THE RISK OF SEA-LEVEL RISE

The United Nations Framework Convention on Climate Change requires nat ions to implement measures for adapting to rising sea level and other effects of changing climate. To decide upon an appropriate response, c oastal planners and engineers must weigh the cost of these measures ag ainst the likely cost of failing to prepare, which depends on the prob ability of the sea rising a particular amount. This study estimates su ch a probability distribution, using models employed by previous asses sments, as well as the subjective assessments of twenty climate and gl aciology reviewers about the values of particular model coefficients. The reviewer assumptions imply a 50 percent chance that the average gl obal temperature will rise 2 degrees C, as well as a 5 percent chance that temperatures will rise 4.7 degrees C by 2100. The resulting impac t of climate change on sea level has a 50 percent chance of exceeding 34 cm and a 1% chance of exceeding one meter by the year 2100, as well as a 3 percent chance of a 2 meter rise and a 1 percent chance of a 4 meter rise by the year 2200. The models and assumptions employed by t his study suggest that greenhouse gases have contributed 0.5 mm/yr to sea level over the last century. Tidal gauges suggest that sea level i s rising about 1.8 mm/yr worldwide, and 2.5-3.0 mm/yr along most of th e U.S. Coast. It is reasonable to expect that sea level in most locati ons will continue to rise more rapidly than the contribution from clim ate change alone. We provide a set of 'normalized' projections which e xpress the extent to which climate change is likely to accelerate the rate of sea level rise. Those projections suggest that there is a 65 p ercent chance that sea level will rise 1 mm/yr more rapidly in the nex t 30 years than it has been rising in the last century. Assuming that nonclimatic factors do not change, there is a 50 percent chance that g lobal sea level will rise 45 cm, and a 1 percent chance of a 112 cm ri se by the year 2100; the corresponding estimates for New York City are 55 and 122 cm. Climate change impact assessments concerning agricultu re, forests, water resources, and other noncoastal resources should al so employ probability-based projections of regional climate change. Re sults from general circulation models usually provide neither the most likely scenario nor the full range of possible outcomes; probabilisti c projections do convey this information. Moreover, probabilistic proj ections can make use of all the available knowledge, including the vie ws of skeptics; the opinions of those who study ice cores, fossils, an d other empirical evidence; and the insights of climate modelers, whic h may be as useful as the model results themselves.