Chinese cities are sinking under their own weight

Hello. Scientists say cities in China are being badly affected by subsidence - or sinking. Our environment correspondent Matt McGrath explains why. With airline passengers stranded in flood-hit Dubai, we hear from some of those affected. For BBC Future, Sandy Ong examines whether filtered water really is healthier than the stuff from your tap. Plus, look out for some powerful press photography, and a runaway elephant.

QUESTIONS ANSWERED

The threat to China's cities

Subsidence is thought to be linked to the rapid expansion of cities. Credit: Getty Images

Nearly half of China's major cities are sinking, satellite data suggests. And a team of researchers from several Chinese universities describe more than 67 million people as living in areas which are rapidly subsiding – meaning at a rate of more than 10mm a year.

What’s causing the problem?

The scale of decline is influenced by a number of factors, including geology and the weight of buildings. But a major element, according to the authors, is groundwater loss. This essentially means the extraction of water underneath or near cities for use by the local population. This has already been seen in several major urban areas around the world including Houston, Mexico City and Delhi.

Why does it matter?

The authors of the study say that a big threat going forward is the exposure of urban populations to flooding, from a combination of subsidence and sea-level rise brought about by climate change. In 2020, around 6% of China had a relative elevation below sea level. In 100 years time, this could rise to 26% of the country in a mid to high carbon emissions scenario.

Can anything be done?

"Tokyo subsided around the port area, up to five metres in the 20th century," said Prof Robert Nicholls, from the University of East Anglia, who was not involved in the research. "But in the 1970s, they provided good piped water from other areas and they also had a law saying you will not use well water and essentially it stopped the subsidence."

THE BIG PICTURE

Love and grief triumph in photo awards

The jury said Cegarra's personal experience had helped him sensitively document the dangers faced by migrants. Credit: Alejandro Cegarra, for the New York Times.

This scenic photo of two lovers might have been taken on honeymoon. But in reality it captures Honduran Rosa Bello and Venezuelan Ruben Soto atop a freight train en route to the US. It was part of a series on migration that helped Alejandro Cegarra win one of the top prizes in a World Press Photo Contest that rewarded several moving - and sometimes distressing - images.

And finally... in the US

Viola the elephant caused commotion in the US state of Montana. The circus animal was being washed near the road when she was startled by the sound of a car, took off and stomped into traffic.