Surface solar radiation is fundamental for terrestrial life. It provides warmth to make our planet habitable, drives atmospheric circulation, the hydrological cycle and photosynthesis. Europe has experienced an increase in surface solar radiation, termed “brightening”, since the 1980s. This study investigates the causative factors behind this brightening. A novel algorithm from the EUMETSAT satellite application facility on climate monitoring (CM SAF) provides the unique opportunity to simulate surface solar radiation under various atmospheric conditions for clouds (clear-sky or all-sky), aerosol optical depth (time-varying or climatological averages) and water vapor content (with or without its direct influence on surface solar radiation). Through a multiple linear regression approach, the study attributes brightening trends to changes in these atmospheric parameters. Analyzing 61 locations distributed across Europe from 1983 to 2020, aerosols emerge as key driver during 1983-2002, with Southern Europe and high elevations showing subdued effects (0-1%/decade) versus more pronounced impacts in Northern and Eastern Europe (2-6%/decade). Cloud effects exhibit spatial variability, inducing a negative effect on surface solar radiation (-3 to -2%/decade) at most investigated locations in the same period. In the subsequent period 2001-2020, aerosol effects are negligible, while cloud effects dominate the observed brightening (2-5%/decade). This study therefore finds a substantial decrease in the cloud radiative forcing over Europe in the first two decades of the 21st century. Water vapor exerts negligible influence in both sub-periods.

The variability of the shortwave radiative fluxes at the surface and top of atmosphere (TOA) is examined in a pre-industrial modelling setup using the Pacific Decadal Oscillation (PDO) as a possible pacemaker of atmospheric decadal-scale variability. Within models from the Coupled Model Intercomparison Project – Phase 6, downwelling shortwave radiation at the surface, the net shortwave fluxes at the surface and TOA, as well as cloud radiative effects show remarkably similar patterns associated with the PDO. Through ensemble simulations designed with a pure PDO pattern in the North Pacific only, we show that the PDO relates to about 20-40% of the unforced year-to-year variability of these shortwave fluxes over the Northern Hemispheric continents. The SST imprint on shortwave-flux variability over land is larger for spatially aggregated time series as compared to smaller areas, due to the blurring effect of small-scale atmospheric noise. The surface and TOA radiative flux anomalies associated with the PDO index range of [-1.64; 1.64] are estimated to reach up to ±6Wm-2 for North America, ∓ 3Wm-2 for India and±2Wm-2 for Europe. We hypothesise that the redistribution of clouds in response to a North Pacific PDO anomaly can impact the South Pacific and North Atlantic SSTs.

Extending across seven countries, the Alps represent an important element for climate and atmospheric circulation in Central Europe. Its complex topography affects processes on different scales within the atmospheric system. This is of major relevance for the decadal trends in surface solar radiation (SSR), also known as global dimming and brightening (GDB). In this study we analysed data from 38 stations in and around the Swiss and Austrian Alps, over a period ranging from the 1980s up to the 2020s, with the aim of characterizing the spatio-temporal variations of the GDB and understanding the causes for such trends in this region. Our results show a different behavior in the SSR decadal trends in the western part of the Alps in comparison to the eastern part. Our results also suggest a remarkable difference between the causes of such trends at the stations at low altitudes in comparison to the stations at higher altitudes. Significant contribution from cloud optical depth and surface albedo to the SSR decadal trends at high elevation sites were found, in contrast to a strong clear-sky forcing that dominates at low elevations. Results from previous literature and available data suggest that cloud optical depth changes at high altitudes and clear-sky forcing at low altitudes could be associated with the indirect and direct aerosol effects, respectively, due to differing pollution levels at low and high elevation sites.

The Pacific Ocean, spanning over 30% of the Earth’s surface, provides an ideal setting for studying the surface radiative balance due to its relatively pristine atmospheric conditions, far from anthropogenic emission sources. In this study we investigated the causes for the decadal trends of surface solar radiation (SSR) observed at eight stations scattered across seven islands in the Western Pacific Ocean, and extrapolated the results to the whole Western Pacific region based on the understanding of physical processes. Our results show a contrast between the causes for SSR trends in the northwestern and in the southwestern Pacific. From the tropical Southern Hemisphere to the Equatorial region, changes in cloud cover play a major role in the SSR decadal trends and interannual variability. The cloud cover in these areas is strongly associated with sea surface temperature (SST) anomalies, especially those induced by El Nino Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). Modes of variability such as ENSO and IPO impact the large-scale dynamics of the atmopshere, which is followed by a redistribution of the regions of deep convection such as the South Pacific Convergence Zone. This consecutively impacts the cloud cover on a regional level and therefore SSR. In the Northern Hemisphere, however, anthropogenic aerosol transported from Eastern Asia play a major role in the decadal SSR trends. These results contribute to an improved understanding of the physical processes relevant for the long-term SSR trends in remote regions.

A document by Boriana Chtirkova. Click on the document to view its contents.

Extending across seven countries, the Alps represent an important element for climate and atmospheric circulation in Central Europe. Its complex topography affects processes on different scales within the atmospheric system. This is of major relevance for the decadal trends in Surface Solar Radiation (SSR), also known as Global Dimming and Brightening (GDB). In this study we analysed data from 14 stations in and around the Swiss and Austrian Alps, over a period ranging from the 1960s up to the 2010s, with the aim of characterizing the spatio-temporal variations of the GDB and understanding the causes for such trends in this region. Our results showed a different behavior in the SSR decadal trends in the western part of the Alps in comparison to the eastern part. We also identified a remarkable difference between the causes of such trends in the stations at low altitudes in comparison to the station at higher altitudes. The SSR trends under cloudy conditions revealed strong evidence for a control of the decadal trends by cloud optical depth at high elevation sites, in contrast with a strong clear-sky forcing at low elevations. Results from previous literature and available data suggest that such phenomena could be associated with the indirect and direct aerosol effect, respectively, due to differing pollution levels.

Internal variability comprises all processes that occur within the climate system without any natural or anthropogenic forcing. Climate driving variables like the surface solar radiation (SSR) are shown to exhibit unforced trends (i.e. trends due to internal variability) of magnitudes comparable to the magnitude of the forced signal even on decadal timescales. We use annual mean data from 50 models participating in the pre-industrial control experiment (piControl) of the Coupled Model Intercomparison Project – Phase 6 (CMIP6) to give quantitative grid-box specific estimates of the magnitudes of unforced trends. To characterise a trend distribution, symmetrical around 0, we use the 75th percentile of all possible values, which corresponds to a positive trend with 25% chance of occurrence. For 30-year periods and depending on geographical location, this trend has a magnitude between 0.15 and 2.1Wm-2/decade for all-sky and between 0.04 and 0.38Wm-2/decade for clear-sky SSR. The corresponding area-weighted medians are 0.69Wm-2/decade for all-sky trends and 0.17Wm-2/decade for clear-sky trends. The influence of internal variability is on average 6 times smaller in clear-sky, compared to all-sky SSR. The relative uncertainties of these estimates, derived from the CMIP6 inter-model spread, are ±32% for all-sky and ±43% for clear-sky SSR trends. Reasons for differences between models like horizontal resolution, aerosol handling and the representation of atmospheric and oceanic phenomena are investigated. The results can be used in the analysis of observational time series by attributing a probability for a trend to comprise a component due to internal variability, given its magnitude, length and location.