Past, current and future solar radiation trends in Europe: Multi-source assessment of the role of clouds and aerosols

Abstract

The increase of surface solar radiation (SSR) observed during the last decades in Europe has raised concerns for its implications for the climate system. Here, we evaluate the past and projected SSR trends in Europe from 1994 to 2054 based on a comprehensive set of ground site observations, five historical gridded datasets and a remarkable ensemble of 30 CMIP6 climate models with projections in four different forcing scenarios. Together, they provide a seamless unprecedented characterisation of the SSR trend in Europe across time and space. The regional SSR trend observed in the period 1994–2023 is 3.1 W/m/decade. All gridded datasets, except the ERA5 reanalysis, deviate from this value by 0.3 W/m/decade, or less. Assuming that clouds and aerosols are the only boosters of the SSR trend, it is found that 20% of the trend is explained by aerosol direct effect (ADE). It is shown that the first aerosol indirect effect (AIE) alone cannot explain the SSR trend related to cloud changes, thus leaving a significant role for changes in cloud coverage. It is estimated that the total SSR trend is approximately explained in 1/5 by ADE, 2/5 by first AIE and 2/5 by cloud coverage changes. However, this fractions are likely affected by biases in aerosol optical depth. The results suggest that the CMIP6 ensemble overestimates (underestimates) the clear (cloudy) SSR trend during 1994–2014. The median SSR trend projected by the CMIP6 models is, on average, 85% smaller than the trend observed during 1994–2023 with small differences between forcings scenarios.