The Miami condo collapse is a devastating reminder of America's artificial land problem

A condo complex in south Florida partly collapsed out of nowhere Thursday morning. At time of writing, at least four people are confirmed dead and 159 others are still missing — making it likely one of the worst building collapse disasters in American history.

It is not yet known for certain what caused the collapse, but one probable culprit was the fact that the building had been built on reclaimed wetland, and as a result, had been sinking into the ground for decades. Scientist Shimon Wdowinski happened to study the area last year, and found that the building (which was built in in 1981) had been sinking at the rate of about 2 millimeters per year back in the 1990s. Even if some other factor was the proximate cause, the sinking surely made it worse — a building in such a situation can easily develop cracks in its foundation or other problems that compromise its structure.

It's illustrative of a major problem in many American cities: reclaimed land. Big chunks of almost all American coastal cities are built on reclaimed land that will likely turn to soup as climate change causes ocean levels to rise.

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Rising seas of course threaten any city that is at a low elevation. But reclaimed land — that is, new land space made by dumping fill into a lake, river, ocean, or wetland — is at particular risk, because it is much less stable than normal land. At best, reclaimed areas are made with concrete, rocks, sand, or other material that can be compacted into a fairly stable shape. At worst, they are made by throwing some dirt over the top of a pile of garbage, and building more city on top of it.

This literal trash foundation is unfortunately how big parts of numerous American cities have been built. Back in the 19th century, when there was little by way of environmental regulations or sane building codes, cites would often just dump their trash into any handy spot, and start slapping up buildings on top of the mess. U.S. Geological Survey maps show big chunks of the cities around San Francisco Bay are built on artificial fill, some of it garbage from the 1840s. The same thing is true of New York City.

Even the best reclaimed land tends to subside over time, as it has not had tens of millions of years to stabilize. For instance, the Kansai International Airport, built on an artificial island by world-class Japanese engineers in the early 1990s, still sank much more quickly than predicted and required expensive upgrades and protection to keep it from falling underwater. That process is accelerated by rising sea levels, as the ocean is effectively coming up to meet the land coming down, and because reclaimed land tends to become destabilized much more easily with more water contact. That in turn increases the vulnerability to earthquakes in seismically-active locations like Tokyo or San Francisco.

As Grace Mitchell Tada writes in The New York Times, in previous earthquakes much of the old San Francisco fill had liquified, causing immense structural damage to streets and buildings above it. Now rising oceans threaten the sea wall along the city's northeast edge even more. This wall was built over a century ago to shore up the city's edges and create over 500 acres of new land, where many of the city's most popular attractions, biggest businesses, and key public services now sit. The aging sea wall has already subsided substantially, and rising seas only increase the danger. The government estimates that another major earthquake is likely to strike the region in the next 22 years, which could destroy the wall outright and possibly cause big chunks of the city to slough off into the bay:

Reclaimed land is just one of a thousand different threats that are either going to be addressed soon or cause enormous disasters eventually. America and the world needs swift action to slash greenhouse gas emissions immediately, and huge upgrades to our infrastructure and social systems to deal with the oncoming damage that is already baked in from decades of political dithering. Every minute of delay will only make future problems worse.