The coastal zone changed considerably during the twentieth century due to growing populations and increasing urbanization. A recent study indicated that in 1990, 23% of the world's population (1.2 billion people) were living within both a 100-kilometer distance and a 100-meter elevation of the coast at densities 3 times higher than the global average. Society is becoming increasingly vulnerable to sea level extremes, as Hurricane Katrina demonstrated. Rising levels will result in more flooding, even if storm intensities do not increase. Improved understanding of the reasons for sea level rise and variability is required to reduce the uncertainties in sea level rise projections, and this improved understanding could contribute to more effective coastal planning and management.