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Sinking coastal land can cause a rise in relative sea level. Groundwater and hydrocarbon extraction, as well as microbial oxidation and soil compaction related to agriculture, are among the human contributions to subsidence. Tectonic forces, including post-glacial rebound, are among the natural causes.
Postglacial Rebound: The Regional View
To make accurate estimates of sea-level rise, changes in gravitation at the local and regional level must be balanced against the gradual, and linear, recovery of ocean basins from the last ice age. The widening of these basins as they slowly spring upward can cause sea levels to drop. The equally linear nature of mass reduction from modern-day ice melt, however, requires researchers to tease apart the two signals—an essential step in parsing the contribution of ice to sea level rise. One modeling study [Wu et al., 2010] found significant rebound over Alaska and West Antarctica, and less pronounced effects over other regions. The study’s authors estimated mass losses from 2002 to 2008 of about 104 gigatons per year in Greenland, 101 in the Alaska-Yukon region, and 64 in West Antarctica (though with a somewhat wide range of uncertainties—plus or minus 23 gigatons per year for Greenland and Alaska-Yukon, and 32 for West Antarctica).
The authors recommended adding 0.66 millimeters per year to GRACE-based global sea-level rise estimates to compensate for the drop in sea level as rebounding ocean basins widened.
Postglacial Rebound, Self-attraction & Loading
The basins that hold Earth’s oceans do not remain static. Over long time scales they shift and deform in response to powerful forces. Because these, too, can alter sea levels, scientists must tease apart their effects from those of melting ice and thermal expansion.
One of the most important of these is post-glacial rebound, also known as glacial isostatic adjustment (GIA). At the close of the last ice age, some 10,000 years ago, the retreat of massive ice sheets from North America and the Eurasian continent lightened the load on the underlying mantle, deep below the Earth’s surface. The mantle is viscous, not solid, so it rebounds slowly, raising the rock layer above—the lithosphere—over thousands of years. This gradual lift, the recovery from the last ice age, continues today, altering the shape of ocean basins. And these alterations widen the basins, lowering apparent sea level by about 0.3 millimeters per year [Douglas and Peltier, 2002]. Researchers must factor this effect into their calculations when estimating the rate of sea level rise due to climate change.
Modern day melting also has a “rebound effect,” also called GIA.
A second effect, called ‘loading,’ is simple to understand: regardless of mantle viscosity, the lithosphere acts a bit like an elastic plate over large distances and short time scales, hence removing the weight of ice allows it to bounce back elastically. The same effect makes the seafloor go slightly up and down as tides move massive amounts of water from one location to another. While GIA acts over long time scales, ‘loading’ is a short-term effect [Tamisiea et al., 2010].
Another subtle effect can lower sea levels locally, even as the amount of water increases. The loss of ice mass means a loss of gravitational attraction; less surrounding water is pulled toward the ice, causing sea levels in the immediate vicinity to drop. The effect is counterintuitive: sea levels in the immediate neighborhood of melting Greenland do, in fact, go down. This effect is called “self attraction.”
Postglacial rebound and changes in loading are but two of the factors that can alter sea level by altering the elevation of the land. Excessive withdrawal of groundwater, for example, can cause the land surface to subside, raising coastal sea levels by comparison. Such interplay of effects might quickly lead to a measurement problem if scientists did not carefully calibrate their observations. They begin by drawing a sharp distinction between absolute and relative sea level—that is, sea level with respect to Earth’s center versus sea level with respect to the coast.
Relative sea level also can rise because the land itself is sinking—visible in extreme form in the California Bay Delta. Some of the islands amid this 2,900-square-kilometer network of water channels and farmland have dropped to eight meters or more below sea level [Mount et al., 2005], due mainly to microbial oxidation and soil compaction—the legacy of more than a century of farming. One low-resolution modeling study suggested the additional strain placed on levees as the land surface subsides, coupled with rising sea levels, are sharply increasing the risk of floods resulting from levee failure [Mount et al., 2005]. These factors contributed to the study’s estimate of a two in three chance of catastrophic flooding in the Delta by 2050, triggered by a 100-year flood or earthquake event.
While subsidence occurs naturally, it can be greatly accelerated by human activity. Groundwater and hydrocarbon extraction, for example, can cause compaction of sediments [Waltham, 2002]. Subsidence related to groundwater withdrawal can be especially pronounced in river deltas with large populations and extensive agriculture, among them Thailand’s Chao Phraya, the Bengal Delta of India and Bangladesh, China’s Yangtze and the Egyptian Nile [Ericson et al., 2006].
Groundwater extraction has been identified as the cause of land subsidence in Vietnam’s lower Mekong Delta, averaging 1.6 centimeters per year, as declines in groundwater levels cause compaction of sedimentary layers [Erban et al., 2014]. Subsidence on the order of 0.88 meters could occur by 2050 if present pumping rates continue; combined with expected sea level rise, this will likely bring a significant increase in flood hazard.
A study of “effective” sea level rise—defined as global sea-level rise combined with sediment deposition and subsidence—in 40 deltas across the world found that about 20 percent were experiencing accelerated subsidence because of groundwater or hydrocarbon extraction [Ericson et al., 2006].
Tectonic subsidence, sometimes related to volcanism, has left behind impressive examples of its effects, including the submerged ancient Roman city of Baia off the Italian coast [Lambeck et al. 2010].