Reconstructing the climate and the air pollution of Rome during the life of the Trajan Column

Abstract

The climate of Rome has been reconstructed on the basis of geophysical factors, documentation taken from latin literature and recent instrumental observations. The daily meteorological statistics, based on their own experience, by Roman writers, gives a clear picture of the climate when the Trajan Column was constructed. Other series that may be related to climatic events, such as the floods of the Tiber, furnish a general overview of the rainy periods in past centuries. Unbroken instrumental records of air temperature, rainfall and wind, taken at the nearby Collegio Romano provide good information on the meteorological situation over the last two centuries. Freeze-thaw cycles are not as frequent today as in the past, and reached a maximum frequency in the mid 19th century. Air pollution in Rome in the past may be considered from the size of the population, the presence of few industrial factories and the occurrence of volcanic eruptions, mainly from nearby Vesuvius, Etna, Stromboli and Vulcano. The surface of the column is covered with an oxalate patina, called ‘scialbatura’, with multiple layering. Two main hypotheses have been proposed for the natural transformation of the marble into oxalate: the biological deterioration due to lichen activity (responsible also for pitting) and chemical reaction of oxalic acid and oxalates present in the atmosphere. Past changes and fluctuations of climate and pollution have been here considered to interpret under a new light these hypotheses, and the critical number of inhabitants for making a lichen-toxic atmosphere and inhibiting the epilithic activity has been deduced. The scialbatura is less soluble (and more resistant) than the original marble; its colour is gray-pink while the Carrara marble is white. The question arises of whether the transformation of the marble into oxalate should be better considered damage (i.e. different colour and chemical composition) or a natural means of conservation. Field observation shows that deterioration of monuments can be reduced by lowering the pollutant concentration to a threshold limit; however, below this limit, lichens can live and chemical weathering (e.g. dissolution, sulphation) may be overwhelmed by biological weathering (e.g. transformation of marble in oxalate, pitting) in a rather uncontrolled way.