I pulled my horse to a stop along the banks of a little stream, which was wedged between two grassy hills speckled with wildflowers and pika holes, to admire the view of the Tibetan-Qinghai Plateau’s rolling hills evolving into snowcapped mountains.

Sewage—refuse liquids or waste matter usually carried off by sewers—is at the front lines of a global movement for clean energy. Innovative U.S. cities are digging into their dirtiest depths to create new sources of power that optimize economic benefits, generate clean energy, and control pollution. This wastewater-to-power movement is just beginning to catch on in China. But with some of the largest and most densely populated cities in the world, the country could be poised to lead a sludge-to-energy revolution.

The first U.S. city to host a docked bike-share system, Washington, D.C., is now home to a rapidly growing influx of dockless bikes, with five companies vying for the market. The docked system still accounts for 87 percent of the shared bikes in the United States, but the number of dockless bikes—which can be located by riders using an app and then left anywhere—is growing rapidly. The data from these location-enabled bikes provide a unique opportunity to measure the point-to-point transportation needs of millions of people in some of the world’s densest cities.

$1 billion sales in two minutes. More than 250,000 purchases every second. Singles’ Day, China’s annual retailing extravaganza, absolutely crushes Black Friday: E-commerce giant Alibaba raked in $17.8 billion in gross sales during last year’s event, more than double the combined total of $6.8 billion in sales during the United States’ 2016 holiday shopping kick-off. The shopping spree lasts only 24 hours, but its environmental impacts will extend for decades.

With air pollution causing more than one million deaths in 2015 and reducing the lifespan of citizens in northern China by three years, clean energy has become a top priority for China’s leaders. China tops the world in wind and solar power installations and the government plans to invest more than $360 billion through 2020 on renewable energy. But the green energy transition needs more than renewable power generation: Long-distance electricity transmission could play a key role in cleaning up China’s brown skies. Our recent study estimated that transmitting a hybrid of renewable and coal power through 12 new high-voltage transmission lines could prevent 16,000 deaths from air pollution exposure, and avoid 340 million tons of CO₂ emissions in China.

In 2011, a group of Hong Kong water activists and researchers traveled the length of the Dongjiang (East) River, which stretches from northeast Guangdong Province into Hong Kong’s New Territories, to investigate the challenges facing the watershed. The Dongjiang basin, which provides nearly 80 percent of Hong Kong’s water supply, has suffered water shortages due to the region’s increasing urbanization and industrialization. They found unchecked wastewater discharges—from agriculture, poultry farms, chemical plants, tanneries, and even an open-air quartz quarry—were dangerously degrading  water quality.