Long-term trends in reconstructed atmospheric aerosol load based on large-scale sunshine duration records since 1900

Abstract. Aerosol emissions from anthropogenic sources have increased considerably since pre-industrial times. However, global climate models exhibit substantial discrepancies in the radiative forcing attributed to anthropogenic aerosols, a primary factor contributing to uncertainties in total anthropogenic forcing estimates. Therefore, achieving a sufficiently accurate quantification of historical aerosol optical depth (AOD) would be crucial to enhance our ability to project future climate changes and formulate effective mitigation and adaptation strategies. This study uses multiple observational networks, utilizing sunshine duration as a proxy for broadband AOD (BAOD), together with cloud cover observations from 2700 sites across the world, to reconstruct BAOD trends since the late 19th century. In particular, this analysis provides, for the first time, historical BAOD estimates for Europe dating back to the early 1900s. The findings include a general trend toward cleaner atmospheres at most European observation sites during both the 1900–1925 and 1926–1959 periods, amounting to regional trends of −0.014 and −0.004 per decade, respectively. The mean-annual BAOD is found to increase at only a few stations during those periods, likely because of local industrialization. Conversely, based on sunshine data from around 400 sites during the 1960–1985 period, the analysis, underscores the role of anthropogenic aerosols in the dimming observed across Europe (0.004 per decade), as well as the modulating relevance of volcanic aerosols. A continuous increase in BAOD is also observed over Southeast Brazil during 1960–1985, with a noticeable higher rate of 0.015 per decade, which is approximately four times as large as that found in Europe. At the same time, Japan experienced a notable decrease in BAOD with a rate of −0.015 per decade, owing to stringent environmental regulations implemented between 1960 and 1985. Meanwhile, Oceania exhibited a modest negative trend of −0.004 per decade during that period. During the 1986–2015 period, commonly referred to as a “brightening phase”, a general decline of annual BAOD is observed in each studied region: higher rate of decreasing aerosol load in Southeast Brazil, Japan, and Europe by −0.010, −0.015, and −0.013 per decade, respectively, compared to much lower rate of −0.003 per decade over Oceania. This comprehensive assessment sheds new light on the historical BAOD variability and highlights region-specific differences, contributing valuable insights that can help refine climate models and advance our understanding of the complex interplay between aerosols and climate.