Reduced aerosols and cloud cover driving Europe’s increasing solar radiation
Over the last 30 years, a complex combination of reduced aerosol pollution, global warming and cloud changes have resulted in solar irradiance levels in Europe increasing significantly, according to a new study co-produced by the Universities of Málaga and Murcia and Solargis, the solar industry’s trusted source for data and software. The data shows that the amount of sunlight reaching the surface of Europe increased each decade by an average of 2% - equivalent to 3.1 Watts - during the period between 1994 to 2023.
This Surface Solar Radiation (SSR) - also known as Global Horizontal Irradiation (GHI) - increase has not been homogeneous across the continent or throughout time. Results show that central-western Europe experienced greater growth than other areas, especially during the last 20 years, with north-eastern France, the Benelux and western Germany experiencing nearly 4% increase between 1994 to 2023, even higher in the last 20 years (5%).
The study - Past, current and future solar radiation trends in Europe: Multi-source assessment of the role of clouds and aerosols - published in Remote Sensing of Environment Journal was conducted over a 24-month period and is one of the largest solar radiation studies undertaken in Europe to date.
The teams at the Universities of Malaga and Murcia and Solargis gathered, cleaned and polished data from 1994 to 2054 based on a comprehensive set of ground site observations, five historical gridded datasets and 30 CMIP6 climate models with projections in four different forcing scenarios.
The Drivers of Change: Clouds and Aerosols
The amount of sunlight reaching the ground is influenced by two principal factors - aerosol pollution levels and cloud cover – the study sheds light on which of the two has been more influential on the long-term solar irradiance changes observed in Europe in the last 30 years.
The study finds that changes in cloud opacity and coverage account for around 80% of the total SSR increase. Reducing aerosol pollution levels over the past 30 years have accounted for the remaining 20%.
Nonetheless, the ‘indirect’ role of aerosols in determining cloud opacity and coverage has been pivotal, elevating the importance of aerosol pollution beyond the former 20%. The reason is that aerosols have a two-fold impact on solar radiation levels: first, they absorb and scatter solar radiation directly, which is known as the Aerosol Direct Effect (ADE). This, in turn, affects the heating and cooling of the atmosphere, which influences cloud properties.
The second impact is the Aerosol Indirect Effect (AIE). Cleaner atmospheres as a result of pollution cleanup have led to clouds becoming less reflective and letting more sunlight pass through, as they are formed by larger but fewer water droplets, altering precipitation patterns and cloud lifetime as well. These, combined with the Thermal Effect – temperatures rising as a result of global warming, which also leads to lower cloud formation – have significantly increased the level of solar radiation reaching the ground in Europe.
“When we embarked on this study we were already aware of the overall increase in solar irradiance levels, but our findings around just how much they have increased – and the factors driving this change - have certainly been surprising,” said José Antonio Ruiz-Arias, Professor at the University of Malaga and lead researcher.
“Changing irradiance levels impact the solar power industry greatly, with a direct influence on the long-term production, sustainability and bankability of projects. Beyond the energy sector, a “brighter” Europe translates into more energy, rising temperatures and altered precipitation patterns. These factors have broad impacts on societal patterns and almost all economic sectors, from farming and tourism to education,” he added.
Looking Forward: A Plateau
The study also looked at SSR forecasting data available for the coming decades until 2054, in an effort to forecast future trends. The results showed that the increase in irradiance is anticipated to slow down in the next three decades.
“Although the historical data does not show evidence that allows anticipating a future plateau in the SSR increase, it appears reasonable to assume that the current SSR increase level cannot be sustained indefinitely and should slow down at some point, which is consistent with results from climate models”, says Ruiz-Arias. “Despite the scientific community’s efforts to predict SSR levels accurately, long-term solar radiation projections carry a high degree of uncertainty because of the complexities surrounding the atmosphere’s non-linear behaviour and our limited knowledge of aerosol levels and their interactions in the atmosphere. It is therefore important for the global solar community to continue improving existing datasets, closely monitoring atmospheric changes and re-evaluating how solar radiance impacts us locally on the ground”.
