"The pollution currently blanketing northern China is an extraordinary and unnatural phenomenon.
This is certainly an environmental disaster and a public health crisis. But it also has the potential to become a huge political problem.
After years of denying the issue really existed, the central government has recently accepted that pollution is of genuine concern. It now publishes figures for the air quality in China's major cities (the accuracy of some is still contested), and in 2013 promised $275bn (£165bn) to tackle the issue in the next five years, setting targets for air quality improvements....
China has lifted hundreds of millions out of poverty; it's also shifted their priorities. The government has now pledged to reduce pollution, and in so doing has raised expectations.
The political risk for the Communist Party comes if those expectations are not met. Unlike other government promises, people will be able to judge success and failure on pollution simply by looking out of their window.
And the scale of the problem is enormous. China's pollution crisis has been a long time coming. The reliance on coal, the rise of the car, along with the soaring energy demand of an ever-growing economy, have all meant more dirty air. Predictably so. Reducing pollution will require wholesale structural change, not just in how and where energy is produced, but how it is priced.
It will probably mean sacrificing some economic growth in favour of quality of life. To do all of that will mean confronting some of the most entrenched vested interests in China's economy, and inside the Communist Party itself.
How the pollution issue is handled in the next few years will tell us a lot about China's government, its changing relationship to its people, and its ability to make difficult decisions."