Projections Of Future Sea Level Rise

The Intergovernmental Panel on Climate Change (IPCC) estimated that sea level would rise by an additional 0.6 to 1.9 feet (0.18 to 0.59 meters) by 2100 (Meehl et al., 2007a). However, this projection was based only on current rates of change and was accompanied by a major caveat regarding the potential for substantial increases in the rate of sea level rise. The 2007 IPCC projections are conservative and may underestimate future sea level rise because they do not include one of the two major processes contributing to sea level rise discussed in this chapter: significant changes in ice sheet dynamics (Rahmstorf, 2010). While the growth of ice sheets—mainly through snow accumulation—is an inherently slow process, the processes that govern ice sheet losses, in particular discharge rates, can be strongly nonlinear, with the potential for sudden changes (Overpeck et al., 2006), as illustrated in Figure 7.5. Thus, there is a real potential for ice sheets to shrink rapidly, causing a rapid rise in sea levels. Unfortunately, we do not yet have a good understanding of the processes that control the flow rates; consequently, the potential for rapid ice sheet losses is not well understood at this time. This uncertainty prevented the IPCC from providing a quantitative estimate of how much ice sheet losses might contribute to sea level rise in the coming century.

Research on current and potential future rates of sea level rise has advanced considerably since the IPCC Fourth Assessment Report, which was based on data published in 2005 or earlier. Some research conducted during the past several years suggests that sea level rise during the 21st century could be several times the IPCC estimates, as shown in Figure 7.6. Empirical techniques (e.g., Grinsted et al., 2009; Rahmstorf, 2007; Vermeer and Rahmstorf, 2009) that relate sea level to historical average temperatures

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FIGURE 7.6 Projection of sea level rise from 1990 to 2100, based on IPCC temperature projections for three different GHG emissions scenarios (pastel areas, labeled on right). The gray area represents additional uncertainty in the projections due to uncertainty in the fit between temperature rise and sea level rise. All of these projections are considerably larger than the sea level rise estimates for 2100 provided in IPCC AR4 (pastel vertical bars), which did not account for potential changes in ice sheet dynamics and are considered conservative. Also shown are the observations of annual global sea level rise over the past half century (red line), relative to 1990. SOURCE: Vermeer and Rahmstorf (2009).

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FIGURE 7.6 Projection of sea level rise from 1990 to 2100, based on IPCC temperature projections for three different GHG emissions scenarios (pastel areas, labeled on right). The gray area represents additional uncertainty in the projections due to uncertainty in the fit between temperature rise and sea level rise. All of these projections are considerably larger than the sea level rise estimates for 2100 provided in IPCC AR4 (pastel vertical bars), which did not account for potential changes in ice sheet dynamics and are considered conservative. Also shown are the observations of annual global sea level rise over the past half century (red line), relative to 1990. SOURCE: Vermeer and Rahmstorf (2009).

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; Poumadere et al., 2008; see further discussion below).

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