to dominate coastal ice losses. Numerical modeling (Nick et al., 2009) further supports this conclusion and suggests that tidewater outlet glaciers adjust rapidly to changing boundary conditions at the calving terminus. Expanded monitoring of both air and sea temperatures at high latitudes and an improved understanding of ice sheet dynamics will be needed to improve scientific knowledge of these processes.

The world’s glaciers and ice caps contain the water equivalent of up to 2.4 feet (0.72 meters) of sea level (Dyurgerov and Meier, 2005). They have consistently been contributing about one quarter of the total sea level rise over the past 50 years, staying roughly proportional to the overall rate of sea level rise (Bindoff et al., 2007). Mountain glaciers are expected to continue to be a significant contributor to sea level rise during this century, and their retreat poses significant risks to populations that depend on glacial runoff as a water source (see Chapter 8). However, unlike the Greenland and Antarctic ice sheets, mountain glaciers are relatively small and do not carry the potential for large and sudden contributions to sea level rise.

There are additional contributions to sea level rise from other human activities such as wetland loss, deforestation, and the extraction of groundwater for irrigation and industrial use. While estimates of the size of these sources are somewhat uncertain, they are believed to be small relative to land ice melting and may be partially offset by the increased storage of water behind dams and in other surface reservoirs over the past century and a half (e.g., Chao et al., 2008). Moreover, the observed recent sea level rise rate of over 0.12 inches (3.3 ± 0.4 millimeters) per year (Cazenave et al., 2010) is consistent with what would be expected from the combination of thermal expansion of the oceans and melting of ice on land (Bindoff et al., 2007). Hence, the overall contribution of other land-based sources to global sea level rise is thought to be small. Nonetheless, small glaciers and ice caps remain important contributors to sea level rise, and their respective contributions need to be better understood.

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