Thank you to Trade Coffee for  supporting this episode of SciShow.

You can go to drinktrade.com/scishowoffer to get a free bag of coffee  with any subscription purchase. This video is not about the climate crisis.

Yes, the climate crisis does mean coastal  cities will experience rising sea levels and more frequent, potentially  devastating flooding. But here’s what they’re not telling you. In many coastal cities, the land is  sinking much faster than the sea is rising.

Making all that flooding  that much more of a problem. The issue? It’s our extremely unavoidable  need for water in the first place.

But if we can put some of that  water back where it came from, we might just be able to stop  these cities from drowning. [♪ INTRO] The technical term for when  the ground sinks is subsidence. And it can cause more than just floods. Shifting ground can damage  the foundations of buildings, and other infrastructure  like roads and sewer systems.

It used to be the sort of  thing people would only notice when that kind of damage  happened and something cracked. Or in really extreme cases like  where massive amounts of groundwater are used for irrigating farms. Like in the San Joaquin Valley in California, where the ground has sunk as  much as 9 meters since the 1920s.

But today we have satellites  that can constantly monitor tiny and gradual changes in ground movement. Interferometric Synthetic Aperture Radar,  or InSAR for short, begins with a satellite bouncing a pulse of radar off the Earth  and recording the returning signal. This essentially measures the distance  between the ground and the satellite.

The satellite does this over and over again over a long period and then compares each image. This can measure very slight  changes in ground movement, even as little as a few millimeters. A 2022 study looked at InSAR data from  99 coastal cities around the world.

Subsidence doesn’t only  affect cities along the coast, but that’s often where the  flood risk is the greatest. They found that 33 of them have areas that  are sinking over 10 millimeters a year. That’s five times faster than sea level  rise caused by the climate crisis.

This means devastating floods will occur  sooner, and will be worse than expected, compared to predictions that  only account for sea level rise. The fastest subsidence rates are in Asia. Rates as high as 28 centimeters per year  have been seen in one area of Jakarta.

Worldwide, 1.2 billion people live in an  area that is prone to high subsidence. Building big heavy buildings  does contribute to this somewhat. A 2023 study calculated the weight  of all buildings in New York at 764 billion kilograms.

Because comparisons are fun, that's more than the total weight of all living humans! And New York City is sinking  at 1 to 2 millimeters a year. But New York is on the heavier  side of cities worldwide, and this isn’t a huge amount of  sinking compared to other places.

Which means something else  has to be the main factor. Globally, the biggest factor  contributing to cities sinking is pumping out groundwater from beneath them. After you build a city, the people  that live there need to drink and take showers, wash  dishes, and have water fights.

So lots of people living in  a place also tends to lead to more groundwater being extracted. When we say groundwater, we’re not talking about some deep  cave that fills up with water. It’s the water that can fill up tiny gaps in porous rock or space  between grains of sediment.

If a layer of ground can hold water in  it, that layer is called an aquifer. But the presence of water in those  little gaps also helps hold the layer up. Some aquifer materials, like fine sediments, will collapse a bit after the water is taken away.

Not only does this mean the ground above sinks, it also smushes the aquifer and causes  it to lose some of its water capacity. So in the future when natural  sources of water refill it, it’s not going to hold as much. Ultimately, the most important thing to stop this is to reduce how much  groundwater we are extracting.

Of course, that’s easier said than done if a city doesn’t have another viable source of freshwater. Globally, 30% of household water and  40% of farm water comes from the ground. Regulating groundwater has worked in many places.

In 2010, Jakarta brought in stricter regulation and licensing of groundwater pumping. As of 2020, subsidence had slowed. Instead of rates as high as 280 millimeters  per year, it was looking more like 20.

Although one suburb was sinking  at more than twice that rate due to continued groundwater extraction. Those are still some of the  fastest rates in the world, and unlicensed pumps are a big problem. So even stricter groundwater  regulations were put in place in 2023.

The issue is that as of 2020,  60% of the people in Jakarta still relied on groundwater. So the goal is to build water  pipes to reach everyone by 2030. Then there’s the Coachella Valley in California, which sank over half a  meter between 1995 and 2010.

So they switched to the Colorado  River for some of their water and slowed subsidence by around 50 to 75%. Of course, that’s not exactly a perfect fix,  given the Colorado River is at its limits. As of 2023, states are being  asked to cut back their use of it.

Some places are going even  further and trying to reverse some of the damage done – by  pumping water back into the ground. This is known as managed aquifer recharge. Essentially you drill a well down to an aquifer, but instead of pumping water  out, you pump water in.

This can be tens of millions of liters a day, and so it can even reverse some subsidence. In Perth, Australia, they  were able to lift the ground by 20 millimeters over about 3 and a half years. While water banking has a lot of potential,  it also has big issues to overcome.

It’s expensive, and you still  need to get that fresh water to pump back in from somewhere. In many places, there’s not an  easy way to stop the ground sinking and ensure plentiful fresh water  for decades and centuries to come. But we are making progress.

Thanks to modern satellites,  we can see the effects on the ground beneath our feet much sooner. Which makes it a lot easier to  try and do something about it. This SciShow video is supported by Trade

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To try it for yourself, you can  go to drinktrade.com/scishowoffer to get a free bag of coffee  with any subscription purchase. Thanks to Trade Coffee for supporting  this video and thank you for watching! [♪ OUTRO]