Well, not really in decline in the way you’re thinking. Megacities, defined by the United Nations as areas with urban populations in excess of 5 million persons (think Tokyo, New York City, Mumbai), are slowly sinking into the earth – according to research from the Netherlands (and reported by the BBC). Coupled with raising sea levels due to climate change, humanity’s most densely packed population centers at risk of longer and deeper floods.
Land or the ground sinks into the Earth’s crust naturally, as any geographer will tell you. One method is tectonic plate movement. Depending on the plate, one may be subsumed under another with one being pushed upwards and the other being pushed under. These geologic processes also cause earthquakes and volcanic activity (see: the Pacific Ring of Fire). As the BBC article points out, this geologic activity may be responsible for about 1 mm of subsidence a year. According to the research, a longitudinal study using radar imagery (measuring elevation), concludes that human activity – particularly groundwater extraction – is the primary culprit for city subsidence.
In most cases, a city’s drinking water supply is sourced from local groundwater. As this water is extracted from underground aquifers, one would assume that heavier buildings and infrastructure would press down upon, and compact, the underlying soils. Of course, the relative amount of compaction would be dependent on the local soil (sand, clay, silt, and other factors). While some cities have reduced, if not almost wholly eliminated, municipal subsidence (the article mentions Tokyo and Venice) by halting groundwater extraction – this option isn’t a realistic solution for coastal megacities in less economically advanced countries (Dhaka, Lagos, Jakarta). In these and other “smaller” cities (between 1 and 5 million persons) municipal budgets are already strained coping with a vast informal housing sector (read: slums and shantytowns), a stagnant infrastructure, and poor administration. Adding a requirement for an entirely new source of drinking water for an entire city would be prohibitively expensive.
However, given rising sea levels, municipal subsidence, some 75% of humanity lives on the coast (but not necessarily in a city), and about half of humanity lives in cities (not necessarily on a coast) – we can easily see the scale of the problem. Fortunately, the problem is somewhat long-term, city subsidence and sea level rise occurs at rates of millimeters a year. However, though the number is small the results are disproportionately large. A National Geographic article, published Sep 2013, cited a OSCE report stating that a 20-inch sea level rise would leave 150 million people and $35 trillion dollars (about 9% of global GDP) at risk of coastal flooding. A city sinking 20-inches, an easy analogy, would take 40 years at 5 mm a year.
Of course, this timeline would compress markedly if cities are to contend with both rising sea levels and their own sinking. The timeline is likely to further compress if urban population growth rates remain high as new residents also demand access to water.
Even more long-term, Z Geography wonders if the growth of megacities will lead to their own decline. Could this natural hazard (coastal flooding) combine with other human-made hazards in cities (violent conflict, poverty) and lead to a period of deurbanization in the next 50, or 100, years? One could argue “yes”, in the true spirit of Malthus, but we shall watch for technological and economic innovation – perhaps a cheaper way to reduce our dependence on groundwater?