Global land-atmosphere exchange of methane and nitrous oxide: magnitude and spatiotemporal patterns

Abstract. Methane (CH₄) and nitrous oxide (N₂O) are two most important greenhouse gases after carbon dioxide, but their regional and global budgets are far from certain, which is largely owing to uncertainties in scaling up field measurements as well as the poor model representation of processes and factors governing CH₄ and N₂O exchange between the terrestrial biosphere and atmosphere. In this study, we applied a process-based, coupled biogeochemical model (DLEM – the Dynamic Land Ecosystem Model) to estimate the magnitudes, spatial and temporal patterns of CH₄ and N₂O fluxes as driven by multiple environmental changes including climate variability, rising atmospheric CO₂, increasing nitrogen deposition, tropospheric ozone pollution, land use change and nitrogen fertilizer use. The estimated CH₄ and N₂O emissions from global land ecosystems were 169.43 ± 32.92 Tg C yr⁻¹ and 12.52 ± 1.52 Tg N yr⁻¹, respectively. Our simulations have indicated a significant (P < 0.01) increasing trend for CH₄ (0.75 ± 0.08 Tg C yr⁻¹) and N₂O (0.14 ± 0.02 Tg N yr⁻¹) during 1981–2010. CH₄ and N₂O emissions increased significantly in most climatic zones and continents, especially in tropical region and Asia. The most rapid increase in CH₄ emission was found in wetlands (including rice fields and natural wetlands) owing to increased rice field area and climate change; N₂O emission increased substantially for all the biome types and the largest increase occurred in upland crops owing to increasing air temperature and nitrogen fertilizer use. Given large increase in CH₄ and N₂O emission at global scale, we suggest that these two gases together with CO₂ have to be simultaneously considered when evaluating if a policy is effective or efficient to reduce global warming in the future.