A factor of 2.5 increase in the global abundance of atmospheric methane (CH 4 ) since 1750 contributes 0.5 Wm −2 to total direct radiative forcing by long-lived greenhouse gases (2.77 Wm −2 in 2009), while its role in atmospheric chemistry adds another approximately 0.2 Wm −2 of indirect forcing. Since CH 4 has a relatively short lifetime and it is very close to a steady state, reductions in its emissions would quickly benefit climate. Sensible emission mitigation strategies require quantitative understanding of CH 4 ’s budget of emissions and sinks. Atmospheric observations of CH 4 abundance and its rate of increase, combined with an estimate of the CH 4 lifetime, constrain total global CH 4 emissions to between 500 and 600 Tg CH 4  yr −1 . While total global emissions are constrained reasonably well, estimates of emissions by source sector vary by up to a factor of 2. Current observation networks are suitable to constrain emissions at large scales (e.g. global) but not at the regional to national scales necessary to verify emission reductions under emissions trading schemes. Improved constraints on the global CH 4 budget and its break down of emissions by source sector and country will come from an enhanced observation network for CH 4 abundance and its isotopic composition ( δ 13 C, δ D (D= 2 H) and δ 14 C). Isotopic measurements are a valuable tool in distinguishing among various sources that contribute emissions to an air parcel, once fractionation by loss processes is accounted for. Isotopic measurements are especially useful at regional scales where signals are larger. Reducing emissions from many anthropogenic source sectors is cost-effective, but these gains may be cancelled, in part, by increasing emissions related to economic development in many parts of the world. An observation network that can quantitatively assess these changing emissions, both positive and negative, is required, especially in the context of emissions trading schemes.

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr À1. Simultaneously, δ 13 C CH4 (a measure of the 13 C/ 12 C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13 C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

Atmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.

In order to investigate the mechanism of arsenic release to anoxic ground water in alluvial aquifers, we sampled ground waters from three piezometer nests, 79 shallow (< 45m) wells, and 6 deep (> 80m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of arsenic (200 -1180 µg l -1 ) are accompanied by high concentrations of iron (3 -13.7 mg l -1 ) and phosphate (1 -6.5 mg l -1 ). Ground water that is rich in manganese (1 -5.3 mg l -1 ) contains < 50 µg l -1 of arsenic. The composition of shallow ground water varies at the 100m-scale laterally and the metre-scale vertically, with vertical gradients in arsenic concentration reaching 200 µg l -1 m -1 . The arsenic is supplied by reductive dissolution of FeOOH and release of the sorbed arsenic to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (< 10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release arsenic to ground water.Arsenic concentrations are high (>> 50 µg l -1 ) where reduction of FeOOH oxide is complete and its entire load of sorbed arsenic is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction incomplete, the sediments are brown in colour and resorption of arsenic to residual FeOOH keeps arsenic concentrations below 10 µg l -1 in the presence of dissolved iron. Sorbed arsenic released by reduction of manganese oxides does not release arsenic to ground water because the arsenic resorbs to FeOOH. Arsenic pollution of ground water is common in the alluvial aquifers of the Bengal Basin because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.

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By comparison of the methane mixing ratio and the carbon isotope ratio (δ13CCH4) in Arctic air with regional background, the incremental input of CH4 in an air parcel and the source δ13CCH4 signature can be determined. Using this technique the bulk Arctic CH4 source signature of air arriving at Spitsbergen in late summer 2008 and 2009 was found to be −68‰, indicative of the dominance of a biogenic CH4 source. This is close to the source signature of CH4 emissions from boreal wetlands. In spring, when wetland was frozen, the CH4 source signature was more enriched in 13C at −53 ± 6‰ with air mass back trajectories indicating a large influence from gas field emissions in the Ob River region. Emissions of CH4 to the water column from the seabed on the Spitsbergen continental slope are occurring but none has yet been detected reaching the atmosphere. The measurements illustrate the significance of wetland emissions. Potentially, these may respond quickly and powerfully to meteorological variations and to sustained climate warming.

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