But scientists have not modeled global risks of sagging – until now. To build their model, Sneed and his colleagues scanned the existing literature on land subsidence in 200 locations around the world. They took these geological factors (high clay content) into account, as well as topology, as subsidence is more likely to occur on flat ground. They took into account population and economic growth, data on water use and climate variables.
The researchers found that, globally, subsidence could threaten 4.6 million square kilometers of land over the next two decades. Although this is only 8% of Earth’s land, humanity tends to build large cities in coastal areas, which are prone to subsidence. They therefore estimate that in the end 1.6 billion people could be affected. The modeling further revealed that worldwide, sag is exposing assets totaling a gross domestic product of $ 8.19 trillion, or 12% of global GDP.
Granted, gradual subsidence is not as destructive as a sudden earthquake or volcanic eruption. “But this will cause these effects or indirect impacts which, in the long term, can produce either damage to structures or infrastructures, or increase the flood zones in these river basins or coastal areas”, explains geoscientist Gerardo Herrera-García of the geological division. and mining. Institute of Spain, main author of the article.
The subsidence is particularly sensitive to climate change, at least indirectly. On a warmer planet, droughts are longer and more intense. “It’s very important,” says Herrera-García. “Because no matter how much annual rainfall you have, the most important problem is that you extended period of drought. Dry reservoirs will cause cities to pump even more water from their aquifers, and once you collapse the structure of an aquifer by neatly stacking those clay grain plates, there’s no turning back. back. For the 1.6 billion people potentially affected by the subsidence – and only by 2040 – the consequences could be dire, leading to both water shortages and flooding of the lowlands .
“These are certainly very surprising results,” says USGS coastal geologist Patrick Barnard, who is studying the subsidence but was not involved in this new work. “Mostly coastal mega-cities – most mega-cities are, in fact, coastal. So that really highlights the problem with coastal flooding. “And urban populations are booming: according to the United Nations, nearly 70% of humans will live in cities by 2050, compared to 50% currently.
Mankind has tended to build its cities where rivers flow into the sea, where subsidence conditions are ideal. Long ago, these rivers deposited clay-laden sediments, on which humans then built themselves. “High-risk areas are found in these types of environments near the outlets of river deltas, and in which there are flat and low sedimentary basins near the coast,” explains Roland Burgmann, geophysicist at the University of California at Berkeley. , who is studying the sag but was not involved in this new work. But you can also find this problem inland, for example in Mexico City, which is built on top of the sediment of an ancient lake, and is therefore suffering from sagging.
Cities built on landfills are also sinking as this material takes hold. In the megalopolis of Bay Area, for example, some areas sink as much as a third of an inch per year. Modeling estimates from researchers at Arizona State University and UC Berkeley argue that by the turn of the century, as many 165 square miles of the bay area could be flooded as the land sinks and the sea rises.
Subsidence becomes even trickier as its effects can vary widely over short distances, depending on factors such as the composition of the local clay or which side of a seismic fault the earth is on. This new global study is therefore ideal for determining risk on a large scale, but scientists will still need to study sag in a finer concentration.