Coastal Populations, Topography, and Sea Level Rise
A frequently cited consequence of global climate change is the potential impact of sea level rise (SLR) on coastal populations. Eleven of the world's 15 largest cities lie along the coast or on estuaries. In the United States, around 53% of the population lives near the coast. In spite of this widespread concern over sea level rise, no accurate worldwide estimate of the number of people likely to be affected by coastal inundation or flooding has been published.
In a recent study with Christopher Small (Columbia Univ. Lamont-Doherty Earth Observatory) and Joel E. Cohen (Rockefeller Univ.), I attempted to provide an improved assessment of human vulnerability to sea level rise by integrating the best currently available information about global population distributions, elevation, and sea level. Population data are compiled from a 1997 study by Tobler and others; topography comes from the EROS Data Center (Sioux Fall, SD) 30 arc second gridded elevations, and the sea level records from the Permanent Service for Mean Sea Level, Bidston Observatory (Fig. 2). We considered the following scenarios:
- Extrapolation of current trends. No climate change occurs and current rates of sea level rise are extrapolated from 1990 to 2100.
- Goddard Institute for Space Studies (GISS) coupled ocean-atmosphere General Circulation Model (GCM) (Russell et al. 1995) at 4° by 5° horizontal resolution. In the GS run, CO2 increases by 1%/yr and sulfate aerosols increase annually up to 2050, with a slight decrease thereafter.
- The IPCC IS92a "best estimate" includes effects of sulfate aerosols.
The two model scenarios show increases of around 40-45 cm in sea level over the next 100 years, which is about three times that of extrapolating the current trend (Fig. 3A).
In addition to permanent inundation due to sea level rise, the coast is at risk to flooding by storms. Average country-wide surge levels for given return periods are taken from a 1993 Delft Hydraulics study. Total flood levels comprise storm surges, tides, and local sea level change, including vertical land motions. In Figure 3B, mean high water is generally less than 3m above present sea level. Most sea level stations show significantly less than 1m subsidence over the next 100 years. Areas experiencing uplift include Scandinavia and Alaska, due to ongoing glacial rebound, and Japan due to tectonic uplift.
Analysis of the spatial distribution of population with respect to topography reveals that the number of people and population density diminishes rapidly with increasing elevation and increasing distance from the shoreline. Approximately 400 million people live within 20 m of sea level and within 20 km of a coast, worldwide. However, this figure is not very precise -- the spatial distribution of coastal populations is not known to better than 20-30 km. In fact, a major conclusion of our study is that the available data are still inadequate to permit quantitatively precise global estimates of the number of people likely to be affected by plausible levels of sea level rise or storm surges in the coastal zone. In the near future, airborne and satellite-based radar and laser altimeters can map coastal topography and its changes at much higher resolutions than those used in this study. Satellite monitoring of coastal land cover transformations will provide means of quantifying habitation patterns, and thus indirectly, population trends.
Russell, G.L., J.R. Miller, and D. Rind 1995. A coupled atmosphere-ocean model for transient climate change studies. Atmos.-Ocean 33, 683-730.
Small, C., V. Gornitz, and J.E. Cohen 2000. Coastal hazards and the global distribution of population. Environ. Geosci. 7, 3-12.
Please address all inquiries about this research to Dr. Vivien Gornitz.