Global and Regional Sea Level Rise Scenarios for the United States
Long-term sea level rise driven by global climate change presents clear and highly consequential risks to the United States over the coming decades and centuries. Today, millions of people in the United States already live in areas at risk of coastal flooding, with more moving to the coasts every year (Melillo et al., 2014). Rising seas will dramatically increase the vulnerability of this growing population, along with critical infrastructure related to transportation, energy, trade, military readiness, and coastal ecosystems and the supporting services they provide (Parris et al., 2012; Hall et al., 2016). One recent study estimates that 0.9 meters (m) of sea level rise would permanently inundate areas currently home to 2 million Americans; 1.8 meters would inundate areas currently home to 6 million Americans (Hauer et al., 2016).
Global mean sea level (GMSL) has increased by about 21 centimeters (cm) to 24 cm (8–9 inches [in]) since 1880, with about 8 cm (3 in) occurring since 1993 (Church and White, 2011; Hay et al., 2015; Nerem et al., 2010). In addition, the rate of GMSL rise since 1900 has been faster than during any comparable period over at least the last 2800 years (Kopp et al., 2016a). As is discussed in detail in this report, scientists expect that GMSL will continue to rise throughout the 21st century and beyond, because of global warming that has already occurred and warming that is yet to occur due to the still- uncertain level of future emissions. GMSL rise is a certain impact of climate change; the questions are when, and how much, rather than if. There is also a long-term commitment (persistent trend); even if society sharply reduces emissions in the coming decades, sea level will most likely continue to rise for centuries (Golledge et al., 2015; DeConto and Pollard, 2016).
While the long-term, upward shift in sea level is an underlying driver of changes to the nation's coasts, impacts are generally expressed through extreme water levels (short-period, lower-probability events both chronic and acute in nature) occurring against the background of this shifting baseline. Higher sea levels worsen the impacts of storm surge, high tides, and wave action (e.g., Theuerkauf et al., 2014), even absent any changes in storm frequency and intensity. Even the relatively small increases in sea level over the last several decades have led to greater storm impacts at many places along the U.S. coast (Parris et al., 2012; Miller et al., 2013; Sweet et al., 2013). Similarly, the frequency of intermittent flooding associated with unusually high tides has increased rapidly (accelerating in many locations) in response to increases in relative sea level (RSL) as shown in Figure 1. At some locations in the United States, the frequency of tidal flooding (events typically without a local storm present) has increased by an order of magnitude over the past several decades, turning it from a rare event into a recurrent and disruptive problem (Sweet et al., 2014; Sweet and Park, 2014; Sweet et al., 2016). Significant, direct impacts of long-term RSL rise, including loss of life, damage to infrastructure and the built environment, permanent loss of land (Weiss et al., 2011), ecological regime shifts in coastal wetlands and estuary systems (Kirwan et al., 2010), and water quality impairment (Masterson et al., 2014), also occur when key thresholds in the coastal environment are crossed (Wong et al., 2014). Some of these impacts have the potential to 'feedback' and influence wave impacts and coastal flooding. For example, there is evidence that wave action and flooding of beaches and marshes can induce changes in coastal geomorphology, such as sediment build up, that may iteratively modify the future flood risk profile of communities and ecosystems (Lentz et al., 2016).