“How dark is your sky?” Estimating artificial light in Ireland from satellite imagery, 2015-2019

VIIRS-DNB satellite data was selected for Western Europe and processed to produce international comparisons in artificial light levels between Ireland and other EU countries. In both January 2015 and January 2019, Irish artificial light levels were significantly lower than the other EU countries included in this analysis.

In January 2015, Irish average emissions were half the next lowest country, the UK (1.25 vs 2.5 units) and almost seven times less than the Netherlands average emissions (8.47). While decreases between 2015 and 2019 are seen in most countries (with the exception of Portugal), in both years Irish national artificial light levels are lower.

When comparing January 2015 to 2019, there have been falls in light levels for Ireland (1.25 to 0.92), the Netherlands (8.47 to 7.43) and in the UK (2.50 to 1.78) while Portugal’s light levels have increased slightly between January 2015 and January 2019 (2.33 to 2.52). As noted earlier, there are some possible reasons for the apparent change in average emissions. 

These results tally with the levels of urbanisation and geographical distributions of the countries. In the UK, there is visibly a much higher level of artificial light than Ireland due to the greater number of cities and large urbanised areas. There is also, however, large rural areas of the UK (the Green Belt) with relatively low urbanisation and light emissions. The high light emissions in the Netherlands is to be expected given its high levels of industrialisation/urbanisation/ population density. 

When comparing cities using CSO/OSI settlement boundaries and NUTS2/NUTS3 boundaries for international cities, Dublin emissions are significantly lower than London and Paris emissions, and have decreased noticeably between January 2015 and January 2019, from 31.66 to 24.29 units. This fall is also seen in levels for Paris (61.49 to 43.52), Inner London-West (61.68 to 53.27) and Inner London-East (55.24 to 46.78). As could be expected, Cork, Limerick, Waterford and Galway all have significantly lower light emission estimates than Dublin City and Suburbs.