Insights of climate-driven changes in CH4 and CO2 source contributions at European coastal observatories

Regional carbon dioxide (CO₂) and methane (CH₄) source contributions were analysed at two European coastal observatories, El Arenosillo (ARN) in southwestern Spain and Capo Granitola (CGR) in southern Italy, using in situ observations and STILT model simulations from 2006 to 2023, enabling source attribution to biogenic, fossil fuel, anthropogenic, and natural contributions. Model-observation agreement was robust, with CO₂ and CH₄ correlation coefficients of 0.599–0.900 and 0.693–0.859, respectively. Footprints revealed ARN's sensitivity to Atlantic and Iberian airflows, while CGR was influenced by the central Mediterranean, southern Italy and North Africa. Background dominated (∼98%), with regional CO₂ sources being biogenic (0.81–0.91%) and fossil fuel (0.47–0.56%), and CH₄ sources anthropogenic (∼1.4%) and natural (0.25–0.29%). STILT-simulated source contributions exhibited consistent seasonal and diurnal patterns at both stations. Total CO2 and CH4 concentrations increased in line with global trends. However, source-specific Theil–Sen trend analysis revealed significant shifts between the sub-periods 2006–2010 and 2019–2023. Overall trends accelerated by a factor of 1.6 for CO₂ and by factors of 2.9 and 3.6 for CH₄ at ARN and CGR, respectively. The biogenic CO₂ trend at ARN increased 22-fold between the early and recent sub-periods, while at CGR it transitioned from negative to positive. At ARN, natural CH₄ trends weakened by 46%, whereas at CGR they reversed in sign, suggesting reduced sink strength or enhanced emissions. Anthropogenic reductions weakened or stabilized. These results highlight shifting greenhouse gas sources in European coastal regions, driven by climate feedbacks, urging broader-scale studies.