Decline of fog, mist and haze in Europe over the past 30 years

Surface solar radiation has undergone decadal variations since the middle of the twentieth century, producing global ‘dimming’ and ‘brightening’ effects1,2. These variations presumably result from changes in aerosol burden and clouds3, but the detailed processes involved have yet to be determined. Over Europe, the marked solar radiation increase since the 1980s is thought to have contributed to the observed large continental warming4, but this contribution has not been quantified. Here we analyse multidecadal data of horizontal visibility, and find that the frequency of low-visibility conditions such as fog, mist and haze has declined in Europe over the past 30 years, for all seasons and all visibility ranges between distances of 0 and 8 km. This decline is spatially and temporally correlated with trends in sulphur dioxide emissions, suggesting a significant contribution of air-quality improvements. Statistically linking local visibility changes with temperature variations, we estimate that the reduction in low-visibility conditions could have contributed on average to about 10–20% of Europe’s recent daytime warming and to about 50% of eastern European warming. Large improvements in air quality and visibility already achieved in Europe over the past decades may mean that future reductions in the frequency of low-visibility events will be limited, possibly leading to less rapid regional warming. Regional amplification of global warming, as observed in Europe over the past 30 years, is a challenging research issue. Models fail to predict the recent 0.5 C/decade European warming trend5. In summer, amplification over land areas may result from feedbacks between air temperature, soil moisture and clouds6–9. Large warming trends have also been found in winter or autumn4,10, associated to a certain extent with circulation changes5,11, but such changes alone could not explain recent extreme temperature anomalies12,13. Atmospheric stability changes due to an increasing greenhouse effect have been suggested as a possible amplifier in a simplified model framework14, but such changes have not yet been shown in observations. The upward trend in surface solar radiation since the 1980s (refs 1,2,15) (in Europe about 0.6Wm−2 yr−1) is a candidate for additional warming, especially in populated regions16. In Europe, decreasing aerosol burden due to emission reductions is a larger contributor than cloud evolution to decreasing optical depths3. However cloud changes, due to relative humidity17 or aerosol changes, are difficult to characterize because of the poor quality of multidecadal surface or satellite-derived observations18. Furthermore, radiation and temperature changes induced by clouds depend critically on the altitude, because high clouds warm and low clouds cool the surface19. Recent studies found significant trends in high cloud cover20,21. Fog frequency trends have been identified