On a recent visit to the Gobi desert, which stretches across China’s western Gansu province, I came upon an unusual sign. In the midst of a dry, sandy expanse stood a large billboard depicting a settlement the government intended to build nearby — white buildings surrounded by lush, green, landscaped lawns, and in the center a vast, gleaming blue reservoir. The illustration’s bright colors were quite unlike the actual surroundings, which consisted of dull sky that faded into a horizon of undulating, parched-brown hillsides.

Still, the billboard’s promise was clear: Through feats of engineering and willpower, specifically the planned construction of a series of aqueducts to bring water from a tributary of China’s Yellow River, the government pledged to build new homes and remake nature. Let there be water.

My companion, the young Chinese environmentalist Zhao Zhong, founder of the nonprofit group, Green Camel Bell, was dubious. He pointed out that not only has the water level of the Yellow River been declining in recent years, in some months no longer reaching the Pacific Ocean, but that the river is now an estimated 10 percent sewage by volume. Watering the desert seemed to him, quite literally, a pipe dream.

Yet the sign conjuring an oasis in the desert does point to a very real dilemma: In order to sustain its rapid development, China needs a lot of water. It can only build as many cities as it can supply with clean water. And the country’s water supply is precariously limited: The Middle Kingdom is home to 20 percent of world’s population, but just 7 percent of its available freshwater resources. Rapid urbanization is quickly increasing demand for fresh water, while climate change threatens to further reduce availability.

Wang Rusong, an expert in urban ecosystems at the Chinese Academy of Sciences and an environmental advisor to Beijing’s mayor, told me when I visited his offices in May that China’s most worrisome environmental challenge is not what it has too much of — pollution, sewage, carbon emissions, etc. — but what it doesn’t have enough of: “The limiting factor in Beijing’s development is water,” he said. And Beijing is hardly alone.

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