Abstract Greenhouse gas (GHG)‐induced climate change is among the most pressing sustainability challenges facing humanity today, posing serious risks for ecosystem health. Methane (CH 4 ) and nitrous oxide (N 2 O) are the two most important GHGs after carbon dioxide (CO 2 ), but their regional and global budgets are not well known. In this study, we applied a process‐based coupled biogeochemical model to concurrently estimate the magnitude and spatial and temporal patterns of CH 4 and N 2 O fluxes as driven by multiple environmental changes, including climate variability, rising atmospheric CO 2 , increasing nitrogen deposition, tropospheric ozone pollution, land use change, and nitrogen fertilizer use. The estimated CH 4 and N 2 O emissions from global land ecosystems during 1981–2010 were 144.39 ± 12.90 Tg C/yr (mean ± 2 SE; 1 Tg = 1012 g) and 12.52 ± 0.74 Tg N/yr, respectively. Our simulations indicated a significant ( P < 0.01) annually increasing trend for CH 4 (0.43 ± 0.06 Tg C/yr) and N 2 O (0.14 ± 0.02 Tg N/yr) in the study period. CH 4 and N 2 O emissions increased significantly in most climatic zones and continents, especially in the tropical regions and Asia. The most rapid increase in CH 4 emission was found in natural wetlands and rice fields due to increased rice cultivation area and climate warming. N 2 O emission increased substantially in all the biome types and the largest increase occurred in upland crops due to increasing air temperature and nitrogen fertilizer use. Clearly, the three major GHGs (CH 4 , N 2 O, and CO 2 ) should be simultaneously considered when evaluating if a policy is effective to mitigate climate change.
Abstract. The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (~biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (T-D, exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories, and data-driven approaches (B-U, including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003–2012 decade, global methane emissions are estimated by T-D inversions at 558 Tg CH4 yr−1 (range [540–568]). About 60 % of global emissions are anthropogenic (range [50–65 %]). B-U approaches suggest larger global emissions (736 Tg CH4 yr−1 [596–884]) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the T-D budget, it is likely that some of the individual emissions reported by the B-U approaches are overestimated, leading to too large global emissions. Latitudinal data from T-D emissions indicate a predominance of tropical emissions (~64 % of the global budget,
GIEMS2 represents the minimum extents of northern wetlands.**GLWD provides a representation of the maximum extent of northern wetlands.***These numbers are derived from CT natural microbial emissions, which include emissions from wetlands, river/lake/pond systems, and possibly wild animals (despite the small amount).
