A publication in Atmosphere

The causes of the increase in the global radiative flux in Belgium

In Research

How has the amount of solar radiation reaching the Earth's surface changed over the last fifty years in Belgium and what are the causes of these variations? A study conducted by researchers at the Laboratory of Climatology (UR SPHERES) of the University of Liège confirms that the increase in the amount of solar radiation reaching the Earth's surface since the 1980s is partly due to decreasing cloud cover. The results - published in the journal Atmosphere(1) - do not necessarily indicate a link with the current climate change.


ssential to life, the amount of solar radiation reaching the Earth's surface, namely the global radiative flux,  governs the climate of our planet by warming its surface. It allows the evaporation of water that feeds the water cycle, as well as photosynthesis which has a major role  in the carbon cycle. Since the 1980s, global radiative flux has significantly increased, after a period of substantial decrease. These variations can be explained by fluctuations in the quantity of aerosols in the troposphere but also by changes in cloud cover. Variations in global radiative flux can thus have consequences on our ecosystems, even strenghtening heat peaks and droughts, and modifying the yield of devices such as solar panels or photovoltaics, which generate electricity or hot water thanks to solar radiation. It is therefore important to study these variations in order to identify them and to understand their causes.

To this end, researchers from the Laboratory of Climatology (SPHERES Research Unit) used the MAR model - the regional climate model (RCM) developed at the University of Liège - which enabled them to reconstruct Belgium's climate from 1900 to the present days. The use of RCMs can partly reduce  the biases in global radiation flux found in  global climate models (GCMs) because they allow a more detailed representation of atmospheric processes. This study whoseresults have just been published in the scientific journal Atmosphere(1), compares the results obtained using the MAR model with radiation and cloud cover measurements carried out by the Royal Institute of Meteorology (notably at Uccle and Saint-Hubert) since the 1950s. "The results showthat the MAR model  successfully simulates the historical variations in solar radiation," explains Coraline Wyard, a PhD student and FRIA grantee at the Laboratory of Climatology and the first author of the article. »

Highs and lows

The first period researchers looked at is between 1959 and 1979. "Duringthis period, we notice an overall decrease in solar radiation, of about -5% for Uccle," says the young doctoral student. The reason for such a decline results from a higher concentration of fine particles (aerosols) in the atmosphere, which are mainly emitted by human activities. » These aerosols act as a parasol by reflecting the sun's rays and they also enhance the development of clouds by acting as condensation nuclei for the fine droplets that constitute the clouds. Thus, this increase in aerosol concentration and consequently in cloud cover resulted in an overall decrease in the global radiative flux.


Time series of mean annual radiative flux at Uccle. We clearly see a decrease in solar radiation received from the 1960s to the late 1970s and a sharp increase since the early 1980s.

After this period of decline, the researchers observed a second period extending from 1980 to 2010. Following the Convention on Long-Range Transboundary Air Pollution (CLRTAP) against the acid rain and sulfur dioxide (SO2),and therefore reducing the presence of fine particles in the atmosphere, scientists recorded a period of increase,  in solar radiation received in Europe and North America (e.g. of about +12% for Uccle). "In this case, the MAR model showed that the decrease in aerosol concentration in the atmosphere was accompanied by a decrease in low and medium cloud cover, especially in spring and summer," explains Xavier Fettweis, FR.S-FNRS Research associate at the Laboratory of Climatology. "However, even after aerosol concentrations stabilized in the early 2000s, the amount of low and medium cloud continued to decrease suggesting the intervention of other factors such as changes in the atmospheric circulation. »


Time series of mean annual cloudiness at Uccle. There is a clear increase in cloud cover from the 1960s to the late 1970s and a decrease since the early 1980s.

The origin of these changes in cloudiness is therefore unclear and other factors, such as the appearance of more anticyclonic conditions or the more frequent inflow of tropical air over our regions, could have intervened. « Further analyses are required », concludes Coraline WYARD.« As the climate of Europe has always been subject to strong decadal oscillations, it is difficult to say whether these changes result from natural climate variability or from the current climate change, especially as the latter can affect the natural oscillations of our climate. »

So don't jump to conclusions about the link between increased radiative flux and climate change. For the moment, the MAR model of the Laboratory of Climatology shows that the increase in radiative flux is mainly due to a decrease in cloudiness resulting from changes in atmospheric circulation. However, if this increase is maintained, it could amplify episodes of high heat and droughts but could also have consequences on the yield of solar panels.

Scientific reference

Wyard C., Doutreloup S., Belleflamme A., Wild M. and Fettweis X. (2018), Global Radiative Flux and Cloudiness Variability for the Period 1959–2010 in Belgium: A Comparison between Reanalyses and the Regional Climate Model MAR., in Atmosphere 9(7), 262.

consult the publication on ORBi


Laboratory of Climatology – UR SPHERES – Faculté des Sciences

Coraline WYARD - coraline.wyard@uliege.be

Xavier FETTWEIS - Xavier.Fettweis@uliege.be

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