Quantifying Atmospheric Humidity in the Western Mediterranean During the Iron Age and the Roman Era

Abstract

Atmospheric moisture is a key hydroclimatic parameter that influenced ancient societies, agriculture, and ecosystems. However, quantitative constraints on pre‐industrial atmospheric humidity values are rarely available. Here, the triple oxygen ( 17O/16O and 18O/16O) and hydrogen ( 2 H/1 H) isotope compositions of lacustrine gypsum are used to quantify changes in summer atmospheric relative humidity (RHs) in the southern Iberian Peninsula from 650 BCE to 500 CE. RHs was 59 ± 4‰ from 450 to 350 BCE, approximately 10% higher than at present, and decreased to 50% ± 2% before the Roman conquest of the Iberian Peninsula in 218 BCE. Drier conditions (49% ± 3%) prevailed from 300 to 500 CE, coinciding with the decline and fall of the western Roman Empire in 476 CE. Long‐term RHs values correlated inversely with the strength of the North Atlantic Oscillation, suggesting that hemispheric climate patterns influenced atmospheric moisture in the western Mediterranean during the Iron Age and Roman times.