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Global sea level has risen 6.7 inches (170 mm) over the past 100 years — and the rate of rise has doubled since the 20th century. Part one of a three-part series designed to help you understand your flood risk.
by Jeff Masters July 12, 2023 August 18, 2024
The modern era of human-caused climate change — the Anthropocene — has also been called the Pyrocene because we’ve entered an age of fire, characterized by large wildfires of increasing size, intensity, and duration. But we propose another term for the modern era: the Aquacene, a time of rapidly increasing flooding from sea level rise and heavier precipitation events, because of human-caused climate change, which will drive millions of people from their homes and fundamentally alter society.
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In part one of this multipart series on understanding your flood risk in the changing climate, we will look at what sea level rise is doing today — with a hint of the dangers ahead. Part two will review eight great books that look at the increasing flood risks in some of the most endangered U.S. places: Miami, New York City, New Jersey, Charleston, Norfolk, Houston, New Orleans, Houston, and the Florida Keys. Part three will provide tools to evaluate future flood risk.
Sea levels are rising at different rates in different places. Globally averaged sea level has risen about 6.7 inches (170 mm) over the past 100 years (1920-2020); over the same period, the contiguous U.S. sea level has risen about 11 inches (280 mm).
Tide gauges provide the longest-running data set for measuring sea level rise, with many locations offering over a century of data. NOAA’s excellent Tides&Currents website has current tide gauge water levels for U.S. locations and a Sea Level Trends page with plots of historic global sea level rise at tide gauges. The figures they give are for relative sea level rise — the combined effect of increased sea level rise from melting glaciers and thermal expansion of water from climate warming, offset by subsidence of the land from groundwater pumping, geological processes, oil and gas operations, and other factors.
Sun et al. 2023, Causes of accelerated High-Tide Flooding in the U.S. since 1950, came up with a detailed description the contribution of subsidence to the increase in high tide flooding observed in the U.S. since 1950. For the Mid-Atlantic region from Cape Hatteras, NC, to Woods Hole, MA, roughly half of the increase in high tide flooding during 2011-2020 was from land subsidence (Fig. 6c in their paper). For the U.S. Gulf Coast and Southeast Coast as far north as Cape Hatteras, only about 17% of the increase in high tide flooding during 2011-2020 was from land subsidence; this fraction was less than 5% along the U.S. West Coast.
Changes in ocean currents and wind patterns also have a strong impact on sea level. For example, the sea level in the tropical Pacific over the past several decades has been rising more quickly toward the west versus the east, as evident in Fig. 1 above. This may be related to a La Niña-like pattern that has emerged over the same period (see our post from April 2023). Moreover, individual El Niño and La Niña events can have a striking short-term influence on sea level. For example, during the past three strong El Niño events, the associated winds and ocean currents raised sea levels along the California coast by four to eight inches for months at a time (see Tweet and Fig. 2 below).
On Jan. 26-29, 1983, during one of the largest El Niños in half a century, six tide gauges along the west coast (San Francisco, Monterey, and Crescent City, California; Charleston and Astoria, Oregon; and Seattle, Washington) reported their highest water levels on record. Peak sea level was 9.4 inches (24 cm) above the level predicted from astronomical factors alone in San Diego (104 years of record), 12.2 inches (31 cm) above predicted in Los Angeles (87 years of record), and 29.9 inches (76 cm) above predicted in Seattle (112 years of record).
The rate of sea level rise has accelerated and is more than double the rate of rise during the 20 th century. Short-term rates of sea level rise are subject to high uncertainty because of changes to wind patterns and ocean currents caused by climate change and natural variability.
A detailed way to look at sea level rise in the U.S. over the past 100 years is from a May 2023 research paper. The study found that sea level rise in the 1970s and 1980s along the U.S. Atlantic coast was only about 1-2 mm/year and was even negative in some places. However, sea level rise has accelerated drastically along the U.S. Southeast and Gulf coasts since 2010 and is now more than 10 mm/year (3.9 inches/decade) — about triple the global rate.
The researchers theorized that the acceleration results from changes in the ocean’s density and circulation and wind patterns, which have caused a clockwise-spinning ocean current system called the North Atlantic subtropical gyre to expand. They concluded that about 40% of the recent acceleration was from climate change and about 60% was from natural variability, so the rate of sea level rise will likely return to the more moderate values predicted by climate models in the coming years.
A June 2023 paper used a modeling study to show that the acceleration of sea level rise along the Southeast and Gulf Coasts to over 10 mm/year from 2010-2022 was a delayed response to a slowdown from 2009-2010 of the Atlantic Ocean current system called the Atlantic Meridional Overturning Circulation, or AMOC. The authors wrote, “If the 2009–10 AMOC slowdown event turns out to be a part of a long-term weakening trend, as projected by climate models under greenhouse gas forcing, sea levels along the East and Gulf Coasts may stay high in the next years. Nonetheless, it is likely that the rapid sea level rise along the Southeast and Gulf Coasts, at a rate of more than 10 mm [per year] during 2010–22, will taper off in the next decade.” However, they theorized that the Northeast U.S. and east coast of Canada would see extra sea level rise of about 1.7 mm/year, as ocean currents adjust to a continued slowdown of the Atlantic Meridional Overturning Circulation because of continued emissions of heat-trapping greenhouse gases. A 2023 paper by Zhang et al., Skillful multiyear to decadal predictions of sea level in the North Atlantic Ocean and U.S. East Coast, predicted that sea level along the Northeast U.S. coast would rise at about 9 mm/year through 2033: about 3.5 inches per decade. An August 2024 study gave a 59% chance of AMOC collapse before 2050.
To put this recent acceleration of sea level rise in perspective, one common rule of thumb is that every 100 mm of sea-level rise triples the frequency of coastal flooding. Another rule of thumb, known as the Bruun Rule, says that a 10 mm rise in sea level leads to a one meter retreat of the coastline. Thus, we can expect that the past decade of 10 mm/year sea level rise along the U.S. Southeast and Gulf coasts has led to a tripling of their coastal flood risk, and has caused about a 10 meter (33-foot) retreat of the coast in places where it is not being defended.
A May 2023 paper in PNAS found that glaciers that end in the ocean are subject to much greater melting than previously assumed. This could be a major Scooby-Doo “ruh-roh” finding for people living on the coast – in an interview with axios.com, University of California professor Eric Rignot, co-author of the study, said, “We have reasons to believe that the current [sea level rise] projections are too low, not too high. They could be as much two times too low.”
That’s all the more reason to take your flood risk seriously.
Bob Henson contributed to this post.
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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a. More by Jeff Masters
