More sunshine in Europe due to cleaner air

By Rob Roebeling (EUMETSAT), Joerg Trentmann and Uwe Pfeifroth (DWD), Christine Traeger Chatterjee (EUMETSAT)

Solar radiation is the main driver of the Earth’s climate and its energy balance. The starting point for understanding our climate system is having very accurate measurements of the amount of solar radiation going towards Earth and the amount of solar radiation reflected and thermal radiation emitted back to space (Hartmann et al., 2013). Simply put, when the amount of radiation coming in exceeds the amount of radiation going out, our climate system heats up. This makes observing and analysing changes in surface solar radiation essential to better understand changes in our climate (Wild, 2013).

Tracing solar radiation in the climate system requires good knowledge of its interactions with atmospheric gases, clouds, and aerosols.

EUMETSATs Satellite Application Facility of Climate Monitoring (CM SAF) provides high quality satellite-based Climate Data Records of solar radiation, clouds, and aerosol optical depth, which are key to analyse and monitor these interactions (see Figure 1).

From the 1950s to the 1980s the amount of solar radiation reaching the Earth surface gradually decreased, a phenomenon referred to as global dimming. Since the 1980s, global dimming came to a standstill and was followed by a period of global brightening that is lasting until today (e.g., Ohmura, 2009; Wild, 2009, 2013). Recent studies state that the trend of dimming was broken because of more stringent air pollution regulations that were, and are still, put in place since the 1980s (Wild, 2009). Specifically, the main reason for the current period of global brightening is attributed to changes in the abundance and characteristics of clouds and aerosols (e.g., Wild, 2009, 2013).

A decrease in aerosol concentrations results in an increase of the amount of incoming solar radiation at the Earth surface and a decrease of the amount of infrared radiances emitted back to the surface, this effect is referred to in literature as the direct-aerosol effect. Beside the direct-aerosol effect, clouds might also play an essential role in the process of global brightening, as they strongly interact with radiation in the troposphere and, thus, are another possible reason for global brightening (e.g., Sanchez-Lorenzo et al., 2017). In fact, increases of aerosol concentration are known to affect the clouds in various ways. Firstly, they lead to clouds with smaller droplets and higher brightness (referred to as the first indirect aerosol effect; Twomey, 1977). Secondly, they lead to clouds having a longer life time, a higher thickness, and less precipitation (referred as the second in-direct aerosol effect; Albrecht, 1989). Figure 2 illustrates the direct and indirect aerosol effects.

Observing clouds and radiation from space

Nowadays satellite-based climate data records help to get a better picture of the full Earth radiation budget (see Figure 1), and could, for example, be used for studying the interaction between aerosols and clouds during the period of global brightening. Hereto, these data records shall cover sufficiently long time-series, be of high quality, be spatially and temporarily continuous, be independent of each other and be consistent with each other. As the availability of high-quality station measurements is either low or entirely absent, satellite-based climate data records are an important source of information in (sub)-global energy balance and water cycle studies.

Trends in aerosol concentrations

In its 6th Assessment Report, the International Panel for Climate Change (IPCC, 2021) states with high confidence, that the reduced aerosol concentrations in the recent decades led to an observable positive trend in shortwave radiation over Europe. As an unfavourable side effect, improved air quality (less aerosols) enhances warming through the direct and in-direct aerosol effects.

Satellites measure aerosol concentrations through a quantity called 'Aerosol Optical Depth' (AOD). The impact of recent improvements in air quality over Europe is illustrated with annual mean aerosol optical depth maps of Figure 3.

It can be seen that, especially over Eastern Europe, air quality has improved significantly since the 1990s. Also, that pollution hot spots, such as the Po valley (Northern Italy), areas over the centre of the United Kingdom, the Netherlands, or over north-western Germany, appear cleaner.

Impact of changes in aerosols concentrations on clouds and radiation

Pfeifroth et al. (2018) studied the actual impact of the in-direct aerosol effect over Europe. Hereto, they analysed decadal trends in satellite climate data records of surface solar radiation (SSR), cloud fraction (CFC), and top-of-atmosphere reflected solar radiation (TRS), during a period that air became gradually cleaner (1995–2015). The study of Pfeifroth et al. (2018) uses satellite-based data records from the EUMETSAT Satellite Application Facility on Climate Monitoring (CM SAF). These data records have shown to be of a high accuracy (e.g., Riihelä et al., 2015; Urraca et al., 2017), are largely independently from each other, and have been used in several studies on climate trends (e.g., Sanchez-Lorenzo et al., 2017).

The analysis of Pfeifroth et al. (2018) shows predictable and coherent temporal behaviour between these three CM SAF climate data records, as can be seen from Figures 4 and 5. Over most areas, the amount of solar radiation reaching the surface, indicated by the red areas in Figure 4 (left panel), has increased since the 1990s. This is particularly true over Eastern Europe and over the Atlantic Ocean.

Simultaneously, a decrease in cloud fractional coverage, indicated by the blue areas in Figure 4 (middle panel), and top-of-atmosphere reflected radiation, indicated by the blue areas in Figure 4 (right panel), was observed.

The changes over the Atlantic Ocean may be related to changes in circulation patterns and natural, wind induced, variations in sea salt aerosols. Similar to anthropogenic aerosols, sea salt aerosols make clouds brighter and have a longer live time (Penner et al., 2001). This seems in line with Figure 3, showing higher aerosol loads in the 1990s than in the 2000s over the Atlantic. Confirmation of this theory needs further research though.

The consistency between the three climate data records is also confirmed by the time-series of SSR, CFC, and TRS (see Figure 5). Over Europe, the decadal trends of cloud fraction and top-of-atmosphere reflected solar radiation over are highly positively correlated, whereas the time-series of surface solar radiation is negatively correlated with the aforementioned two. The results of Pfeifroth et al (2018) indicate that, besides reduced aerosol concentrations (IPCC, 2021), positive trends in surface solar radiation may also be attributed to a decreases in cloud coverage since the 1990s, and thus, hint at reduction of the first in-direct aerosol effect (clouds became less bright) and a reduction of the second in-direct aerosol effect(clouds had a shorter life time).

Benefits

This Climate Use Case shows that satellite-based climate data records, that are high quality, long-term, and independent, provide a good basis for developing a better understanding of changes in the energy budget of Europe during a period that air became gradually cleaner. By comparing time-series of three quantifiers of this budget, it is demonstrated that part of the observed brightening over Europe may be explained by reduced amounts of clouds. This, is confirmed by two climate data records of cloud cover derived from observations of different satellites, i.e., from NOAA-AVHRR (CLARA-A2 CFC) and Meteosat First Generation-MVIRI (COMET CFC). Although not directly part of the study of Pfeifroth et al. (2018), their findings are in line with observed changes in the aerosol concentrations over Europe during the same period.

Data used

• A satellite-based data record of surface solar radiation, derived from AVHRR on NOAA data over the period 1982-2015, from the Climate Monitoring Satellite Application Facility (CM SAF) that is cites as:
  Karlsson, Karl-Göran; Anttila, Kati; Trentmann, Jörg; Stengel, Martin; Meirink, Jan Fokke; Devasthale, Abhay; Hanschmann, Timo; Kothe, Steffen; Jääskeläinen, Emmihenna; Sedlar, Joseph; Benas, Nikos; van Zadelhoff, Gerd-Jan; Schlundt, Cornelia; Stein, Diana; Finkensieper, Stephan; Håkansson, Nina; Hollmann, Rainer; Fuchs, Petra; Werscheck, Martin (2017): CLARA-A2: CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data — Edition 2, Satellite Application Facility on Climate Monitoring, DOI:10.5676/EUM_SAF_CM/CLARA_AVHRR/V002, https://doi.org/10.5676/EUM_SAF_CM/CLARA_AVHRR/V002 (accessed on 2021-11-11)
• A satellite-based data record of surface solar radiation, derived from MVIRI on Meteosat First Generation and SEVIRI on Meteosat Second Generation data over the period 1982-2015, from the Climate Monitoring Satellite Application Facility (CM SAF) that is cites as:
  Pfeifroth, Uwe; Kothe, Steffen; Trentmann, Jörg; Hollmann, Rainer; Fuchs, Petra; Kaiser, Johannes; Werscheck, Martin (2019): Surface Radiation Data Set — Heliosat (SARAH) - Edition 2.1, Satellite Application Facility on Climate Monitoring, DOI:10.5676/EUM_SAF_CM/SARAH/V002_01, https://doi.org/10.5676/EUM_SAF_CM/SARAH/V002_01 (accessed on 2021-11-11)
• A satellite-based data record of top of atmosphere radiation, derived from MVIRI on Meteosat First Generation and SEVIRI on Meteosat Second Generation data over the period 1983-2015, from the Climate Monitoring Satellite Application Facility (CM SAF) that is cites as:
  Urbain, M., Clerbaux, N., Ipe, A., Tornow, F., Hollmann, R., Fuchs, P., and Werscheck, M., 2017: Top of Atmosphere Radiation MVIRI/SEVIRI Data Record, Satellite Application Facility on Climate Monitoring, DOI:10.5676/EUM_SAF_CM/TOA_MET/V001, https://doi.org/10.5676/EUM_SAF_CM/TOA_MET/V001 (accessed on 2021-11-11).
• A satellite-based data record of aerosol properties, derived from MERIS, OLCI, ATSR-2, AATSR, SLSTR, and/or IASI data over the preiod 1995-2020 from the Copernicus Climate Change Service (C3S) that is cites with DOI: 10.24381/cds.239d815c, https://doi.org/10.24381/cds.239d815c (accessed on 2022-02-04).

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