Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).

For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters.

Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning.

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The global methane budget 2000–2017

. 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 (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (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 top-down inversions at 558 Tg CH4 yr−1, range 540–568. About 60 % of global emissions are anthropogenic (range 50–65 %). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher than all other RCP scenarios. Bottom-up approaches suggest larger global emissions (736 Tg CH4 yr−1, range 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 top-down budget, it is likely that some of the individual emissions reported by the bottom-up approaches are overestimated, leading to too large global emissions. Latitudinal data from top-down emissions indicate a predominance of tropical emissions (∼ 64 % of the global budget, The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute 30–40 % on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center ( http://doi.org/10.3334/CDIAC/GLOBAL_METHANE_BUDGET_2016_V1.1 ) and the Global Carbon Project.

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The global methane budget 2000–2012

Global Methane Budget 2000-2012

Ammonia emission from field applied manure and its reduction-invited paper

A review of recent advances in semiconductor laser based gas monitors

Volatilization of ammoniacal nitrogen from newly applied anaerobically digested sewage sludge was measured in the field using an aerodynamic method in May and Oct. for 5- and 7-day periods, respectively. Sludge was applied in a circular area of 0.405 ha at rates of 116,480 kg/ha (150 kg NH₄⁺-N/ha) and 134,400 kg/ha (89 kg NH₄⁺-N/ha) in the May and Oct. experiments, respectively. Windspeed and atmospheric ammoniacal N was measured at 10, 50, 100, and 150 cm above the soil surface providing time averaged vertical profiles at the center of the area from which ammoniacal N flux was estimated. Flux was determined on the premise that NH₃ molecules leaving a horizontal surface must be carried through a vertical plane by horizontal air flow. Fluxes generally followed a diurnal pattern with maximums occurring at about midday. Flux generally decreased with time in an exponential manner. Thus, the “half life” of ammoniacal N applied in the sludge was 3.6 and 5.0 days for the May and Oct. experimental periods, respectively. Of several meteorological variables measured, air temperature appeared to be most closely related to flux rate especially in the two or three days following application. It was estimated that, during the 5-day experimental period in May, 60% of the 150 kg ammoniacal N/ha applied in sludge was volatilized. During the 7-day experimental period in Oct. 56% of the 89 kg ammoniacal N/ha applied was volatilized. Sampling the sludge layer and soil beneath it during and after the Oct. experimental period resulted in considerable variability in ammoniacal N estimates and, hence, in volatilization estimates.

Ammonia Volatilization from Sewage Sludge Applied in the Field

As part of the Canadian Northern Wetlands Study (NOWES) measurements of methane flux were made at the Kinosheo Lake tower site for a 1-month period during the 1990 summer intensive. The measurements were made with a diode-laser-based methane sensor using the eddy correlation technique. Measurements of the methane fluxes were made at two levels, 5 or 18 m. Approximately 900 half-hour average methane flux measurements were obtained. Weak temporal and diurnal trends were observed in the data. Fluxes averaged over the study period showed an overall methane emission of 16 mg CH4 m(exp -2)/d with a daytime average of 20 mg CH4 m(exp -2)/d and a nighttime average of 9 mg CH4 m(exp -2)/d. The effect of emission footprint was evident in the data. A strong relationship between the daily average methane flux and wet bog temperature at 20-cm depth was observed.

Eddy correlation measurements of methane fluxes using a tunable diode laser at the Kinosheo Lake tower site during the Northern Wetlands Study (NOWES)

Increasing atmospheric carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) levels have prompted research on management of the soil C and N pools. The impact of C and N fertilizer addition on N 2 O and CO 2 field emissions is not clear. We determined N 2 O and CO 2 fluxes from a 1-ha bare soil plot using micrometeorological methods with the objective of evaluating the effect of management practices (cultivation, irrigation, fertilizer, and sucrose applications) on the relative importance of both trace gases. Research was conducted at the Elora Research Station (Typic Hapludalf) in Ontario, Canada, over 7 mo. The N 2 O concentration gradients were measured using a Tunable Diode Laser Trace Gas Analyzer and the CO 2 gradients using an Infra-Red Gas Analyzer. The transport coefficients were calculated using a Bowen Ratio Energy Balance and two wind profile approaches. These three approaches resulted in similar hourly fluxes. Daily N 2 O fluxes for nonevent periods were 12 ng m -2 s -1 in 1991, and 2 ng m -2 s -1 in the summer of 1992, while CO 2 fluxes before treatments in 1991 were 0.18 mg m -2 s -1 . Sucrose addition (420 kg C ha -1 ) resulted in the highest N 2 O and CO 2 daily emissions measured during the experiment at 3100 ng m -2 s -1 and 0.5 mg m -2 s -1 , respectively. Peak emissions of 250 ng N 2 O m -2 s -1 were measured after wetting of dry soil (WFP 0.6). Carbon dioxide fluxes varied between 0.01 and 0.5 mg m -2 s -1 being the predominant gas contributing to an equivalent CO 2 global-warming potential, but addition of sucrose increased the contribution of N 2 O to twice the contribution of CO 2 The combined effect of C and N additions (e.g. manure and legume) on the N 2 O emissions in irrigated or high rainfall areas should be considered in the efforts of atmospheric C sequestering.

Nitrous Oxide and Carbon Dioxide Fluxes from a Bare Soil Using a Micrometeorological Approach

A diode laser based gas monitor suitable for measurement of trace gas exchange using micrometeorological techniques

This paper presents the first micrometeorological-based measurements of methane (CH4) emissions from Asian rice paddies of which we are aware. The research features the tunable diode laser trace gas analyzer system (TGAS) recently developed at the University of Guelph. CH4 fluxes were measured between March 9 and 24, 1992, from an irrigated rice paddy field at the International Rice Research Institute (IRRI), the Philippines. The daytime CH4 flux averaged 6.0 μg m−2 s−1. The CH4 fluxes displayed a diurnal trend similar to daily soil temperature curves, with peak emissions of about 8 μg m−2 s−1 in the early afternoon. A tenfold increase in CH4 emissions (to about 70μg m−2 s−1) during a brief weeding experiment resulted from soil disturbance. Up to 25 μg m−2 s−1 of CH4 were released during a drying of the field, after which unsuitable soil redox potentials apparently suppressed methanogenesis. The CH4 flux was also arrested when the field was flooded with oxygen-rich water during a heavy rainstorm.

G. E. Kidd

Papers

Ammonia Volatilization from Sewage Sludge Applied in the Field

Eddy correlation measurements of methane fluxes using a tunable diode laser at the Kinosheo Lake tower site during the Northern Wetlands Study (NOWES)

Nitrous Oxide and Carbon Dioxide Fluxes from a Bare Soil Using a Micrometeorological Approach

A diode laser based gas monitor suitable for measurement of trace gas exchange using micrometeorological techniques

Tunable diode laser measurements of methane fluxes from an irrigated rice paddy field in the Philippines