ADAPTING TO SEA-LEVEL RISE - RELATIVE SEA-LEVEL TRENDS TO 2100 FOR THE UNITED-STATES

Global sea levels have slowly risen during this century, and that rise is expected to accelerate in the coming century due to anthropogenic global warming. A total rise of up to 1 m is possible by the year 2100 (relative to 1990). To deal with this change, coastal managers requir e site-specific information on relative (i.e., local) changes in sea l evel to determine what might be threatened. Therefore as a first step, global sea-level rise scenarios need to be transformed into relative sea-level change scenarios which take account of local and regional fa ctors, such as vertical land movements, in addition to global changes. Even present rates of relative sealevel rise have important long-term implications for coastal management-projecting existing trends predic ts a relative sea-level rise from 1990 to 2100 of up to 0.4 m and 1.15 m for the Mid-Atlantic Region and Louisiana, respectively. Ignoring s ea-level rise will lead to unwise decisions and increasing hazard with time. This article adapts the Intergovernmental Panel on Climate Chan ge (IPCC) global scenarios for sea-level rise (Warrick et at, 1996) to three relative sea-level rise scenarios for the contiguous United Sta tes. These scenarios cover the period 1990 to 2100 and provide a basis to assess possible proactive measures for sea-level rise. However, th ey are subject to the same uncertainties as the global scenarios as mo st of the sea-level rise will occur decades into the future. When cons idering what should be done now in response to future sea-level rise, given these large uncertainties, if is best to identify (I) low-cost, no regret responses which would maintain or enhance the choices availa ble to tomorrow's coastal managers; and (2) sectors where reactive ada ptation would have particularly high costs and where allowance for fut ure sealevel rise can be considered a worthwhile ''insurance policy. ' ' Sea-level rise will impact an evolving coastal landscape which alrea dy is experiencing a range of other pressures. Therefore, to be most e ffective, responses to sea-level rise need to be integrated with all o ther planning occurring in the coastal zone.