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Why sea level rise varies from place to place

Multiple, overlapping factors can mean big differences in flood risk

By
9:30am, August 15, 2018
Maldives

SLOWLY BUT SHORELY  The Maldives (its capital, Malé, shown) has long been a poster child for vulnerability to rising seas, with more than 400,000 people living across low-lying atolls in the Indian Ocean. A 2017 study suggests that weakened monsoon winds have curtailed ocean circulation, leading to sea levels rising faster than the global average around the island nation.

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In the 20th century, ocean levels rose by a global average of about 14 centimeters, mainly due to melting ice and warming waters. Some coastal areas saw more sea level rise than others. Here’s why:  

Expanding seawater

As water heats up, its molecules take up more space, contributing to global sea level rise. Local weather systems can influence that effect. In 2017 scientists reported in Geophysical Research Letters that weakening monsoon winds have resulted in hotter surface ocean temperatures in the northern Indian Ocean, causing local sea level rise. Those weaker winds curtailed ocean circulation that normally brings cooler water up from the deep. Surface waters in the Arabian Sea, for example, got warmer than usual and expanded, raising sea levels near the island nation of Maldives at a slightly faster rate than the global average.

Glacial rebound

Heavy ice sheets covered much of the Northern Hemisphere about 20,000 years ago. Regions once compressed beneath the weight of all that ice, such as the northeastern United States, have been slowly rebounding. In those areas, sea levels appear to be rising more slowly, because the land is rising as well.

But regions that once lay at the edges of the ice sheets, such as the Chesapeake Bay region, are now sinking as part of that ongoing postglacial shift. That’s because the weight of the ice squeezed some underlying rock in the mantle and caused the surface of the land to bulge, much like the bulging of a water bed when a person sits on it. Now, with the ice gone, the bulge is sinking — accelerating the impacts of sea level rise on the communities that sit atop it.

Sea change

Across the world, local sea level rise varies thanks to the influence of temperature, gravity and even Earth’s spin. This map by the National Oceanic and Atmospheric Administration offers a snapshot of current trends. For example, sea levels are rising faster on the U.S. East Coast than on the West Coast.

Sinking land

Tectonic activity such as the 2004 magnitude-9.1 Sumatra-Andaman earthquake (SN: 8/27/05, p. 136) may tilt the land and alter relative sea level rise, as it did in the Gulf of Thailand. And human activities, such as extracting groundwater or fossil fuels, can also cause land to sink.

Earth's rotation

The planet’s rotation deflects fluids in motion, causing ocean water to swirl counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. As water moves around coastlines, this Coriolis effect can cause bulges of higher water in some areas and troughs in others. Output from rivers can exacerbate this effect, scientists reported in the July 24 Proceedings of the National Academy of Sciences. As rivers flow into the ocean, the water gets pushed by the swirling currents to one side, causing water levels to rise higher there than on the side behind the current.

Melting ice sheets

Massive glaciers exert a gravitational pull on nearby coastal waters and cause them to rise higher than they otherwise would. When glaciers melt, their mass redistributes, weakening their gravitational pull and causing the nearby water levels to drop. The melting ice in Antarctica, for example, causes more sea level rise on faraway New York than on the closer beaches of Sydney, scientists reported in 2017 in Science Advances.

Citations

C.G. Piecuch et al. River-discharge effects on United States Atlantic and Gulf coast sea-level changes. Proceedings of the National Academy of Sciences. Vol. 115, July 24, 2018, p. 7729. doi:10.1073/pnas.1805428115.

E. Larour, E.R. Ivins and S. Adhikari. Should coastal planners have concern over where land ice is melting? Science Advances. Vol. 3, November 15, 2017. doi:10.1126/sciadv.1700537.

P. Swapna et al. Multidecadal weakening of Indian summer monsoon circulation induces an increasing northern Indian Ocean sea level. Geophysical Research Letters. Vol. 44, October 28, 2017, p. 10,560. doi: 10.1002/2017GL074706.

R.E. Kopp et al. Temperature-driven global sea-level variability in the Common Era. Proceedings of the National Academy of Sciences. Vol. 113, March 15, 2016, p. 1434. doi:10.1073/pnas.1517056113.

B.D. DeJong et al. Pleistocene relative sea levels in the Chesapeake Bay region and their implications for the next century. GSA Today. Vol. 25, August 2015, p. 4. doi:10.1130/GSATG223A.1.

S. Saramul and T. Ezer. Spatial variations of sea level along the coast of Thailand: Impacts of extreme land subsidence, earthquakes and the seasonal monsoon. Global and Planetary Change. Vol. 122, November 2014, p. 70. doi:10.1016/j.gloplacha.2014.08.012.

Further Reading

L. Hamers. Antarctica has lost about 3 trillion metric tons of ice since 1992. Science News. Vol. 194, July 7, 2018, p. 6.

T. Sumner. India’s monsoon winds trace back nearly 13 million years. Science News. Vol. 190, September 3, 2016, p. 12.

T. Sumner. Sea levels could rise twice as fast as previously predicted. Science News. Vol. 189, April 30, 2016, p. 13.

T. Sumner. 20th century sea level rose at fastest rate since founding of Rome. Science News. Vol. 189, April 2, 2016, p. 20.

B. Bower. Handed-down tales tell of ancient sea level rise. Science News Online, September 22, 2015.

S. Perkins. Earthshaking event. Science News. Vol. 168, August 27, 2005, p. 136.

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