Sea-level Rise

Sea-Level Rise and Coastal Communities

Sea-Level Rise and Coastal Communities

Citation: Walker, J.S., Kopp, R.E., Little, C.M., and Horton, B.P., 2022, Timing of emergence of modern rates of sea-level rise by 1863: Nature Communications, 13:966. doi: 10.1038/s41467-022-28564-6

With most of the world’s population living in coastal zones, more accurate predictions of future sea-level changes can help avoid unnecessary property damage and protect the populations residing in these more dynamic areas. One way to improve these predictions is by gaining a better understanding of how quickly sea-level is changing, how this rate has changed over time, and the factors that could be responsible for these changes.

What Causes Sea-Level Changes?

Two main components influence sea-level in a given area: global sea-level changes and regional or local sea-level changes. As the names would imply, global sea-level changes affect sea-level around the world, while regional or local sea-level changes are area- or site-specific. As global temperatures rise, ocean water warms and expands (known as thermal expansion), and glaciers and ice-sheets that were previously located on land melt and run off into the oceans. These two factors cause global sea-level rise. 

On the other hand, differences in more regional ocean circulation patterns, such as changes in the Gulf Stream along the U.S. Atlantic coast, and vertical land movements, such as those from tectonic activity or sagging of the land (known as land subsidence) from increasing groundwater withdrawal, cause sea-level changes only in the specific areas affected by these factors. Together, global and regional events can result in variations in sea-level, and consequently rates of sea-level change, around the world.

How Quickly is Sea-Level Rising?

In order to determine how quickly sea-level is rising globally, Walker et al. compared records of past and present sea-levels from several different areas around the world. They considered the rates of sea-level change at each site across two time periods: before the Industrial Revolution (from 0 to 1700 CE) and after the Industrial Revolution (from 1700 to 2000 CE), when global temperatures (and sea-levels) began to rise. The researchers were able to precisely determine when modern sea-level rise began by comparing the rate of sea-level change before and after these two time periods, and found post-Industrial Revolution rates to be much higher than previous rates.

From these comparisons, Walker et al. found that the modern rate of sea-level rise emerged by 1863 CE when the global rate of sea-level rise reached 0.4 ± 0.2 millimeters per year (mm/yr). This rate exceeded the pre-Industrial Revolution (0 to 1700 CE) global rates of between -0.3 ± 0.2 mm/yr and 0.2 ± 0.3 mm/yr. They also found that average global rates of sea-level rise between 1940 to 2000 CE increased to 1.4 ± 0.2 mm/yr.



Map showing locations of study sites and graph showing rates of sea-level rise pre- and post-Industrial Revolution
Locations of the sites used in the study by Walker et al. along the Atlantic coast of North America and in Europe showing the spatial variability in the time of emergence of modern rates of sea-level rise. The graph shows the rates of sea-level rise at each site before and after the Industrial Revolution. Average rates of sea-level rise pre-Industrial Revolution (0-1700 CE) are shown as open circles, average rates of sea-level rise post-Industrial Revolution (1700-2000 CE) are represented by filled in black circles, and average rates of sea-level rise from 1940 to 2000 are shown by colored lines. Image Source: Walker et al., 2022.

Do Modern Rates of Sea-Level Rise Differ Regionally?

In order to see how these rates of sea-level rise varied regionally, Walker et al. then removed the global signals of sea-level change from sea-level records at sites in the North Atlantic. This removal made it possible to observe only the sea-level changes resulting from regional or local factors. For the period from 1700 to 2000 CE, rates of regional sea-level rise were positive in the mid-Atlantic, muted or absent in the northeastern and southeastern U.S., and potentially negative in Canada and Europe. As a result, modern rates of sea-level rise were found to appear earlier at sites in the mid-Atlantic than at sites in the northeastern and southeastern U.S, while they appeared last at sites in Canada and Europe.

So, What Does This All Mean?

Although local or regional sea-levels may vary, sea-level is rising globally, which poses a serious issue for coastal communities around the world. When compared to mid-nineteenth century rates, the rate of global sea-level rise has more than tripled in recent decades, which will cause coastal areas to be flooded even more rapidly than they were in the past. While these floods may initially only occur during rainstorms or more severe events like hurricanes, as sea-levels continue to rise the amount of inhabitable land might shrink and coastal communities will be displaced. It is thus important to understand how sea-level is changing and has changed in the past in order to better prepare for these events in the future.


Featured image from B137 on Wikimedia Commons.

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