Spatial distribution pattern and long-term trend of atmospheric methane in the Atlantic-Mediterranean transition region based on TROPOMI and GOSAT measurements

Methane (CH4) spatial distribution and its trends in the Atlantic-Mediterranean transition region of southwestern Europe were studied using TROPOMI and GOSAT observations. TROPOMI XCH4 provided insights into the distribution across the entire Iberian Peninsula, highlighting the high XCH4 mixing ratios in the two main valleys and the southern sub-plateau, characterized by agricultural and livestock activities. The marine-continental region in the southwestern Iberian Peninsula was identified as a major CH4 hot spot. Focusing on this region, the seasonal distribution of CH4 and its trends were studied. The lowest XCH4 levels were recorded in a semi-natural landscape in a mountain range with an average mixing ratio of 1871.9 ± 18.5 nmol mol−1, while the highest trend was recorded at 10.9 ± 0.7 nmol mol−1 year−1 for the period 2019–2023. Conversely, the highest XCH4 levels were obtained over an inland wetland, averaging 1888.7 ± 15.8 nmol mol−1; while recording the lowest regional trend, 6.4 ± 0.9 nmol mol−1 year−1. Aiming to identify potential regional changes in the XCH4 trend, GOSAT observations were used, revealing a long-term trend of 8.7 ± 0.3 nmol mol−1 year−1 from 2009 to 2023. Focusing on the first and last five years of the series, trends of 6.5 ± 0.9 nmol mol−1 year−1 and 14.9 ± 1.2 nmol mol−1 year−1 were detected, indicating a trend acceleration ratio of 2.3. An analogous pattern was found in the Atlantic Ocean and central Mediterranean at the same latitude, with ratios of 2.7 and 2.4 respectively. Furthermore, these outcomes were corroborated by ground-based observations from the Azores and Lampedusa stations, which belong to the NOAA network. Despite CH4 increasing globally and its trend accelerating, this work highlights the complexity of its dynamics, with regional variations in its mixing ratios, horizontal distribution, and temporal trends.