China's cities sinking

China is home to around 1.4 billion people - which is more than a sixth of the world’s entire population. The country’s rapid urbanisation in recent years has seen skyscrapers thrown up and groundwater extracted at massive rates to meet people’s needs. But now a new report published in the journal Science has found that this expansion is causing many of China’s biggest cities to literally sink into the ground. Robert Nicholls is director of the Tyndall Centre for Climate Change Research at the University of East Anglia. He authored a commentary on this research…

Robert - A group of Chinese scientists, a very large group of Chinese scientists, studied 82 cities around China, the biggest cities. And they used satellites to measure how fast the land in these cities is sinking or rising. And they found that many of them are sinking sometimes quite rapidly, more than a centimetre per year.

Chris - Oh goodness, that is a lot.

Robert - Yeah, absolutely. I mean, it's a substantial change and when you add it up, it can have quite big sort of consequences. If that continues.

Chris - Why do they think it's happening?

Robert - Several reasons, but probably the biggest one is groundwater withdrawal. These cities are expanding very rapidly and when you have increasing demands for groundwater, you pump it out. And actually the soils beneath these cities, when the geology's right, doesn't happen for every city, you get consolidation. The land literally falls as the water is taken out.

Chris - Lots of cities, though, rely on groundwater for their supplies. Perth, Western Australia has seen its groundwater levels drop considerably. Cape Town in South Africa relies very heavily on it. America, parts of America, has lots and lots of groundwater extraction. So is this likely to be happening everywhere and it's just that the Chinese have spotted it, or is this a uniquely Chinese problem?

Robert - It's not happening everywhere because many areas that contain water underground, like limestone, when you take the water out, they don't drop. I mean, in the south of England, we take water out of chalk and that doesn't consolidate. In the case of China, it's the geology. They've got sands, muds, clays that are geologically recent and they're very old compared to you and I, but they're geologically recent. And when you take the water out, the actual density of the sediment increases. It's called consolidation and that causes the land to sink. But it's not only happening in China. Deltas are particularly prone to this. And there are many, many deltas in Asia, particularly around the Himalayan plateau.

Chris - What's the natural evolution of this then? Does it mean that, because of what you've just said, these buildings will settle eventually and the retreat will stop? Or are they going to continue for the foreseeable to sink like this?

Robert - They won't sink forever. So if you keep on pumping groundwater, eventually all the potential to settle has been used up. But Tokyo, for example, subsided up to five metres in the 20th century. So you can get very large changes. Parts of the Central Valley of California have gone down more than 10 metres, and that's not in an urban area. So you can get very large changes. Why is it damaging? Well, it's not normally uniform. So you find that buildings will literally fall down or in the central area of a city, where maybe the demand for groundwater is highest, will become a bowl and it will flood much more frequently. So it'll mess up the hydrology. And if you are near the coast, if the land sinks, the sea will flood. It is as if the sea level roses, the land sinking, will have the same effect.

Chris - So what can China do about it? Should they be looking for other sources of water? Do they need to be building their buildings differently? What's the solution?

Robert - First of all, it's important to recognise this is happening and then you ask, what can we do about it? One solution is to try and provide other sources of water so that they don't exploit the groundwater. That, however, may not be possible. So the first thing is, can we stop removing groundwater? If not, can we adapt to these changes? But then at least you're not being surprised by them. And back to your point, maybe you can build buildings that won't fall over if they subside or on the coast. Maybe you have to build to keep the sea out or maybe you have to move. I think there are multiple solutions and it will vary from place to place.

Chris - It sounds expensive.

Robert - I think it is costly. Oh, without doubt. I mean, often this problem is seen as a very local problem. And I think one of the arguments we make in our piece is that it's a big problem and as you say, it's costly, but recognising it is going to lower the cost because you're going to start to plan proactively.