New satellite images of the Earth at night give the most complete view of global human settlement yet obtained.

The Operational Linescan System (OLS) detects visible and near-infrared emissions resulting from human activity. These maps not only reveal a fascinating view of worldwide ‘light pollution’, but they also serve as indirect tracers of international greenhouse-gas production.

The OLS sensors are part of the Defense Meteorological Satellite Program, and have been operational for two decades. But previous images suffered limited areal coverage and were unable to filter out misleading signals, such as temporary light from fires.

Using new techniques to process digitally archived data, C. D. Elvidge and colleagues (Glob. Change Biol. 3, 387-395; 1997) have circumvented these problems. By analysing a series of images acquired at different times during 1994-95, they have built up maps of the permanent light and heat emission from the Americas, Asia and Europe (shown here). Moreover, by selecting observations acquired when there was little moonlight (which can bounce off clouds and produce spurious signals), they can achieve optimal sensitivity to faint, small sources at ground level.

Variations in the density of the emitted light are striking but not surprising — certainly, the major cities cannot hide. However, the maps also facilitate comparisons between the illuminated area of each country, and statistics such as population and carbon emission.

The OLS data suggest that, allowing for certain biasing factors, these statistics are directly correlated with the area of lit regions. So the results may provide a way of gauging different national contributions to greenhouse-gas production, and the wider implications for climatic change.