China will spend $275 billion over the next five years improving air quality — roughly the same as the GDP of Hong Kong, and twice the size of the annual defence budget.

There's an old hypothesis, known as the environmental Kuznets curve, that suggests that countries will sacrifice clean air and water in favor of economic development — but only up to a certain point. Once they get rich enough, the calculus shifts, and countries start spending more on environmental goals. China appears to have reached that point.

There are plenty of reasons for the about-face. Not only is China's air pollution triggering angry protests, but it appears to be hurting the economy, too. One recent MIT study suggested that coal pollution in northern China had shaved as much as 5.5 years off life expectancy. (Some statisticians, like Andrew Gelman, have criticized that precise estimate, though everyone agrees coal pollution has a sharply negative effect on health.)

The Economist, meanwhile, wonders whether China's anti-pollution binge will curtail the nation's coal use and carbon pollution quickly enough to help the world avoid drastic global warming. (Since 2000, China has accounted for about two-thirds of the growth in the world's greenhouse-gas emissions.) That's a much trickier question.

On one hand: Several Chinese cities are beginning to experiment with cap-and-trade policies that will set hard ceilings on the amount of greenhouse gases that factories and power plants may emit. But the targets themselves can often get weakened in negotiations between state-owned firms and local officials. What's more, the Chinese government has been cracking down on and jailing environmentalists, the sort of outsiders who would typically help ensure these policies are actually working at the local level.

This follow-up Economist article suggests that China has plenty of reason to care about climate change: "China will suffer as much as anywhere. Already its deserts are spreading, farmland is drying out and crop yields are plateauing. Climate change may make matters worse. It has 80 million people living at sea level who are vulnerable to rising oceans and higher storm surges."

But that, in itself, isn't reason to think that a sharp reduction in emissions is inevitable — even with all those billions.