A Church—Understanding Sea-Level Rise and Variability 7.1 Introduction Global mean sea level rose by approximately 1.8 mm/year over the last 50 years, increasing to approximately 3.1 mm/year during the 1990s (Church et al. 2004; Holgate and Woodworth 2004; Cazenave and Nerem 2004). Thermal expansion of ocean water accounts for approximately 0.4 mm/year of sea-level rise for the past four to ive decades, rising to approximately 1.5 mm/year during the last decade (Levitus et al. 2005; Ishii et al. 2006; Willis et al. 2004; Lombard et al. 2006). Contributions from water on land are probably small, with sequestration by dams more or less balanced by release of groundwater, but uncertainties are large (Chapter 8). The most important source of the remainder is likely land ice which, if it were all to melt, would cause sea-level rise of more than 60 m: with contributions of 88 and 11% from the big ice sheets in Antarctica and Greenland respectively, and 1% from glaciers and ice caps (Table 7.1) 2 . Many glaciers and ice caps are currently retreating and have contributed a sea-level rise of 0.3– 0.45 mm/year over the last 50 years, rising to 0.8 mm/year over the last decade (Dyurgerov and Meier 2005) 3 . This leaves an unexplained contribution of approx- imately 1 mm/year, which probably comes from the big ice sheets. Cryospheric Contributions to Sea-Level Rise and Variability Konrad Steffen, Robert H. Thomas, Eric Rignot, J. Graham Cogley, Mark B. Dyurgerov 1 , Sarah C.B. Raper, Philippe Huybrechts, and Edward Hanna 7 1 Deceased. 2 Clariication of terminology: two general categories of ice are considered as contributors to global sea-level rise. (1) The ice sheets of Antarctica and Greenland, both of which drain into the surround- ing ocean via a number of ice streams or outlet glaciers. Ice sheets are suficiently thick to cover most of the bedrock topography. (2) Glaciers and ice caps, mostly outside Antarctica and Greenland. A glacier is a mass of ice on the land lowing downhill under gravity and an ice cap is a mass of ice that typically covers a highland area. 3 Note that estimates of sea-level rise associated with transfer of ice to the ocean are calculated by divid- ing the equivalent freshwater volume by ocean area. Elastic depression of the ocean bed would reduce sea-level rise estimates given in this chapter by about 6% (G. Milne, personal communication). Understanding Sea-Level Rise and Variability, 1st edition. Edited by John A. Church, Philip L. Woodworth, Thorkild Aarup & W. Stanley Wilson. © 2010 Blackwell Publishing Ltd. c07.indd 177 6/2/2010 5:40:46 PM