"We have added another piece of information showing that the timescales on which humans have changed the composition of the atmosphere are extremely short compared to the natural time cycles of the climate system," says Thomas Stocker of the University of Bern in Switzerland, who led the research.

A recent U.S. study predicts that in the next decade as much as one third of the smog-forming nitrogen oxide emissions in California's air will come from China. South Korean researchers published a study last year showing that more than 40% of sulphur dioxide and other toxic pollutants in that country's air appeared to have originated from China.

One difficulty has been that the State Environmental Protection Administration has only 200 professional employees as opposed to 10,000 that work for its counterpart in the U.S., the Environmental Protection Agency.

Top Chinese officials are also making renewed calls on local provinces to rethink growth priorities that have allowed the construction of plants along river banks and in residential areas.

But China's industrial engine has proven largely incapable of policing itself. Many of its biggest companies remain state-run, and many government agencies continue to play the role of both regulator and owner, leading to potential conflict of interests.