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

Satellite data provides insight on Ireland’s light emissions in the period 2015-2019

This Frontier Series publication from the Central Statistics Office (CSO) uses satellite data to provide estimates of Irish artificial light emissions at national, county and local levels. Comparative statistics for locations in the UK, France, Germany, the Netherlands and Portugal are also provided. This is the first report from the CSO to be based on satellite data and it seeks to show how Big Data such as satellite imagery could be used to produce high-quality relevant statistics.

The CSO Remote Sensing and Satellite Imagery Group was established partly with the objective of identifying possible official statistics that could be produced using satellite imagery, with an emphasis on open source and low cost data sources. Measuring artificial light was quickly identified as an area of investigation.

Artificial Light at Night (ALAN) is a matter of increasing public interest. In recent years, concerns about excessive levels of artificial lighting have driven research and legislation in countries including France and the UK. In 2009, the UK Government’s Royal Commission on Environmental Pollution published “Artificial Light in the Environment” which raised concerns about the impact of artificial light on animal behaviour, human sleep patterns, energy waste and carbon emissions associated with the misuse of artificial light. The United States NOAA published research in Nature in 2010 linking excessive use of artificial light in cities with increased levels of air pollution. There is also increasing scientific literature about the adverse impact of artificial light on human health.

The data used for estimating artificial light emissions for this report is from the US National Oceanic and Atmospheric Administration's (NOAA) Visible Infrared Imaging Radiometer Suite Day-Night Band (VIIRS-DNB) system, which measures atmospheric light. The VIIRS-DNB monthly composite data was provided from the NOAA website and from the Colorado University of Mines, a research partner of the NOAA in analysing the VIIRS-DNB dataset. The data is free to use, once acknowledgement is made to NOAA in any publication. This dataset is produced by the Earth Observation Group (EOG) at the NOAA.

The CSO applied GIS (Graphical Information Systems) techniques with Ordnance Survey (OSI) mapping information to extract light emissions data for Irish (and international) locations. Statistics were produced for January 2015 and January 2019.

The results show the following across the time period:

• Ireland has lower artificial light emissions than other European countries such as the UK, Portugal and the Netherlands
• Irish cities have lower light emissions than international counterparts such as London and Paris
• Dublin county has much higher light emissions than any other county
• The Pembroke South Dock and the North Inner City have high light emissions
• South and West Kerry and West Mayo have very low light emissions
• There is an apparent decrease in light emissions when comparing January 2015 and January 2019. However, care must be taken  in making comparisons and it cannot be necessarily said that light emissions have decreased. Firstly, there are differences in the number of cloud-free observations in each month. Therefore, the months may not be directly comparable. Another factor may be the increased use of LED lighting which the VIIRS-DNB system measures as less intense than traditional lighting.

Measurement units

This report uses the NOAA average radiance (brightness) values estimates for artificial light from cleaned datasets (processed for cloud cover) combined with CSO geospatial boundaries. All units used for monthly average light levels (radiance) in this report are nanoWatt per square centimetre per square radian (nW/cm2/sr). For brevity, these are simply referred to as units.