I saw a video clip someone Facebooked recently showing how bad Beijing's smog is. It was a hyperbolic news segment that noted that "this is what an average day looks like here" while also mentioning at the end that they filmed on the record worst day of the year.
China has a real pollution problem, and it's terrible, but it doesn't worry me. That's a soluble problem. It's one we've solved before. As an Occupy Democrats post put it, "Woah! Think Beijing is smog-choked? Well NYC used to be just as bad. Many Americans have forgotten just how smoggy many of our cities were until Nixon signed the Clean Air Act in 1970. It took some time to really get off the ground, but its positive effects today are crystal clear."
The people of China have been desperate to climb out of poverty, so they made a choice to sacrifice the environment in the short-term. Just like developed countries around the world, once their citizens acquire a certain amount of wealth, they will start to prioritize a cleaner environment, and then take the steps to make it happen. (This is, of course, a simplification. Not everyone in China was able to participate in that choice. It's a choice they've made as a society, and not a democratic one.)
Of course, this argument only holds water if there are no long-term negative externalities to this much pollution. As we know now, there are, of course: 1) the pain and death directly caused by air pollution and 2) climate change.
The first point is terrible but needs to be compared to the pain and death caused by poverty. I haven't done that math, but again, economists would say that millions of people are choosing the former over the latter. They would rather have pollution than poverty. The alternative to pollution in China is not just beautiful blue skies and scenic rural landscapes. It's the poverty that comes with those, until the country is able to eventually afford to adjust its development in a cleaner direction.
The second point, climate change, we're finally starting to see some action on. That's why it was reassuring to see the landmark Paris deal two months ago. It's not the final step, but it's still a step in the right direction.