Explainer: Why sea levels aren't rising at the same rate globally | Science News for Students

Explainer: Why sea levels aren't rising at the same rate globally

A spinning planet, melting ice sheets and warmer waters all contribute to sea level rise
Jan 10, 2019 — 6:35 am EST
an aerial photo of a flooded street in Ho Chi Minh City, scooters and cars are driving through the floodwaters

Traffic keeps flowing on a flooded street in Ho Chi Minh City in Vietnam. Sea levels are rising around the world, but they are rising more quickly in some places than in others.

xuanhuongho/iStockPhoto

The sea is coming for the land. In the 20th century, ocean levels rose by a global average of about 14 centimeters (some 5.5 inches). Most of that came from warming water and melting ice. But the water didn’t rise the same amount everywhere. Some coastal areas saw more sea level rise than others. Here’s why:  

Swelling seawater

As water heats up, its molecules spread out. That means warmer water takes up slightly more space. It’s just a tiny bit per water molecule. But over an ocean, it’s enough to bump up global sea levels.

Local weather systems, such as monsoons, can add to that ocean expansion.

Monsoons are seasonal winds in southern Asia. They blow in from the southwest in summer, usually bringing a lot of rain. Monsoon winds also make the ocean waters circulate. This brings cool water from the bottom up to the surface. That keeps the surface ocean cool. But weaker winds can limit that ocean circulation.

Weaker monsoons in the Indian Ocean, for instance, are making the ocean surface warmer, scientists now find. Surface waters in the Arabian Sea warmed more than usual and expanded. That raised sea levels near the island nation of the Maldives at a slightly faster rate than the global average. Scientists reported these findings in 2017 in Geophysical Research Letters.

Land a-rising

Heavy ice sheets — glaciers — covered much of the Northern Hemisphere about 20,000 years ago. The weight of all that ice compressed the land beneath it in areas such as the northeastern United States. Now that this ice is gone, the land has been slowly rebounding to its former height. So in those areas, because the land is rising, sea levels appear to be rising more slowly.

But regions that once lay at the edges of the ice sheets are sinking. These areas include the Chesapeake Bay on the East Coast of the United States. That’s also part of a postglacial shift. The weight of the ice had squeezed some underlying rock in the mantle — the semisolid rock layer below the Earth’s crust. That caused the surface of the land around the Chesapeake Bay to bulge. It’s a bit like the bulging of a water bed when a person sits on it. Now, with the ice gone, the bulge is going away. That’s speeding up the impacts of sea level rise for the communities that sit atop it.

a map showing trends in sea level rise globally
Lots of factors, local and worldwide, can affect how quickly seas will rise in different places. This 2018 map shows how fast the seas are rising and falling. The arrows indicate that sea levels are rising faster on the East Coast of the United States than on its West Coast.
RJGC, ESRI, HERE, NOAA, FAO, AAFC, NRCAN

Land a-falling

Earthquakes can make land levels rise and fall. In 2004, a magnitude-9.1 earthquake made land in the Gulf of Thailand sink. That has worsened the rate of sea level rise in this area. Adding to the problem are some human activities, such as pumping up groundwater or drilling for fossil fuels. Each process can cause the local land to sink.

The spin of the Earth

Earth spins at about 1,670 kilometers (1,037 miles) per hour. That’s fast enough to make the oceans move. Ocean water swirls clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (This is due to a process known as the Coriolis effect.) As water moves around coastlines, the Coriolis effect can make water bulge in some places, and sink in others. The flow of water from rivers can exaggerate this effect. As their waters flow into the ocean, that water gets pushed to one side by the swirling currents. That makes water levels in that area rise more than on the side behind the current. Scientists reported that finding in the July 24 Proceedings of the National Academy of Sciences.

Glaciers begone

Melting glaciers also can add water to the oceans. But these huge ice slabs affect sea levels in other ways, too.

Huge glaciers can exert a gravitational tug on nearby coastal waters. That pull piles up water near the glaciers, making it higher than it otherwise would be. But when those glaciers melt, they lose mass. Their gravitational pull is now weaker than it had been. So the sea level near the melting glaciers drops.

But all that melted water has to go somewhere. And that can lead to some surprising effects, according to a 2017 report in Science Advances. Melting ice in Antarctica, for instance, could actually make sea levels rise faster near distant New York City than in nearby Sydney, Australia.

Editor’s Note: This story was updated on January 15, 2019, to correct that ocean water swirls clockwise in the Northern Hemisphere and counterclockwise in the south, rather than the other way around.   

Power Words

(more about Power Words)

Antarctica     A continent mostly covered in ice, which sits in the southernmost part of the world.

circulation     (adj. circulatory) A term that refers to the pumping of some fluid repeatedly throughout a system of vessels.

crust     (in geology) Earth’s outermost surface, usually made from dense, solid rock.

current     A fluid — such as of water or air — that moves in a recognizable direction.

earthquake     A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

Earth’s crust     The outermost layer of Earth. It is relatively cold and brittle.

fossil fuel     Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.

glacier     A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

groundwater     Water that is held underground in the soil or in pores and crevices in rock.

ice sheet     A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

magnitude     (in geology) A number used to describe the relative size of an earthquake. It runs from 1 to more than 8 and is calculated by the peak ground motion as recorded by seismographs. There are several magnitude scales. One of the more commonly used ones today is known as the moment magnitude. It’s based on the size of a fault (crack in Earth’s crust), how much the fault slips (moves) during a quake, and the energy force that was required to permit that movement. For each increase in magnitude, an earthquake produces 10 times more ground motion and releases about 32 times more energy. For perspective, a magnitude 8 quake can release energy equivalent to detonating 6 million tons of TNT. (in astronomy) A measure of a star brightness.

mantle     (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

monsoon     A system of winds that influences the climate of a large area. In southern Asia, monsoons blow from the southwest in the summer and usually bring heavy rains.

Proceedings of the National Academy of Sciences     A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sea level     The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

seawater     The salty water found in oceans.

Sumatra     A part of the island nation of Indonesia, this is one of its bigger islands (and indeed, the sixth largest island in the world). 

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

Citation

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

Journal:​ ​​ 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.

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

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

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

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

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