This winter, the air quality over the north China plain has been record breaking - but not in a good way. From Jan 10 to16, the air quality in the Beijing-Tianjin-Hebei region was so bad it was actually off the scale used to monitor air quality. The air pollution reached levels that the World Health Organization describes as hazardous and people were advised to stay indoors. On Thursday, heavy haze and smog descended over most of northern and eastern China again.

It is reasonable to ask why the air pollution was so bad this winter. Weather conditions and topographic factors have been given as reasons, but, although contributing factors, these are not to blame. The worsening air pollution is linked to an energy mix that relies heavily on coal and to motor vehicle emissions.

But despite the astonishing growth of motor vehicles in our cities, it is the burning of coal that is the biggest cause of air pollution. More than half of the country's thermal power plants are located in the eastern region and China's coal consumption more than doubled between 2000 and 2011, from 1.5 billion tons in 2000 to 3.8 billion tons in 2011, accounting for almost half the world's total coal consumption. And with coal occupying nearly 70 percent of the country's primary energy consumption, it has become critical to reduce the use of coal if we are to solve the nation's overall air pollution problem.