Multiple, overlapping factors can mean big differences in flood risk
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:
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.
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.
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.
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.
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.