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Soil bacteria known as methanotrophs are the sole biological sink for atmospheric methane (CH4), a potent greenhouse gas that is responsible for ∼ 20 % of the human-driven increase in radiative forcing since pre-industrial times. Soil methanotrophy is controlled by a plethora of factors, including temperature, soil texture, moisture and nitrogen content, resulting in spatially and temporally heterogeneous rates of soil methanotrophy. As a consequence, the exact magnitude of the global soil sink, as well as its temporal and spatial variability, remains poorly constrained. We developed a process-based model (Methanotrophy Model; MeMo v1.0) to simulate and quantify the uptake of atmospheric CH4 by soils at the global scale. MeMo builds on previous models by Ridgwell et al. (1999) and Curry (2007) by introducing several advances, including (1) a general analytical solution of the one-dimensional diffusion–reaction equation in porous media, (2) a refined representation of nitrogen inhibition on soil methanotrophy, (3) updated factors governing the influence of soil moisture and temperature on CH4 oxidation rates and (4) the ability to evaluate the impact of autochthonous soil CH4 sources on uptake of atmosphericCH4. We show that the improved structural and parametric representation of key drivers of soil methanotrophy in MeMo results in a better fit to observational data. A global simulation of soil methanotrophy for the period 1990–2009 using MeMo yielded an average annual sink of 33.5 ± 0.6 Tg CH4 yr-1. Warm and semi-arid regions (tropical deciduous forest and open shrubland) had the highest CH4 uptake rates of 602 and 518 mg CH4 m-2 yr-1, respectively. In these regions, favourable annual soil moisture content (∼ 20 % saturation) and low seasonal temperature variations (variations < ∼ 6 ∘C) provided optimal conditions for soil methanotrophy and soil–atmosphere gas exchange. In contrast to previous model analyses, but in agreement with recent observational data, MeMo predicted low fluxes in wet tropical regions because of refinements in formulation of the influence of excess soil moisture on methanotrophy. Tundra and mixed forest had the lowest simulated CH4 uptake rates of 176 and 182 mg CH4 m-2 yr-1, respectively, due to their marked seasonality driven by temperature. Global soil uptake of atmospheric CH4 was decreased by 4 % by the effect of nitrogen inputs to the system; however, the direct addition of fertilizers attenuated the flux by 72 % in regions with high agricultural intensity (i.e. China, India and Europe) and by 4–10 % in agriculture areas receiving low rates of N input (e.g. South America). Globally, nitrogen inputs reduced soil uptake of atmosphericCH4 by 1.38 Tg yr-1, which is 2–5 times smaller than reported previously. In addition to improved characterization of the contemporary soil sink for atmospheric CH4, MeMo provides an opportunity to quantify more accurately the relative importance of soil methanotrophy in the global CH4 cycle in the past and its capacity to contribute to reduction of atmospheric CH4 levels under future global change scenarios.

Warm periods in Earth’s history offer opportunities to understand the dynamics of the Earth system under conditions that are similar to those expected in the near future. The Middle Pliocene warm period (MPWP), from 3.3 to 3.0 My B.P, is the most recent time when atmospheric CO₂ levels were as high as today. However, climate model simulations of the Pliocene underestimate high-latitude warming that has been reconstructed from fossil pollen samples and other geological archives. One possible reason for this is that enhanced non-CO₂ trace gas radiative forcing during the Pliocene, including from methane (CH₄), has not been included in modeling. We use a suite of terrestrial biogeochemistry models forced with MPWP climate model simulations from four different climate models to produce a comprehensive reconstruction of the MPWP CH₄ cycle, including uncertainty. We simulate an atmospheric CH₄ mixing ratio of 1,000 to 1,200 ppbv, which in combination with estimates of radiative forcing from N₂O and O₃, contributes a non-CO₂ radiative forcing of 0.9 W·m−2 (range 0.6 to 1.1), which is 43% (range 36 to 56%) of the CO₂ radiative forcing used in MPWP climate simulations. This additional forcing would cause a global surface temperature increase of 0.6 to 1.0 °C, with amplified changes at high latitudes, improving agreement with geological evidence of Middle Pliocene climate. We conclude that natural trace gas feedbacks are critical for interpreting climate warmth during the Pliocene and potentially many other warm phases of the Cenezoic. These results also imply that using Pliocene CO₂ and temperature reconstructions alone may lead to overestimates of the fast or Charney climate sensitivity.

Methane emissions from tropical wetlands represent ~ 20% of the total methane contributions to the atmosphere annually and have been the main driver of variation during recent years. Despite the high number of wetlands in Mexico, methane emissions from this source remain poorly quantified. To address this gap, we estimate CH 4 emissions from Mexican wetlands using the output of 13 global models, providing both a constrained value and an assessment of variation of the spatial and temporal fluxes. Our analysis reveals an annual rate of 1.76 ± 2.55 g CH 4 m −2 y −1 (mean ± 1SD) for the period spanning 2000 to 2017, with total annual emissions of 134.1 ± 17.2 Gg CH 4 y −1 (mean ± 1SD). The spatial variation in emissions is apparently related to the country´s precipitation distribution, with lower emission rates in the arid northern regions and higher emissions in the humid southeastern area. Annual variation in emissions is not related to environmental drivers such as precipitation, temperature and evapotranspiration, although intra-annual emissions show marked seasonality controlled by precipitation. Our nationwide estimate of CH 4 wetland emissions represents 12.3% of emissions from fossil fuel in Mexico. We identify key areas for future research including the need for more direct measurements of methane emissions across the country and more accurately delineating the extent of Mexican wetlands.

Soil bacteria known as methanotrophs are the only biological sink for atmospheric methane (CH4). Soil methanotrophy is controlled by a plethora of factors, including temperature, soil texture, moisture and nitrogen inputs, resulting in spatially and temporally heterogeneous rates of soil methanotrophy across the globe. As a consequence, the exact magnitude of the global soil sink, its temporal and spatial variability and the attribution of the main drivers of change, remain poorly constrained. For this reason, a new model to estimate global atmospheric CH4 uptake by the soil MeMo (Methanotrophy Model) was developed by introducing several advances (i.e the effect of N input via fertilizers) to previous existing models in the light of recent findings. The improved structural and parametrical representation of key drivers of soil methanotrophy in MeMo results in a better fit to observational data in a latitudinal distribution comparison with previous models, representing the first validation of global methanotrophy models. MeMo was then employed to simulate and quantify the uptake of atmospheric CH4 by soils at the global scale through different time periods. The new model runs showed a constant increase in global CH4 uptake since the last glacial maximum to the preindustrial era (from 7 to 17 Tg CH4 y-1), a sharp increase in the last 100 years (from 17-35 Tg CH4 y-1) and likely increase in the future, depending on the scenario (from 23-89 Tg CH4 y-1). The changes were further attributed to fluctuations in atmospheric CH4 concentration during the paleo-record and the last century, however in recent decades temperature and nitrogen inputs started to have a larger influence on regional trends which are likely to be more pronounced in future RCPs.