A global atmospheric methane record from a tropical ice core.

Abstract

Tropical wetlands are widely considered the largest natural source of atmospheric methane (CH₄)^1-4. However, uncertainties about wetland extent and CH₄ production have led to large variations in modelled CH₄ emission trends^2,3. Most historical reconstructions rely on data from polar ice cores, which cannot fully resolve the tropical contribution^5,6 to the CH₄ budget. Here we present a 2,000-year record of atmospheric CH₄ concentrations from ice cores drilled from the South Peak summit of Nevado Huascarán (Summit Core A, SCA; -9.122° S, -77.605° W; 6,768 m asl). We find that the trends and magnitudes of our CH₄ record are broadly consistent with polar records⁷. Our δ¹³C-CH₄ measurements (from approximately 1530 CE to 1999 CE) align with isotope values⁸ consistent with a dominant tropical CH₄ source. Integration of our record into an atmospheric four-box model suggests a sustained equatorial dominance of CH₄ source strength over the past two millennia. Our findings indicate that equatorial CH₄ emissions are higher than previous estimates based only on polar ice core data, supporting the long-standing hypothesis that low-latitude CH₄ emissions dominated pre-industrial (PI) CH₄ variability^5,6. These results demonstrate the importance of tropical ice cores on the reconstruction of CH₄ variability and latitudinal distribution.