As the sea levels rise, humanity's habit of building on the coast means that many of the world’s major cities will slowly drown. This may be some way off in most people’s minds, but in certain parts of the world it is already a reality. New research published in Nature Communications reveals that the low-lying wetlands of Louisiana are experiencing sea level rise four times the global average.

The researchers measured the sea level rise across 274 sites in Louisiana by combining shallow subsidence rates with GPS measurements of deeper subsidence, as well as satellite observations of the sea surface in the Gulf of Mexico. From this, they calculated that for the past six to 10 years, the sea has been rising on average half an inch a year – four times the global average.

In the wetlands of Louisiana, however, the rising of the sea is compounded by the fact that the ground is also sinking. The multitude of levees and barriers constructed throughout the region, which are meant to prevent storm surges and flooding, have the added impact of preventing sediment and mud from being washed out of the river systems into the estuary. This is worrisome as the influx of fresh soil from inland is crucial for maintaining the ecosystem of wetlands and marshes, as well as replacing that lost to erosion by the sea.

Not only that, but as the sea levels continue to rise due to climate change, even more sediment will be washed out from the wetlands to the sea, increasing this rate of loss.

Some communities in the marshes, such as this one in Delacroix, are already largely abandoned. Torbjörn Törnqvist

The researchers think that what's currently happening in Louisiana may serve as a template for what will eventually occur in other parts of the country. “In the westernmost part of coastal Louisiana, many of the sites we studied are on track to drown,” explains Torbjörn Törnqvist, co-author of the study, in a statement. “This is why it is such an important setting to assess what may happen elsewhere later in this century, when global sea level rise accelerates.”

The worry is that as the wetlands sink below the waves, the land and communities behind it will be left open and exposed to more severe weather. In many parts of the world, these coastal habitats – from wetlands to mangroves – act as natural flood defenses and barriers. In fact, the degradation of the marshlands surrounding New Orleans is thought to have been a major contributor to the massive damage experienced when Hurricane Katrina hit.