Cities and the millions of people in them are collapsing. There are more of them than thought

Decreasing sea level at a rate of more than three millimeters per year affects 45% of the 82 Chinese cities surveyed in a study recently published in the journal Science, with 16% falling even faster than ten millimeters per year.

Scientists found this out from satellite measurements of deformations of the earth’s surface carried out between 2015 and 2022. The main cause is the abundant use of groundwater, the absence of which in the sediment dramatically accelerates the process of its compression.

According to British scientist Robert Nicholls, who participated in the review process of the study and has been dealing with the topic for a long time, the study showed that the problem has larger dimensions than expected.

“China already included the solution to the problem in its national plan before this study. But it showed that more cities are sinking than was thought, and that this can have an impact on hundreds of millions of people,” says the expert in an interview with Seznam Zprávy.

Many other cities around the world have a similar problem.

Robert Nicholls

The topic of declining cities is often presented as something long overlooked, not fully explored and deserving of more attention. Is it just my feeling or is it real?

I would say that the world knows that cities are collapsing. The first articles about it started to appear years ago. First it was Bangkok, Jakarta, Shanghai, Taipei, Tokyo and so it went on. But in my opinion, the problem is still often seen as a local one, even though – as the Chinese study also showed – it is national and probably international. Although it always has only local consequences, it occurs in so many places that the effects go beyond the local framework.

Is there any estimate, even if not perfect, of what percentage of the world’s population lives in cities at risk of this problem?

There are some estimates. For example, UNESCO conducted a study in which many scientists from all over the world tried it and it mentions about 20%. But beware, this is an estimate relating only to the decline caused by the pumping of groundwater. At the same time, the loss of altitude also occurs for other reasons connected with, for example, the extraction of minerals, so there are even more people at risk of subsidence as such.

Which parts of the world are most at risk of urban decline? Is it China?

Yes, it certainly belongs to the most endangered countries. But it also applies to other Asian countries.

There are an awful lot of river deltas with young geological bedrock. These are areas such as the deltas of the Ganges, the Brahmaputra and the Yangtze River. When you build a city in these deltas and start pumping groundwater to the surface on a large scale, the cities begin to sink. In addition to the geological conditions, the fact that there are many growing cities in Asia that need more and more groundwater also plays a role.

If I go back specifically to China, it also has the largest population living on the coast in the world. This is also why it is one of the least threatened countries by this problem.

How exactly does groundwater pumping to the surface contribute to lowering sea level?

Lowering itself is a natural process in places with young geological bedrock. It occurs in such a way that when one layer is deposited, it has a lot of water in it, which is gradually pushed out by the weight of newer layers. As water decreases from a given layer, it is compressed. This happens due to the fact that the water envelops the individual grains of the soil, which, thanks to it, do not have to be in direct contact. But when the water is squeezed out, the grains get closer together until the layer reaches a thick consistency that is really hard to compress any more.

As I said, it’s a natural geological process, but pumping underground water to the surface speeds it up. Instead of happening gradually, it happens essentially instantaneously or in a very short time. And once compression occurs, there’s nothing you can do about it. There is no force that forces the soil grains to divide again. It is an irreversible process.

So such a slump can only occur in cities with soft ground…

Yes, it only happens in places with certain geological conditions. Especially where there are thick geologically young layers containing rather less permeable materials.

This means that, for example, it cannot happen here in Prague, because the subsoil here is rocky and settled a long time ago, so these processes have already ended there. Theoretically, you can observe subsidence in the sediments of the river, but in your case they only cover very small areas, on which, moreover, almost nothing is likely to be built.

But if you look, for example, at the Yangtze River Delta, which is 100 kilometers wide, it’s a different matter. There are entire cities built on such subsoil, such as Shanghai.

Is Europe avoiding this problem altogether?

It’s not as prominent there, but it’s not completely avoided. It is felt, for example, by the coast of the Netherlands, especially around Rotterdam and Groningen. It concerns northern Italy, where Venice is the most famous example, but the affected area is larger. The third example is the Greek Thessaloniki and its surroundings. Then there are many other places, but these three are the most affected.

Why lowering sea level threatens cities that are right next to the sea is probably obvious. But what about inland cities?

Land subsidence inland is such that if it happened everywhere at the same rate, it probably wouldn’t even be a problem. Unfortunately, it usually happens that the groundwater is sucked up from several points, around which a depression begins to form with the highest drop in the middle and a smaller one at the edges. When the difference in subsidence is large enough, damage to underground infrastructure or house foundations may occur, which may eventually fall.

Another very important thing is that when you have “bowls” like this in your city, it disrupts the hydrology. Then when a heavy downpour comes, all the water flows to the bottom of the bowl and has nowhere to drain. In such a case, the city either has to have a pumping system or put up with more frequent floods.

Are the problems with underground infrastructure and buildings still a question of the future, or are they already really manifesting themselves somewhere?

I would say that, for example, it is already a relatively significant problem in Chinese cities. Therefore, China is considering to stop pumping groundwater and included a solution to the problem in its national plan even before this study. But it showed that more cities are sinking than previously thought, and that this could have an impact on hundreds of millions of people.

What can you do? Is stopping groundwater abstraction the only possible solution?

I think it’s similar to climate change. You can either address its nature and stop producing greenhouse gases, in this case pumping groundwater to the surface, or you can try to adapt.

In the case of declining cities, in my opinion, adaptation with technological solutions is not impossible. I can imagine, for example, more durable building foundations or systems that pump out water after heavy rains. But first, it is necessary to have a good map of what and how it sinks. We need to ask what area is failing, why it is failing, how big a problem it is, and then address how to fix it.

One more question about coastal cities. Can you calculate how much the rise in ocean levels caused by climate change and how much the drop in sea level contributes to the gradual sinking of cities under water?

Yes, we did a 2021 piece of research on that, looking at how much vertical soil movement contributes to the problem. We found that if you look at it by looking at the number of people who are going to experience the effects of rising ocean levels, and you factor in the drop in elevation, it increases the number three to four times. This means that the effect of decay is significant, which was the main message of the paper.

This is not to say that climate change is not a problem. On the contrary, it means that it is even worse than we thought. We should address both issues.

Do you feel that your findings have been heard?

I think that awareness of this problem is increasing and that our work has helped that. The study had a lot of citations, so it was definitely read, influenced people’s thinking and subsequent work. At the same time, I don’t think that one study can solve the whole problem. A lot of work still needs to be done.