From space, Earth's night is adorned with a veil of luminous points that is gradually gaining intensity. This increase in overall brightness, however, masks opposing trends, with some areas seeing their nights become darker instead.
An international team studied satellite images taken between 2014 and 2022. Their results indicate that artificial nighttime lighting has increased by 16% worldwide over this period. This general rise is mainly driven by developing countries, where access to electricity is becoming widespread. At the same time, areas where brightness is decreasing are also expanding each year, painting a contrasted picture of the evolution of our nights.
The darkening zones generally stem from sudden events such as conflicts or natural disasters. In Ukraine, for example, a clear decrease was observed after the start of the Russian invasion in 2022. Other territories, like France, are experiencing a reduction in their lighting through voluntary energy-saving policies, with a decrease of 33%.
To obtain these measurements, scientists used data from NASA's
Black Marble tool. This instrument relies on algorithms processing information from the VIIRS sensor, carried on polar-orbiting satellites. These algorithms filter out disturbances such as moonlight reflection, auroral glow, or cloud shadows, to precisely isolate the signature of artificial terrestrial lighting.
However, this method has its limits. The sensors used are not very sensitive to the blue light typical of LEDs, which nevertheless represent a growing share of modern public lighting. Thus, when a city installs white LEDs to replace older models, and therefore with a blue component, an observer on the ground perceives a gain in brightness, while the satellite may record an apparent decrease. This divergence shows the value of cross-referencing sources for a complete understanding of the subject.
Maps showing the evolution of artificial light at night (ALAN) worldwide.
Two types of changes are distinguished: abrupt changes and gradual changes, with the date of the last change indicated by colors.
Detailed examples illustrate the causes of these variations: urbanization, conflicts, agriculture, public policies, gas flaring, power outages, or economic crises.
The global maps are simplified by area, while the zooms offer a finer level of detail.
The increase in nighttime brightness has multiple repercussions. It affects human health by disrupting sleep cycles and alters the biological rhythms of many species. For astronomers, this luminous veil complicates the observation of the starry sky, forcing them to seek ever more remote sites.
In contrast, the progression of artificial light can also signal economic and social advances. In regions of Africa or Asia where access to electricity was limited, the appearance of new points of light often corresponds to an improvement in living conditions.
The work, published in the journal
Nature, therefore offers a vision of the evolution of our nights. By combining satellite observations and regional analyses, it allows for almost real-time tracking of the impact of major events, from pandemics to wars, and even voluntary energy savings.