2014-02-03

Contrasting trends in light pollution across Europe

Two weeks ago, Jonathan Bennie and colleagues (Environment and Sustainability Institute, University of Exeter, UK) released a paper (free download on nature.com) that caught my attention, because it's results are pretty unexpected. Analyzing satellite data (DMSP) from 1995-2000 and 2005-2010, Bennie et al. found that light pollution has grown significantly across Europe in the last 15 years, but not everywhere: Some large areas and even entire countries experienced a decline in night brightness. This is good news for stargazers like me (and for society as a whole) but raises some questions.

Light pollution increase (red) and decline (blue), from Bennie et al. 2014
The most important one: Why did the measured brightness decline in some places, and increase in others? The reasons for the measured "dimming" certainly vary from country to country, some examples are given by the authors.

Bennie et al. 2014
For example, large areas in eastern Europe suffered a severe decline in industry in the aftermath of the collapse of the Soviet Union (like Ukraine). But lighting trends seem to have been very different since then. Czech Republic and Slovakia provide the most extreme example: Once a single country until 1992, the two faced similar economic challenges since then, and both are members of the European Union since 2004. But while the Czech Republic increased the amount of night lighting within the past 15 years, Slovakia is the country with the most net decline in brightness according to the satellite data.

Belgium, a country in western Europe, is dimming as well - mainly due to the shut-down of its unique motorway-lighting in recent years. The Scandinavian countries also show a net dimming, I assume because of shut-down or improvement of outdoor lighting. Germany shows a mixed trend: more artificial light wasted in the south, less in the north and east.

Figuring out what caused the dimming of individual places is a time consuming task. For their "calibration region" in southeast England, Bennie and colleagues found that 15% of the apparent decreases in brightness (51% of the area) were predominantly attributed to areas associated with mineral extraction industries where production declined or ceased, while 45% of the patches (22% of area) were existing small urban areas in which street lighting infrastructure was modernised during the period of measurement.

Mean pixel brightness (left) and 10-year change (right), excerpt of larger map above. Note the difference between Belgium and France. (from Bennie et al 2014).

The largest net increase of light pollution is measured in countries like Portugal, Spain, France, Italy, Ireland and Greece, interestingly the countries most affected by the economic crisis in recent years. So as a first guess I'd say that economic problems are not the major cause for dimming - but one might be inclined to think that the waste of resources indicated by light pollution is a telltale sign of what's going wrong in these countries. After all, the light measured by satellites is largely wasted - unless used by alien spaceships as "lighthouse" beacons...

There are, however, limitations to the interpretation of satellite data, one being the fact that the sensors on board DMSP are less sensitive to the more bluish light of modern LED luminaires. This might mimic a reduction in brightness where there is in fact an increase. Then again Bennie et al. claim to have employed a better statistical technique to yield more robust results than previous studies. But read for yourself - you can download the original article here.
 

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