CHINA, GREEN OR BLACK? As the country continues its rapid development, this may prove to be the defining question of the twenty-first century. In When a Billion Chinese Jump: How China Will Save Mankind — Or Destroy It, Jonathan Watts has given us the most comprehensive investigation yet, and the way you know he’s an honest reporter is he’s not sure.

I read this beautifully reported book just after finishing a short piece of my own on the subject, one equally equivocal but much less fully reported. It’s a tribute to China’s opaqueness that even a trip like the one Watts makes, which takes him to every corner of the country, isn’t enough to really reach a conclusion. On the one hand, the People’s Republic is burning truly awesome quantities of coal — the equivalent of Britain’s entire electric grid is added each year to China’s power network. Watts recounts the standard story of days when the sun never manages to break through the gloomy haze (and the heartbreaking story of having to hide with the dead coal miners’ widows that he’d interviewed so the security police wouldn’t arrest them for telling the truth). But he also talks to the entrepreneurs and innovators bringing renewable energy to China faster than any place on Earth — the country added record amounts of wind power last year, and the world’s busiest solar panel factories can be found in haze-shrouded cities near Shanghai.

Watts also makes clear for Western readers the particular tragedy of this moment in China: the country is making great strides in pulling hundreds of millions of people out of truly dire rural poverty. But it’s doing it the old-fashioned way (the way we did it, by burning lots of fossil fuel) at precisely the moment the planet has come to realize that our atmosphere simply can’t stand another U.S.-sized burst of carbon. And to make the tragedy complete, China is particularly vulnerable to the climate change that coal-burning produces, what with its already melting glaciers, already dwindling rivers, already rising seas.

It’s impossible to travel the country and not feel both excited by the dynamism of its growth and saddened by the implications, for this generation of Chinese and for the world. Watts, a veteran reporter for London’s Guardian, captures this conundrum with more power than anyone since Mark Hertsgaard in his book Earth Odyssey, now nearly two decades old. In the afterword of the book Watts tips his hand: there’s more to fear than hope for, he says. “The planet’s environmental problems were not made in China, but they are sliding past the point of no return there,” he writes. “It is unreasonable to ask China to save the world, but the country forces mankind to recognize we are all going in the wrong direction.”

If you want to understand the century you’re living in, this book is required reading. We’re used to thinking of America as the most pivotal country on the planet, but in environmental terms this century belongs to China, for better or for worse.