Today’s Image of the Day from NASA Earth Observatory features India’s Krishna Delta, where sinking land – or subsidence – complicates the issue of sea level rise. Worldwide, river deltas are sinking at up to twice the rate of sea level rise.

“The combination of anthropogenic subsidence and increasing rates of sea level rise is a five-alarm fire for many delta cities,” said Manoochehr Shirzaei of Virginia Tech.

“Places like New Orleans, Kolkata, Yangon, Bangkok, Ho Chi Min City, and Jakarta will undoubtedly face increasing pressures from flooding and saltwater intrusion.”

There are a number of factors that combine to cause subsidence, including the added weight of buildings, the natural settling of sediments, and groundwater or oil extraction.

When it comes to predicting sea level rise,  accounting for subsidence is not straightforward because it is hard to predict human settlement patterns. There are also other uncertainties which make it difficult to project when, where, and how much the sea level will rise in the coming decades and centuries.

“People tend to think that sea level is like a bathtub with the water level simply rising and falling depending on how much water is coming out of the faucet,” said paleoclimatologist Anders Carlson of the Oregon Glaciers Institute. 

“In reality, it’s more like a spinning bathtub that’s changing shape, moving up and down, and has water pouring into and out of different drains and over the sides. Where the water will ultimately slosh over the edge of the tub is influenced by many things, making it difficult to say where the overtopping will occur.”

According to NASA, despite the complexities, the scientific understanding of the factors that control sea level has improved dramatically in recent decades, as have measurements of past sea level change and projections of future change.

“We can tell you how much the ocean has warmed in recent decades, and how much more space the water takes up. We have satellites and other tools that have measured that,” said Ben Hamlington, the current lead of NASA’s sea level change team. “The same thing is true for several of other factors that influence sea level, such as the mass of the ocean, the salinity, and how much water is stored on land.”

“The reason people within the scientific community are working so hard on regional sea level rise projections is that if we can get them right, it will give cities and nations a chance to prepare.”

“Even if some of the more distant projections are inexact, they still provide critical constraints that could end up being the difference between places that successfully adapt to rising seas and those that experience the most damaging consequences.”

Image Credit: NASA Earth Observatory 

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By Chrissy Sexton, Earth.com Staff Writer