## 1 Introduction  
Solar radiation is the main energy source for the Earth-atmosphere system, while the incoming surface solar radiation (SSR) at the Earth's surface plays an important role for various biological, physical and chemical processes, being strongly related to climate and its changes [1, 2]. SSR is driven either by astronomical factors, such as the Sun-Earth distance, the solar zenith angle and position (latitude) or by the composition of the atmosphere, surface conditions, and human activities [3, 4]. As a result, and based on the combination of all these factors, great spatiotemporal changes in the SSR are created. These changes have strengthened the interest of the scientific community due to possible implications for global climate change, but also the steadily increasing use of solar energy [5, 6, 7]. The interdecadalincreases or decreases of SSR, commonly known as global dimming and brightening (GDB), started to gain recognition in the late 1980s [8,9]. GDB may be a regional or global phenomenon and it has been proven that its two major drivers are clouds and aerosols, which largely determine the transparency of the atmosphere [10, 11, 12]. GDB has implications for various sectors ranging from economy, solar power, hydrology and agriculture to climate. The growth of plants and the yield of crops depend on sunshine that creates photosynthesis, but also on temperature, which is strongly dependent on SSR [13, 14, 15]. For example, the increasing diffuse radiation, occurring during dimming phases due to increasing air pollution and cloudiness, enhances photosynthesis in tall vegetation, while low vegetation does not benefit so much [16]. GDB affects the hydrological cycle [17] and changes the diurnal range of surface temperature. Yet, probably the most important effect of GDB is on climate. Thus, [18] noticed that since the 1980s, the lack of solar dimming and its masking effects, allowed the warming by greenhouse gases to become more obvious, resulting in a more rapid increase of temperature than in previous decades. Because of its various and important effects, GDB has become a significant topic of international research, being investigated using either observations and measurements or modelling tools. Direct measurements of SSR have the advantage of providing higher accuracy, but smaller spatial representativeness, while satellite- and model-based products, or combined products, ensure a great representativeness, but need to be evaluated against ground-based truth. Initially, ground-based observations of SSR over North America and Europe have shown a negative trend from the 1960s to the 1980s, referred as 'dimming' [10, 19, 20, 21, 22], and a positive trend, i.e., a 'brightening', from the middle 1980s through to 2000 [5, 11, 23, 24]. However, solar dimming continued to exist until 2000 over India and Southeast Asia [25, 26, 27, 28]. Beyond 2000, it was found that the brightening continued over Europe and N. America and levelled-off at sites in Japan, while in India and China there was a dimming [5]. Besides, it has been suggested [22] that the greenhouse warming after 2000 may be less modulated by GDB than in previous decades, which may be considered in relation to the recent global warming hiatus observed at the beginning of the 21 st century [29, 30, 31]. The GDB results from studies based on surface measurements are complemented by others obtained using satellite observations and models, which provide the necessary spatial coverage and representativeness. Thus, GDB results obtained on regional/global scale, using model simulations and/or satellite data [10, 12, 32, 33, 34, 35, 36, 37, 38], provided evidence that the dimming and brightening during the 1980s and 1990s, respectively, were global rather than local, but also reported contrasting trends in SSR even over neighboring regions.