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

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).