Artificially lit surface of Earth at night increasing in radiance and extent

Abstract

A central aim of the "lighting revolution" (the transition to solid-state lighting technology) is decreased energy consumption. This could be undermined by a rebound effect of increased use in response to lowered cost of light. We use the first-ever calibrated satellite radiometer designed for night lights to show that from 2012 to 2016, Earth's artificially lit outdoor area grew by 2.2% per year, with a total radiance growth of 1.8% per year. Continuously lit areas brightened at a rate of 2.2% per year. Large differences in national growth rates were observed, with lighting remaining stable or decreasing in only a few countries. These data are not consistent with global scale energy reductions but rather indicate increased light pollution, with corresponding negative consequences for flora, fauna, and human well-being.

Fig. 1. Geographic patterns in changes in artificial lighting.

Changes are shown as an annual rate for both lit area (A) and radiance of stably lit areas (B). Annual rates are calculated based on changes over the four year period, that is, $\sqrt[4]{{\mathit{A}}_{2016}/{\mathit{A}}_{2012}}$, where A₂₀₁₆ is the lit area observed in 2016. See fig. S28 for total radiance change instead of stable light radiance change.

Fig. 2. Absolute change in lit area from 2012 to 2016.

Pixels increasing in area are shown as red, pixels decreasing in area are shown as blue, and pixels with no change in area are shown as yellow. Each pixel has a near-equal area of 6000 ± 35 km². To ease interpretation, the color scale cuts off at 200 km², but some pixels had changes of up to ±2000 km².

Fig. 3. Patterns in lit area, radiance, and lighting change for the world and five selected countries.

(A) The 2014 lit area (in km²) for each (logarithmically spaced) bin of radiance in nWcm⁻² sr⁻¹. (B) Normalized cumulative distribution of light in 2016 (that is, what fraction of the total light is emitted below the given radiance). (C) Mean change in radiance from 2012 to 2016 for each bin.

Fig. 4. Relationships between light and economic parameters.

(A) National sum of lights (SOL) per capita compared to per capita GDP, (B) sum of lights versus national GDP, (C) change in lit area from 2012 to 2016 versus change in GDP (one outlier not shown), and (D) change in sum of lights from 2012 to 2016 versus change in GDP. Colors and symbols indicate per capita GDP in 2016: <$2000 (red triangles), $2000 to $6000 (green squares), $6000 to $17,000 (blue stars), and >$17,000 (black circles). Solid lines show an extrapolation based on the value of the median country.

Fig. 5. Change in lighting technology in Milan, Italy, observed from space.

Color astronaut photographs from 2012 (A) and 2015 (B) courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center, with identification and georeferencing by the European Space Agency, the International Astronomical Union, and Cities at Night (48). (C) Change from 2012 to 2016 in the DNB radiance band.

Fig. 6. Expansion of DNB lighting from September 2012 (cyan) to September 2016 (red) in Doha, Qatar.

Newly lit areas are expressed as bright red, as they were not lit (black) in 2012.

Fig. 7. Comparison of radiance changes from 2012–2016 in dimly lit areas of Germany and Peru.

Histograms of radiance changes of pixels in the 5 to 6.1 nWcm⁻² sr⁻¹bin for Germany (A) and rapidly brightening Peru (B). Pixels were assigned to this bin on the basis of their radiance in 2014. The vertical line shows the value of 1 (no change from 2012 to 2016).