by emissions produced in developing countries."

PNAS, 2010. DOI: 10.1073/pnas.0906974107 (About DOIs).