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).

Multi-year empirical (smoothed) distributions for daily highest water levels in Norfolk, Va. (Sweet and Park 2014) for the 1960s and 2010s, showing extent that local RSL rise has increased the flood probability relative to impact thresholds defined locally by the National Weather Service (http://water.weather.gov/ahps) for minor (~0.5 m: nuisance level), moderate (~0.9 m) and major (~1.2 m: local level of Hurricane Sandy in 2012) impacts, relative to mean higher high water (MHHW) tidal datum of the National Tidal Datum Epoch (1983–2001). b) Due to RSL rise, annual flood frequencies (based upon 5-year averages) in Norfolk for recurrent nuisance tidal floods with minor impacts are accelerating, as shown by the quadratic trend fit (goodness of fit [R2]=0.84). Flood rates are rapidly increasing in similar fashions along dozens of coastal cities of the U.S. (e.g., Sweet et al., 2014; Sweet and Park, 2014; Sweet and Marra, 2016).