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We examine sunsets painted by famous artists as proxy information for theaerosol optical depth after major volcanic eruptions. Images derived fromprecision colour protocols applied to the paintings were compared to onlineimages, and found that the latter, previously analysed, provide accurateinformation. Aerosol optical depths (AODs) at 550 nm, corresponding toNorthern Hemisphere middle latitudes, calculated by introducing red-to-green(R / G) ratios from a large number of paintings to a radiative transfer model,were significantly correlated with independent proxies from stratosphericAOD and optical extinction data, the dust veil index, and ice core volcanicindices. AODs calculated from paintings were grouped into 50-year intervalsfrom 1500 to 2000. The year of each eruption and the 3 following years weredefined as "volcanic". The remaining "non-volcanic" years were used toprovide additional evidence of a multidecadal increase in the atmosphericoptical depths during the industrial "revolution". The increase of AOD at550 nm calculated from the paintings grows from 0.15 in the middle 19thcentury to about 0.20 by the end of the 20th century. To corroborateour findings, an experiment was designed in which a master painter/colouristpainted successive sunsets during and after the passage of Saharan aerosolsover the island of Hydra in Greece. Independent solar radiometricmeasurements confirmed that the master colourist's R / G ratios which wereused to model his AODs, matched the AOD values measured in situ byco-located sun photometers during the declining phase of the Saharanaerosol. An independent experiment was performed to understand thedifference between R / G ratios calculated from a typical volcanic aerosol andthose measured from the mineral aerosol during the Hydra experiment. It wasfound that the differences in terms of R / G ratios were small, rangingbetween −2.6% and +1.6%. Also, when analysing different parts ofcloudless skies of paintings following major volcanic eruptions, anystructural differences seen in the paintings had not altered the resultsdiscussed above. However, a detailed study on all possible sources ofuncertainties involved (such as the impact of clouds on R / G ratios) stillneeds to be studied. Because of the large number of paintings studied, wetentatively propose the conclusion that regardless of the school,red-to-green ratios from great masters can provide independent proxy AODsthat correlate with widely accepted proxies and with independentmeasurements

Further evidence of important environmental information content in red-to-green ratios as depicted in paintings by great masters