Climate models predict increases in the depth of thaw over much of the permafrost region. By 2050 seasonal thaw depths are projected to increase by more than 50% in the permafrost regions to the far north including Siberia and northern Canada, while in the southern extents increases of 30-40% are predicted (Stendel and Christensen, 2002; ACIA, 2005; Sasonova et al., 2004). In eastern Siberia permafrost degradation is projected to begin as early as 2050. The increases in thaw depth are associated with warming at the high northern latitudes (e.g., Lawrence and Slater, 2005; Yamaguchi et al., 2005; Kitabata et al., 2006).
The total area covered by continuous, near-surface permafrost (less than 4 m deep) is also projected to decrease and shrink poleward during the 21st century. This is demonstrated in Figure 1 of Stendel and Christiansen (2002), which shows model results for the A2 scenario. Predicted median values for this change are 18, 29, and 41% by 2030, 2050, and 2080 respectively (ACIA, 2005). The size of the decrease varies by model and by warming scenario. For example, Washington et al. (2009) show a range of permafrost loss with the largest losses occurring in their SRES A2 (high emission scenario) and the least in their low emission (450 ppm CO2) scenario. (See Figure 4.19.)
Estimates of near-surface permafrost degradation rates to warming forced by the A1B GHG emissions scenario is on the order of 81,000 km2 per year (Lawrence et al., 2008a). Rates of permafrost degradation may be influenced by rapid Arctic sea ice loss. One climate simulation of such loss predicted warming rates in the western Arctic of 3.5 times greater than the current 21st century climate change trends. This warming signal penetrated inland up to 1,500 km and, although most apparent in autumn, is there year-round. This warming leads to substantial ground heat storage, which in turn degrades the permafrost (Lawrence et al., 2008b).
The coastal zone has changed profoundly during the 20th century, primarily due to growing populations and increasing urbanization. In 1990, 23 percent of the world’s population (or 1.2 billion people) lived both within a 100 km distance and 100 m elevation of the coast at densities about three times higher than the global average. By 2010, 20 out of 30 mega-cities are on the coast with many low-lying locations threatened by sea level rise. With coastal development continuing at a rapid pace, society is becoming in-