When sea level rises, it doesn’t come close to doing so evenly around the world, and pair of new studies released last week suggest that the United States could be squeezed in a rising sea level sandwich over the next few decades.
According to a U.S. Geological Survey study, sea level is rising three to four times faster along parts of the East Coast than elsewhere around the globe. A National Research Council study says the same is true for much of the California coast, where sea level will also climb faster than the global average, potentially increasing damage to coastal real estate and natural resources from higher storm surges and storm-driven wave action.
Rates of sea level rise are increasing three to four times faster along portions of the U.S. Atlantic Coast than globally, according to a new U.S. Geological Survey report published in Nature Climate Change.
The USGS study indicates that, if global temperatures continue to go up, the rates of sea level rise on the Atlantic Seaboard will also increase. The sea-level rise along the East Coast isn’t just as simple as melting ice. Instead, it may be linked with changes in water temperature (warm water expands) and a slowing of the Atlantic Ocean circulation, linked with changes in water temperature, salinity and density in the subpolar north Atlantic.
“Many people mistakenly think that the rate of sea level rise is the same everywhere as glaciers and ice caps melt, increasing the volume of ocean water, but other effects can be as large or larger than the so-called ‘eustatic’ rise,” said USGS Director Marcia McNutt. “As demonstrated in this study, regional oceanographic contributions must be taken into account in planning for what happens to coastal property.”
Overall, many climate models forecast that global sea level will rise about two to three feet by the end of the century at the current rate of warming, but it won’t climb at the same rate everywhere. Differences in land movements, strength of ocean currents, water temperatures, and salinity can cause regional and local highs and lows in sea level.
“Cities in the hotspot, like Norfolk, New York, and Boston already experience damaging floods during relatively low intensity storms,” said Dr. Asbury (Abby) Sallenger, USGS oceanographer and project lead. “Ongoing accelerated sea level rise in the hotspot will make coastal cities and surrounding areas increasingly vulnerable to flooding by adding to the height that storm surge and breaking waves reach on the coast.”
To determine accelerations of sea level, USGS scientists analyzed tide gauge data throughout much of North America in a way that removed long-term (linear) trends associated with vertical land movements. This allowed them to focus on recent changes in rates of sea-level rise caused, for example, by changes in ocean circulation.
The report, Hotspot of accelerated sea-level rise on the Atlantic coast of North America, was published in the journal Nature Climate Change.
On the West Coast, sea level could rise by as much as three feet during the next century, according to the National Research Council report. Sea levels off Washington, Oregon, and northern California will likely rise less, about 60 centimeters during the same period of time. However, an earthquake magnitude 8 or larger in this region could cause sea level to rise suddenly by an additional meter or more.
Along the U.S. west coast, sea-level rise depends on the global mean sea-level rise and regional factors, such as ocean and atmospheric circulation patterns, melting of modern and ancient ice sheets, and tectonic plate movements.
California state agencies have been directed to plan for sea-level rise and coastal impacts and asked the Research Council to establish a committee to assess sea-level rise. Oregon, Washington, and several federal agencies joined California to sponsor the study. The report estimates sea-level rise both globally and for those three states for the years 2030, 2050, and 2100.
The latest estimates are considerably higher than the increases projected by the United Nation’s Intergovernmental Panel on Climate Change’s projection made in 2007. The IPCC report predicted an increase of about six to 18 inches, with another six inches on top of that if there is rapid melting of ice sheets.
The biggest increases are forecast for the area south of Cape Mendocino, where sea level could rise anywhere from 1 to 10 inches by 2030, and by up to three feet by 2100.
The projections for the California coast south of Cape Mendocino are slightly higher than global projections because much of the coastline is subsiding. The lower sea levels projected for northern California, Washington, and Oregon coasts are because the land is rising largely due to plate tectonics. In this region, the ocean plate is descending below the continental plate at the Cascadia Subduction Zone, pushing up the coast.
Extreme events could raise sea level much faster. For example, an earthquake magnitude 8 or greater north of Cape Mendocino, which occurs in this area every several hundred to 1,000 years, with the most recent in 1700, could cause parts of the coast to subside immediately and the relative sea level to rise suddenly by a meter or more.
“As the average sea level rises, the number and duration of extreme storm surges and high waves are expected to escalate, and this increases the risk of flooding, coastal erosion, and wetland loss,” said Robert Dalrymple, committee chair and Willard and Lillian Hackerman Professor of Civil Engineering at Johns Hopkins University.
Most of the damage along the west coast is caused by storms, particularly the confluence of large waves, storm surges, and high tides during El Niño events. Significant development along the coast — such as airports, naval air stations, freeways, sports stadiums, and housing developments — has been built only a few feet above the highest tides.
For example, the San Francisco International Airport could flood with as little as 40 centimeters of sea-level rise. The report also included one model suggesting sea-level rise could cause the incidence of extreme water heights in the San Francisco Bay area to increase from about nine hours per decade to hundreds of hours per decade by 2050, and to several thousand hours per decade by 2100.