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.

Today, China feeds 1.38 billion people—approximately one-fifth of the global population—with about 10 percent of the world’s arable land. Compared to the United States, where each acre of land feeds one citizen on average, China has only 0.2 acres per person. This people-to-land mismatch helps explain why 9.3 percent of China’s population was undernourished in 2015.

From thick smog blanketing the cities, to acid rain corroding soils and ruining crops, air pollution from China’s coal industry is inflicting heavy damage on the country’s environment, its economy, and its citizens’ health. But these clouds of smoke have obscured the less visible (but no less severe) water challenges facing the coal sector and the region it operates in. 

Chinese cities are being encircled by untreated sludge—a pungent, viscous mixture of human excreta and stormwater runoff. Four years ago in Beijing, trucks loaded with untreated sludge from the city’s largest wastewater treatment plant were illegally dumping mountains of sludge in the outskirts of the capital as “free fertilizer” for peri-urban farms. Similarly, in the southern China city of Guangzhou, disposing toxic sludge into the nearby river required nothing more than a hired boat. From the far western desert city of Urumqi to the eastern metropolis of Shanghai, more and more of China’s cities are struggling to manage mountains of sludge and municipal waste, as well as floods of wastewater and stormwater runoff.