Abstract. We examine sunsets painted by famous artists as proxy information for the aerosol optical depth after major volcanic eruptions. Images derived from precision colour protocols applied to the paintings were compared to online images, and found that the latter, previously analysed, provide accurate information. Aerosol optical depths (AODs) at 550 nm, corresponding to Northern 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 stratospheric AOD and optical extinction data, the dust veil index, and ice core volcanic indices. AODs calculated from paintings were grouped into 50-year intervals from 1500 to 2000. The year of each eruption and the 3 following years were defined as "volcanic". The remaining "non-volcanic" years were used to provide additional evidence of a multidecadal increase in the atmospheric optical depths during the industrial "revolution". The increase of AOD at 550 nm calculated from the paintings grows from 0.15 in the middle 19th century to about 0.20 by the end of the 20th century. To corroborate our findings, an experiment was designed in which a master painter/colourist painted successive sunsets during and after the passage of Saharan aerosols over the island of Hydra in Greece. Independent solar radiometric measurements confirmed that the master colourist's R / G ratios which were used to model his AODs, matched the AOD values measured in situ by co-located sun photometers during the declining phase of the Saharan aerosol. An independent experiment was performed to understand the difference between R / G ratios calculated from a typical volcanic aerosol and those measured from the mineral aerosol during the Hydra experiment. It was found that the differences in terms of R / G ratios were small, ranging between −2.6 % and +1.6 %. Also, when analysing different parts of cloudless skies of paintings following major volcanic eruptions, any structural differences seen in the paintings had not altered the results discussed above. However, a detailed study on all possible sources of uncertainties involved (such as the impact of clouds on R / G ratios) still needs to be studied. Because of the large number of paintings studied, we tentatively propose the conclusion that regardless of the school, red-to-green ratios from great masters can provide independent proxy AODs that correlate with widely accepted proxies and with independent measurements.

Abstract. This work is a follow-up study of a research carried out since 2005 and presents evidence supporting the findings of an earlier paper (Zerefos et al., 2007), which postulated that sunsets painted by famous artists provide independent proxy information on the aerosol optical depth after major volcanic eruptions. The series of these and additional paintings have been revisited and comparisons between coarser digital images with those derived from precision colour protocols, match together confirming the earlier results as discussed in the text. It was also found that aerosol optical depths (AODs) at 550 nm calculated by feeding Red-to-Green (R/G) ratios from a large number of paintings to a radiative transfer model, were well correlated with independent proxies from stratospheric AOD and optical extinction data, the dust veil index and others. AODs calculated from paintings have been grouped into 50 yr intervals from 1500 to 2000. From each 50 yr time period the year of the eruption and the 3 following years have been excluded. The remaining years have been termed "non-volcanic" and they provide additional evidence of a multidecadal increase in the atmospheric optical depths during the industrial "revolution". The increase of AOD at 550 nm calculated from the paintings, is estimated to range from 0.15 in the middle 19th century to about 0.20 by the end of the 20th century. To corroborate our findings, an experiment was designed in which a master painter/colourist painted successive sunsets during and after the passage of Saharan aerosols over the island of Hydra in Greece. Independent solar radiometric measurements confirmed that the master colourist's R/G ratios which were used to model his AODs, matched to the AOD values measured in situ by the co-located sunphotometers at the declining phase of the Sahara aerosol. Our work concludes that regardless of the school, red-to-green ratios from great masters can provide independent proxy AODs that correlate with widely accepted proxies and with independent measurements.

Using the red-to-green (R/G) ratio in paintings of sunsets from 1500-2000 with an updated technique, Zerefos et al. (2014) create a historical record of aerosol optical depth (AOD) values that are well-aligned with other techniques. This current paper builds off of an earlier paper (Zerefos et al., 2007) where an analysis of sunsets was performed on digital images of 554 photos that were downloaded from several art galleries and museums. This previous study found that despite the style or the school of the painter, the R/G ratios at low solar zenith angles correlated well with modelled AOD values following large volcanic eruptions. In their 2014 paper, the authors were able to analyse 124 digital images from the Tate gallery and compare with the analysis of the pictures from the website, and found very similar results. In addition, the team ran an experiment where an artist painted a sunset during and after the Greek island of Hydra was impacted by a Saharan dust storm, and the AOD was measured simultaneously. During the high dust event there was a 30% difference between the AOD derived from the painting and the measured, however there was only a 0.02 difference in the AOD between the painting and the measured values during the low dust event. For both events, there was strong agreement with AOD from the painting and from a high resolution picture of the painting. This paper presents a very novel way to create a historical record of AOD, which can be used to identify when the global atmosphere as impacted by volcanoes, as well as the impact of the industrial revolution. The authors th th estimate that the AOD increases from 0.15 in the middle of the 19 century to about 0.20 by the end of the 20 century. As they conclude, "The main conclusion of the paper is that nature speaks to the hearts and souls of artists. When colouring sunsets the R/G ratios perceived by the brain contain important environmental information."