Altitude affects geophysical and biological hazards for humans. Surprisingly, the global systematics of population and topography are not well known. We describe here a quantitative global analysis of human population and elevation. As of 1994, the median person lives at an altitude of 194 m above sea level. The median square kilometer of populated land occurs at 408 m elevation. The median person lives at a population density of 262 persons/km2, although the median square kilometer of populated land is occupied at a population density of 4.3 persons/km2. Altogether, 99.5% of the world's ice-free land is occupied at a density of at least 1 person per 147 km2. An estimated 1.96 x109 people, or 34% of the world's population, live within 100 vertical meters of sea level. As elevation declines from 800 m toward sea level, the number of inhabitants increases faster than exponentially while the occupied land area increases approximately linearly. The net effect is a gradual increase in average population density with decreasing elevation, with an abrupt increase in average density in the lowest 100 m. Occupied land area between sea level and 100 m contains a disproportionate percentage of the world's population compared to other elevations. Local population densities are also far higher at these low elevations than elsewhere on Earth.
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