Sea level is rising: Do we know why?

The gradual rise of sea level is one of the most troubling aspects of global change, especially because it is likely to accelerate in the future as global warming progresses. Understanding the linkage between warming climate and sea-level rise therefore is important and has been the subject of much study (e.g., refs. 1–5). Two processes are involved: an increase of the mass of water in the oceans (the eustatic component), derived largely from the melting of ice on land, and an increase of the volume of the ocean without change in mass (the steric component), largely caused by the thermal expansion of ocean water. Neither of these components is understood fully, and observations are not sufficient yet to develop a precise assessment of the causes of present-day sea-level rise let alone a projection of future rise. In fact many of the analyses produce conflicting results. The article by Munk (6) in this issue of PNAS enlightens and sharpens our understanding of the enigma before us by examining a number of geophysical constraints on the process.

Societal and economic impacts of sea-level rise are evident already, and the consequences of continued rise are substantial (2). Beach erosion and shoreline retreat affect valuable real estate in developed nations and the livelihood of many waterfront communities in developing countries. Shoreline retreat may pinch out coastal wetlands against developed areas, or the wetlands may be harmed irreversibly if the rate of sea-level rise exceeds the rate at which the biota can adapt. Rising sea level can influence the rate of salt-water incursion into coastal aquifers, expansion of the salt-water wedge in estuaries, and the probability of damage from storm surges along coastlines. More than 100 million people live within 1 m of the mean sea level (7), and the problem is especially …