Credit: TEUN VOETEN

An index that both climate scientists and policy makers anxiously keep track of is national emission rates of carbon dioxide, particularly for China, which has quickly been catching up with the US, hitherto the world's largest emitter of carbon dioxide. Despite all efforts to the contrary, the rates for both China and the world are rapidly going up, rather than down. Jay Gregg of the University of Maryland, US, and colleagues suggest that paying attention to China is justified as 54% of the growth of global carbon dioxide emissions between the years 2001 and 2006 has come from increases in Chinese emissions (Geophys. Res. Lett. 35, L08806; 2008).

However, the researchers also estimate large uncertainties, on the order of 15–20%, in the Chinese annual trends. Discrepancies between some of the changes announced in the official Chinese statistics are hard to reconcile with current understanding. For example, between 1996 and 2000 a decrease in carbon dioxide emissions was reported, whereas electricity generation, industrial output and gross domestic product were all reported to have increased. In addition, official numbers for emissions in the year 2000 were recently revised upwards by 23%.

Whether China has already overtaken the US in terms of national carbon dioxide emissions in 2004, or will not do so before 2010, is therefore uncertain. But what does an answer to this particular question mean? A comparison of carbon emissions by country is not necessarily informative, when the populations differ by a factor of four. In addition, an estimated fraction of 7–14% of China's carbon dioxide emissions is ultimately outsourced from the US, in the sense that many energy-intensive products that are manufactured in China are eventually consumed in North America. And other western countries import goods from China, too.

China's sizable and growing contribution to global carbon emissions cannot be ignored: countries are still political units where legislation can steer development, at least to some degree. But part of this contribution is due to demand from developed countries for energy-intensive goods. In today's globally entangled economies it is not entirely straightforward to attribute carbon emissions.