suggest that a sea level rise of up to nearly 5 feet (1.4 meters) is possible by 2100. By incorporating this empirical effect into models, Horton et al. (2008) estimates a sea level rise of 2 to 2.6 feet (0.62 to 0.88 meters) by 2100. In other work, Rohling et al. (2008) find that a rise rate of up to 5 feet (1.6 meters) per century is possible, based on paleoclimatic evidence from past interglacial periods (including the most recent interglacial period, 110,000 years ago, when global temperatures were 3.6°F [2°C] higher than today and sea levels were 13 to 20 feet [4 to 6 meters] higher). Kopp et al. (2009) estimate that sea level peaked at 22 to 31 feet (6.6 to 9.4 meters) higher than today during the last interglacial period and had a 1,000-year average rise rate between 1.8 and 3 feet (0.56 to 0.92 meters) per century. Pfeffer et al. (2008) used geophysical constraints of ice loss to suggest that a 2.5-foot (0.8-meter) sea level rise is more likely, with a 6.5-foot (2-meter) rise the maximum to be expected by 2100. Others (Siddall et al., 2009) suggest that a 2.5-foot (0.8-meter) rise is the most we could experience by 2100, based on a model that is fit to data only since the last glacial maximum.
The differences among these estimates highlight the uncertainties involved in sea level rise projections; however, there is widespread consensus that substantial long-term sea level rise will continue for centuries to come (Overpeck and Weiss, 2009). A considerable amount of sea level rise is to be expected simply from past CO2 emissions as the ocean heat content catches up with radiative forcing (see Chapter 6); furthermore, the risk of ice sheet collapse, and the attendant large rates of sea level rise, will increase if GHG concentrations in the atmosphere continue to increase. The task of determining how much sea level rise to expect, when to expect it, and its regional character is a critical scientific challenge given the large numbers of people, assets, and economic activity at risk, and the substantially different planning and management challenges managers would face if they had to prepare for and adapt to a sea level rise of 2, 4, or 8 feet over the course of one century. While the risks cannot be quantified at present, the consequences of extreme and rapid sea level rise could be economically and socially devastating for highly built-up and densely populated coastal areas around the world, especially low-lying deltas and estuaries (Anthoff et al., 2010; Lonsdale et al., 2008; Nicholls et al., 2007; Olsthoorn et al., 2008; Poumadère et al., 2008; see further discussion below).
As noted above, sea level rise will not be uniform across the globe. Regional variations in the rate of sea level rise occur for a number of reasons. Some coasts are still adjusting to the disappearance of glaciers—the weight of glacial ice pushed them down, and they are still rising in response to the loss of ice. In other regions, coasts may be