Solar radiation warms and illuminates our planet. It is the primary driver behind the movement of clouds and wind, helps keep us warm, and governs activity through daily and seasonal cycles. However, over the past 30 years, a complex combination of reduced atmospheric aerosol pollution (tiny particles suspended in the atmosphere), global warming, and changes in cloud cover has led to a significant increase in solar radiation levels in Europe, according to a new study co-authored by the University of Málaga (UMA), the University of Murcia (UMU), and Solargis, a solar-sector company specializing in data and software.

The data show that solar radiation reaching the European surface increased by 2.4 watts per square meter per decade (equivalent to a total of 4.8%) during the period from 1994 to 2023.

However, this increase has not been uniform across the continent or over time. The results indicate that central-western Europe experienced greater growth than other areas, particularly over the last 20 years. Northeastern France, the Benelux region, and western Germany experienced increases of nearly 11% between 1994 and 2023 (4.7 watts per decade), with even higher rates during the last 20 years (6.8 watts per decade).

The study, “Past, Present and Future Trends of Solar Radiation in Europe: A Multi-Source Assessment of the Role of Clouds and Aerosols,” published in the journal Remote Sensing of Environment, was conducted over a 24-month period and, according to its authors, is one of the most comprehensive solar radiation studies carried out in Europe to date.

The research teams from the Universities of Málaga and Murcia, together with Solargis, collected, curated, and refined data spanning from 1994 to 2054. Their analysis was based on a comprehensive set of ground-based observations, five historical spatially distributed gridded datasets, and 30 climate models from the CMIP6 project, including projections under four different climate forcing scenarios.

Solar radiation reaching the ground after passing through the atmosphere is influenced by two main factors: aerosol levels suspended in the atmosphere and cloud cover. The study sheds light on which of these two has been more influential in the changes in solar radiation observed in Europe over the last 30 years.

In particular, it concludes that changes in cloud opacity and cloud cover account for approximately 80% of the total increase in solar radiation. The reduction in aerosol pollution levels over the past 30 years explains the remaining 20%.

Nevertheless, the indirect role of aerosols in determining cloud opacity and coverage has been fundamental, raising the true importance of aerosol pollution beyond the aforementioned 20%. The reason is that aerosols have a dual impact on solar radiation levels.

First, aerosols directly absorb and scatter solar radiation — this is known as the direct aerosol effect. In turn, this influences atmospheric heating and cooling, which affects cloud properties.

The second impact is the indirect aerosol effect, whereby a cleaner atmosphere, with lower aerosol pollution, causes clouds to reflect less light back into space and allow more to reach the Earth’s surface. This occurs because such clouds are composed of larger, though fewer, water droplets, which also alters precipitation patterns and cloud lifetime. This, combined with the thermal effect (i.e., rising temperatures as a result of global warming, which also leads to reduced cloud formation), has considerably increased solar radiation levels in Europe.

“When we began this study, we were already aware of the general increase in solar radiation levels, but our findings regarding how much they have increased, and the factors driving this change, have certainly been surprising,” states José Antonio Ruiz-Arias, Professor in the Department of Applied Physics I at the University of Málaga and principal investigator of this research.

The study also analyzed projected solar radiation data for the coming decades through 2054 in an effort to anticipate future trends. The results suggest that the increase in radiation levels is expected to slow over the next three decades.

“Although historical data do not show evidence that would allow us to anticipate a future stabilization in the increase of solar radiation, it seems reasonable to assume that the current growth rate cannot be sustained indefinitely and should slow at some point, which is consistent with climate model results,” Ruiz-Arias explains.

“Despite the scientific community’s efforts to accurately predict solar radiation levels, climate projections carry a high degree of uncertainty due to the complexities surrounding the non-linear behavior of the atmosphere and our limited knowledge about aerosol levels and how they interact with other atmospheric constituents. Therefore, it is important that the global solar community continues improving existing datasets, closely monitoring atmospheric changes, and reassessing how solar radiation impacts us at the local level,” concludes the professor from the Faculty of Science at the University of Málaga.