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 ( 17 O/ 16 O and 18 O/ 16 O) and hydrogen ( 2 H/ 1 H) isotope compositions of lacustrine gypsum are used to quantify changes in summer atmospheric relative humidity (RH s ) in the southern Iberian Peninsula from 650 BCE to 500 CE. RH s 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 RH s 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.