But that’s not where the majority of people live: Over the same time period, Earth’s coastal inhabitants saw the seas rise by an average of 7.8 to 9 millimeters annually (about half an inch). Nicholls and his colleagues found that on average, Earth’s coastlines actually experienced a slightly lower relative lift of about 2.6 millimeters per year (0.1 inch) between 19, because so much land is still rising due to glacial rebound. Satellite measurements put climate-driven sea level rise at about 3.3 millimeters per year (around an eighth of an inch). To estimate the rate of sea level rise experienced along thousands of sections of coastline worldwide, Nicholls and his colleagues compiled data from four key sources: satellite observations of climate change-fueled sea level rise model estimates of how land is adjusting from the last ice age data on natural subsidence in 117 river deltas, and estimates of human-caused subsidence in 138 large coastal cities. “We wanted to actually understand what is the human experience of relative sea level rise” by taking subsidence into account worldwide, Nicholls says. But prior to the new study the effect hadn’t been assessed at a global scale. The problem of subsidence and its effects on sea level rise-where the land sinks, the oceans rise relative to the shore by the same amount-is well documented for certain cities.
In places where people are concentrated, these activities, particularly groundwater removal, often cause the land to subside much more quickly than it would via geological processes alone: Over the 20th century, parts of Jakarta, New Orleans, Shanghai, and Bangkok sank between six and 10 feet. Preventing river flooding, a good thing in itself, also stops rivers from spreading sediments that slowly rebuild the land. In coastal river deltas, land subsides slowly as freshly deposited sediments are compacted.īut in addition to those natural processes, human activities, including groundwater withdrawal, oil and gas extraction, sand mining, and the construction of flood barriers around rivers, can all cause the ground to sink. Parts of Earth are still adjusting to the disappearance of glaciers that blanketed it during the last ice age, springing up in some places and sinking in others. Some of the factors contributing to the fall (or rise) of Earth’s coastlines are beyond human control. The silver lining, such as it is: Where coastal land is sinking, it’s in large part because of human activities, like groundwater withdrawal, that coastal cities can do something about. “We are talking about not a forecast we are saying this is happening today,” says lead study author Robert Nicholls of the University of East Anglia’s Tyndall Centre for Climate Change Research. Sinking land makes coastal residents around the world disproportionately vulnerable to rising seas: The typical coastal inhabitant is experiencing a sea level rise rate three to four times higher than the global average. Now, an international team of researchers has demonstrated that this one-two punch is more than a local problem. Cities like New Orleans and Jakarta are experiencing very rapid sea level rise relative to their coastlines-the land itself is sinking as the water is rising. But on a local scale, subsidence, or sinking land, can dramatically aggravate the problem. Sea levels are rising globally as Earth’s ice sheets melt and as warming sea water expands. The world’s coastal residents are experiencing more extreme sea level rise than is widely appreciated because they are concentrated in places where the land is sinking rapidly, a study published Monday in Nature Climate Change has found.
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