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Future sea level
The rate of global mean sea-level rise (~3 mm/yr; SLR) has accelerated compared to the mean of the 20th century (~2 mm/yr), but the rate of rise is locally variable. Factors contributing to SLR include decreased global ice volume and warming of the ocean. On Greenland, the deficiency between annual ice gained and lost tripled between 1996 and 2007. On Antarctica the deficiency increased by 75%. Mountain glaciers are retreating and the cumulative mean thickness change has accelerated from about −1.8 to −4 m in 1965 to 1970 to about −12 to −14 m in the first decade of the 21st century. From 1961 to 2003, ocean temperatures to a depth of 700 m increased and portions of the deeper ocean are warming.
The Intergovernmental Panel on Climate Change (2007) projected sea level would reach 0.18 to 0.59 m above present by the end of the 21st century but lacked an estimate of ice flow dynamics calving. Calving was added by Pfeffer et al. (2008) indicating 0.8 to 2 m of SLR by 2100 (favouring the low end of this range). Rahmstorf (2007) estimated SLR will reach 0.5 to 1.4 m by the end of the century. Pielke (2008) points out that observed SLR has exceeded the best case projections thus far. These approximations and others indicate that global mean SLR may reach 1 m by the end of this century. However, sea level is highly variable and planners considering local impacts must take this variability into account.
==Introduction==
Global mean air temperature is increasing in relation to global warming.〔Goddard Institute for Space Studies (2009) (Global Temperature Trends: 2008 Annual Summation ); Last viewed 22 May 2009〕 As a result, Earth’s ice volume is decreasing and the heat content of the ocean is increasing. Global mean sea level is therefore rising, and the rate of rise has accelerated. Sea-level rise (SLR) presents challenges to coastal communities and ecosystems, and planners are engaged in assessing management options.〔(California Executive Order S-13-08 ) (2008) Ordering the California Resources Agency to complete the first California Sea Level Rise Assessment Report. 14 November; Last viewed 25 May 2009〕 Accordingly, it is desirable to have an estimate of SLR this century to properly design mitigation and adaptation strategies. An approximation of SLR by the end of the century will allow: 1) estimates of coastal erosion and changes in vulnerability to coastal hazards; 2) assessments of threats to coastal ecosystems; and 3) development of climate risk management policies. This paper reviews recent studies of global warming, sea-level observations, global ice volume, ocean heating, and estimates of SLR by the end of the 21st century.
Based on current scientific understanding, we conclude that a global mean rise of approximately 1 m around the end of the century is indicated by present research and constitutes an appropriate planning target at this time. However, sea-level rise will have important local variability that planners should consider as knowledge of that variability improves. Global mean sea-level may rise significantly more than 1 m, but is unlikely to raise significantly less. Important questions remain regarding the melt rate of ice in West Antarctica and southern Greenland. Also unknown are the actual levels of natural climate variability and greenhouse gas accumulation that will be reached this century. However, even if atmospheric composition were stabilized today, global warming and SLR would continue. Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels.
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