Sea level changes near coastlines include tides, a kind of wave caused by the gravitational effects of the sun and moon, along with the Earth’s rotation.
Tidal changes across various time scales, though cyclic, can be superimposed upon the background rise in global sea level; add regional rise driven by ocean circulation, and the highest tides can begin to cause flooding in coastal zones that were previously unaffected, or that experienced such flooding at lower frequencies.
An instructive example comes from the Italian city of Venice, which became iconic for its canals and gondolas as sea level rose gradually over the centuries. Its founding settlements in the fifth century were intended to offer protection against barbarian raids. Built on a marshy lagoon and numerous islands, the city has adapted to gradual immersion, tracking sea level closely by the stains left on buildings and, starting in the 20th century, by tide gauges.
A 2009 modeling study [Carbognin et al.], however, found that rising relative sea level caused by modern day climate change might be pushing the maritime city toward an unsustainably high frequency of flood threat, as high tide levels increase. The study showed that, while the city itself is sinking due to natural processes at 0.05 centimeters per year, relative sea level could rise between 17 and 53 centimeters by the end of the century. This could push high tides above 110 centimeters, the point at which lagoon gates are shut to protect the city from flooding, at a far higher frequency—from the present four instances each year to a range of 20 to 250, depending on assumptions in different modeling scenarios. In this case, cyclic “high water events” caused by astronomical high tides, low-pressure systems, winds from the southeast and other factors are driven even higher by global sea-level rise induced by climate change, hovering in the background.
Storm surge is a higher-than-normal rise of coastal waters, above the astronomical high tide. Its main causes are strong winds within a tropical storm or hurricane, both literally pushing seawater and creating huge waves that travel to the coast and break there. Low atmospheric pressure also induces a dome of water near the storm center. Precipitation and the Coriolis effect may also play a role.
While making direct causal connections between globally averaged climate change and individual storms remains very difficult, improved modeling techniques offer a rare chance to forecast the potential imprint of the global climate signal on short-term, localized meteorological events as the next few decades unfold.
One modeling study [Tebaldi et al., 2012] examined long-term data from 55 tide gauges along the coasts of the contiguous United States. The study’s authors also used more detailed data from the gauges, from 1979 to 2008, to derive historic patterns of “extreme high water events.” They combined those data with sea-level-rise projections to estimate potential storm surge effects through the middle of the 21st century.
By 2050, the authors reported, their modeling showed some of the gauge locations would see high-water events yearly that, today, are considered likely to occur once a century (having a one percent chance of occurring in any given year). Once-a-century events, meanwhile, would become decadal (a 10 percent chance of occurring in any given year). Most locations would experience a higher frequency of storm surge events once considered rare, the study says.
Relative sea level also can rise because the land itself is sinking—visible in extreme form in the California Bay Delta. Some of the islands amid this 2,900-square-kilometer network of water channels and farmland have dropped to eight meters or more below sea level [Mount et al., 2005], due mainly to microbial oxidation and soil compaction—the legacy of more than a century of farming. One low-resolution modeling study suggested the additional strain placed on levees as the land surface subsides, coupled with rising sea levels, are sharply increasing the risk of floods resulting from levee failure [Mount et al., 2005]. These factors contributed to the study’s estimate of a two in three chance of catastrophic flooding in the Delta by 2050, triggered by a 100-year flood or earthquake event.
While subsidence occurs naturally, it can be greatly accelerated by human activity. Groundwater and hydrocarbon extraction, for example, can cause compaction of sediments [Waltham, 2002]. Subsidence related to groundwater withdrawal can be especially pronounced in river deltas with large populations and extensive agriculture, among them Thailand’s Chao Phraya, the Bengal Delta of India and Bangladesh, China’s Yangtze and the Egyptian Nile [Ericson et al., 2006].
Groundwater extraction has been identified as the cause of land subsidence in Vietnam’s lower Mekong Delta, averaging 1.6 centimeters per year, as declines in groundwater levels cause compaction of sedimentary layers [Erban et al., 2014]. Subsidence on the order of 0.88 meters could occur by 2050 if present pumping rates continue; combined with expected sea level rise, this will likely bring a significant increase in flood hazard.
A study of “effective” sea level rise—defined as global sea-level rise combined with sediment deposition and subsidence—in 40 deltas across the world found that about 20 percent were experiencing accelerated subsidence because of groundwater or hydrocarbon extraction [Ericson et al., 2006].
Tectonic subsidence, sometimes related to volcanism, has left behind impressive examples of its effects, including the submerged ancient Roman city of Baia off the Italian coast [Lambeck et al. 2010].