Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of [CH4] and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the [CH4] plateau. Thermogenic emissions did not resume to cause the renewed [CH4] rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.

Samples from two Greenland ice cores (NEEM and NGRIP) have been measured for methane carbon isotope ratios (δ13C‐CH4) to investigate the CH4 mixing ratio anomaly during Greenland Interstadial (GI) 21.2 (85,000 years before present). This extraordinarily rapid event occurred within 150 years, comprising a CH4 mixing ratio pulse of 150 ppb (∼25%). Our new measurements disclose a concomitant shift in δ13C‐CH4 of 1‰. Keeling plot analyses reveal the δ13C of the additional CH4 source constituting the CH4 anomaly as −56.8 ± 2.8‰, which we confirm by means of a previously published box model. We propose tropical wetlands as the most probable additional CH4 source during GI‐21.2 and present independent evidence that suggests that tropical wetlands in South America and Asia have played a key role. We find no evidence that boreal CH4 sources, such as permafrost degradation, contributed significantly to the atmospheric CH4 increase, despite the pronounced warming in the Northern Hemisphere during GI‐21.2.

Methane (CH4) emission from ruminant livestock, the single most important anthropogenic source of emissions of this gas, has both environmental and nutritional implications. Methane emissions can be accurately measured using enclosure facilities, but if data are to be extrapolated to production situations, caution needs to be exercised as there are limitations. The sulphur hexafluoride (SF6) tracer technique for estimating CH4 emissions exploits the knowledge that most of CH4 is excreted at the mouth and nostrils of the animal. Estimations are based on known release rate of SF6 from a permeation tube introduced into the reticulorumen of an animal and the ratio of CH4/SF6 concentrations in aliquots of excreted gases sampled unobtrusively. The technique remains the method of choice for emission estimations in conditions being similar to those under production. Nevertheless, the accuracy and precision of the technique has been debated. Most of the evaluations of the tracer technique involving sheep and cattle indicate that it agrees well with the respiration chamber methods in their overall mean emission estimates, but a greater variation for SF6 tracer emission estimates remains a common feature. Growing evidence suggests that the large variability of tracer-based CH4 emission estimates is related to the experimental conditions and improper handling of the technique (e.g. inaccurate assessment of background SF6 and CH4 concentrations), and more recently the instability of the sample flow rate has been associated with inaccuracy and imprecision of the technique. Fortunately, the identified and suspected sources of variation are controllable. Here, we discuss some potential issues related to the technique and recommend measures of quality assurance and control when applying the technique.

Predictable release of sulphur hexafluoride (SF6) tracer gas from permeation tubes into the reticulo-rumen is necessary to estimate methane emissions from ruminants using the SF6 tracer technique. Any discrepancy between the laboratory determined rate of SF6 release from permeation tubes and the actual rate of release in the reticulo-rumen would bias calculated methane emissions. The purpose of this investigation was to determine the effect of temperature, submersion and orientation on the rate of SF6 release from permeation tubes. Four experiments were undertaken. Experiment 1 determined that release of SF6 increased by 2.5 +/- 0.14% per degree Celsius increase in temperature between 37 and 41 degrees C (P < 0.001). Experiment 2 determined that the Arrhenius equation can be used to describe the temperature dependence of SF6 release rate from permeation tubes between 0 and 70 degrees C, consistent with a change in release rate of 2.3 +/- 0.08% per degree Celsius change in temperature. Experiment 3 determined that submersion of permeation tubes in water did not affect the rate of SF6 release (P = 0.13). Experiment 4 determined that SF6 release rate was not influenced by permeation tube orientation (P = 0.42). In addition we determined the activation energy of permeation, E, describing the overall temperature dependence of SF6 permeation flux from permeation tubes, to be 18,424 +/- 680 J/mol. This research implies that the short-term release rate of SF6 from permeation tubes within the reticulo-rumen will vary in response to temperature change due to animal, diet and/or environmental factors. A short term decrease in temperature of reticulo-rumen contents, induced by drinking cold water, is expected to have a larger influence on the accuracy of estimated methane emission derived from time-averaged sampling periods less than 24 h. Use of the SF6 technique to detect differences in enteric methane emissions due to diet or between animal species may be confounded by diet or genetic effects on body temperature. Unless the effect of temperature is managed through careful implementation of the technique, substantial errors could be caused as illustrated by the following example: a +2 degrees C error in calibration temperature (41 degrees C), and a -2 degrees C discrepancy between the actual (37 degrees C) and assumed reticulo-rumen temperature (39 degrees C), could bias estimated methane emissions by approximately +10%. (C) 2014 Elsevier B.V. All rights reserved.

Implementations of the sulphur hexafluoride (SF6) tracer technique to determine methane (CH4) emission rates from individual ruminant animals involve measuring levels of both CH4 and SF6 in background air. In well-ventilated settings, including grazing, background sampling is straightforward and the algebraic correction for background levels is then usually minor. In a recent paper in this journal (Vol. 170, p. 265–276), Williams et al. drew attention to the much more careful consideration that is needed for background sampling in experiments that use the SF6 tracer technique with housed animals when both CH4 and SF6 levels can build up unevenly within the housing. This note builds on that study to show specifically and rigorously: (a) what is meant by background air, and that background corrections to CH4 emission estimates are unaffected by the recycling of CH4 and SF6 through inhalation of self-exhaled gases; (b) that in studies of the role of various treatments on CH4 emission rates, the siting of background samplers can crucially impact on findings; and, in particular, (c) that reports of a possible dependence of estimated CH4 emission rates upon the rate of SF6 release in the rumen are called into question due to the sensitivity of those findings to the siting of background samplers.

The sulphur hexafluoride (SF6) tracer technique has been widely applied to determine CH4 emission rates by ruminants since its development in the mid-1990s. It remains the only viable method for determining emission rates from individual grazing animals. Essential parts of the method for each participating animal are pre-insertion into the rumen of a source of SF6 with known release rate and breath sample collections near the nose and mouth for CH4 and SF6 analysis. Breath samples are accumulated over an 'averaging period' of usually 24 h to yield estimates of CH4 emissions. As a tracer, SF6 is biologically inert and has a very low detection limit (i.e., 10(-12)), enabling release rates of a few tens of mu l/h to be sustained for many months by an initial SF6 charge of similar to 1 g. Any departure from a uniform SF6 collection rate, such as through SF6 interactions in the digestive tract, could introduce variability into the inferred CH4 emission rate, which has the potential to explain reports of higher variability in CH4 emission rates estimated with this technique compared with whole animal chamber techniques. Our study examined SF6 and CH4 excretion rates for their variability using a novel automated gas chromatography system that isolated and analysed 20 min breath samples collected successively for 6 d from each of 9 housed sheep. We found that that SF6 was not excreted into the breath stream at a uniform rate, but its daily pattern of excretion was strongly correlated with that of CH4, suggesting that some SF6 is retained within the digestive tract and later ventilated with eructated gases following feeding. Methane emission rates can be estimated for different averaging periods through different combinations of the 20 min data. Methane emission rate estimates for each sheep are independent of averaging period between 3 h and 6 d, although inter-period variability is highest for averaging periods less than 1 d. Improved understanding of the SF6 tracer technique supports it as a reliable unbiased estimator of enteric CH4 emission rate in ruminants.This paper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson. (C) 2011 Elsevier B.V. All rights reserved.

Measurements of near-sea-level tropospheric Δ14CO2 have been made at Wellington, New Zealand since December 1954; these measurements comprise the longest such record available. The Δ14C rose from −10‰ in 1955 peaking at 695‰ in 1965 as a result of “bomb 14C” production, before falling thereafter to the present day (2005) value of 73‰. The Δ14C peak occurred about 1 year later in the southern hemisphere than in the northern hemisphere. The post-1965 fall is due to the transfer of 14C-enriched CO2 to the biospheric and oceanic pools together with ongoing release of 14C-free CO2 from fossil fuel combustion, during an era without major atmospheric nuclear-weapon tests. Time series analysis of the data using Loess decomposition and filtering indicates an approximately exponential decline in excess Δ14CO2 over 1967–2005 with an e-folding time of 18 years. The seasonal cycle from 1954 until 1980 had a maximum in the late (austral) summer, a minimum in winter, with peak-to-trough amplitude that peaked at 20‰ in 1966. For the period 1980–1989, a new seasonal cycle emerged, with a maximum in winter and a minimum in late summer/early autumn and peak-to-trough amplitude of 3.5‰, transitioning to a new seasonal structure after about 1990.

Attempts to evaluate the sulphur hexafluoride (SF6) tracer technique to estimate CH4 emissions from ruminants have yielded mixed results. These studies either used SF6 permeation tubes with a long history of use in animals, involved small number of animals or used partial animal enclosure. Our study was conducted with a relatively large number of experimental sheep and controlled variables regarding the permeation rate (PR) of SF6. Twenty four sheep housed in a covered yard and fed lucerne silage to achieve common feed intakes among individuals in the study were administered fresh SF6 permeation tubes. Following 10 d acclimatisation in pens, sheep were staggered in 3 groups of 8 each in order to match availability of 8 respiration chambers. Each group were transferred to individual metabolic crates and habituated to breath collection harnesses for 3d before breath samples were collected daily over 6 consecutive d for CH4 emission estimation using the SF6 'Tracer' technique. Sheep were then brought into respiration chambers for CH4 measurements over 4 consecutive d ('Chamber'). During sheep occupation, chamber inlet and outlet gas streams were sampled into evacuated yokes, as for the tracer technique procedures. Samples were analysed for CH4 and SF6 mixing ratios by gas chromatography as for the Tracer technique, which were then used to estimate CH4 emissions using tracer technique procedures (i.e., Tracer in chamber). Paired t-tests based on within sheep data were used for pairwise comparisons of CH4 emission estimates between techniques. Daily CH4 emissions for the Tracer. Chamber and Tracer in chamber procedures were 14.8 +/- 2.4, 13.9 +/- 1 and 16.1 +/- 2.8 g, respectively. Although Tracer and Chamber emission estimates did not differ, Tracer estimates were associated with much larger among- and within-animal variability than Chamber values, and the relationship between Chamber and Tracer estimates was poor. Rate of recovery of SF6 from chamber gases calculated by dividing the calculated daily emission of SF6 (i.e., net mixing ratio of SF6 x chamber ventilation rate) by the known PR of SF6 was 10% lower than that for CH4. In sheep, the average CH4 emission estimate using the SF6 tracer technique matches that obtained from chambers, but the correlation between estimates is poor, possibly due to a mismatch in routes of excretion of tracer and trace gases.This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson. (C) 2011 Elsevier B.V. All rights reserved.

Recent developments in applying carbon-isotope information to better understand regional and global methane budgets infer a strong role by a highly fractionating seasonal sink such as atomic chlorine. Specifically, OH as the predominant seasonal sink cannot account for the 'phase ellipses' based on observed seasonal cycles of methane mixing ratio and isotope ratio, delta C-13. Although a strong role by atomic chlorine is inferred empirically, open questions remain about the interplay between sources and sinks in determining the properties of phase ellipses. This paper employs a simple didactic model of the seasonal cycling of atmospheric methane to understand such interplay. We demonstrate that a single seasonal sink and seasonal source act together to imprint anti-phase seasonalities on atmospheric methane and delta C-13, which lead to phase ellipses that collapse onto a straight line with slope characteristic of that sink. This explains empirical findings of these anti-phase relationships in three-dimensional modelling studies. We also demonstrate that multiple seasonal sinks acting with a seasonal source can yield surprising properties for the phase ellipse that not only explain some features of phase ellipses reported in modelling studies but also have the potential to explain marked inter-annual variation in phase ellipses based on observation.

The doubling of atmospheric methane (CH4) during the twentieth century due largely to growth in anthropogenic emissions has made CH4 the second largest contributor behind carbon dioxide to anthropogenic forcing of climate change. However, the global CH4 budget and its decadal evolution remain poorly quantified despite re-evaluations that include the IPCC Fourth Assessment Report. Potentially, the aggregation of national anthropogenic emission inventories as reported to the United Nations Framework Convention on Climate Change could document the changing anthropogenic emission since 1990, as could other "bottom-up'' inventories such as EDGAR. As an examination of the recent CH4 budget evolution, we compare two constructions of CH4 source history, one based on an aggregation of national emission inventories, the other version 4 of EDGAR, each in combination with alternative natural CH4 emissions, for consistency with observed atmospheric mixing ratio and carbon isotope content (delta C-13(CH4)). We conclude that despite the utility of isotopic constraints on budget evolution, the level of uncertainty in sink strengths and their isotopic fractionation limits the confidence in constructing anthropogenic emission histories over recent decades.

After nearly a decade without growth in atmospheric methane, there are indications of renewed growth from 2007. Reports of this renewal portray it as global in extent, and due wholly or largely to growth in emissions. Surface methane mixing ratios and constituent δ13C values have been measured approximately twice monthly at Baring Head, New Zealand (41°S, 175°E) since 1989. Surface mixing ratios have been measured continuously at Lauder, New Zealand (45°S, 170°E) since 2007. Also at Lauder, tropospheric-mean mole fractions of methane have been retrieved from ground-based near-infrared solar spectra since 2004. These mixing ratio datasets are consistent with growth rates of about 7.5 and 4.9 ppb year−1 during 2007 and 2008. We consider the possible origins of this growth based on their imprint on δ13C values.

A 79-day rotational grazing experiment was conducted over the summer and autumn of 2007 to compare effects of grazing willow (Salix spp.) fodder blocks, a combination of small trees (i.e., 1.0m) and herbage, or perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) control pasture on breath methane (CH4) emissions, concentrations and solubility of CH4 and sulphur hexafluoride (SF6) tracer gas in blood, and haematology variables in young growing female sheep (i.e., hoggets). Measurements of gases in blood followed a double equilibration technique with two (n=20) replicate per treatment. Ten ewe hoggets in each replicate were dosed on day 22 with intraruminal slow release SF6 capsules, an inorganic tracer gas used to calculate CH4 emissions. Breath samples were collected over 5-day periods in weeks 5 (period 1) and 11 (period 2). Total condensed tannin (CT) concentrations calculated in the diet selected by the willow fodder block sheep was 12g CT kg/dry matter intake, with negligible amounts in control pasture hoggets. Compared to control pasture, grazing willow fodder blocks reduced CH4 emission/kg metabolic body weight (BW0.75) by 20% in period 1 (P<0.01), but not in period 2. Blood CH4 concentrations (ng/mL blood) were similar for both groups on day 36, but higher (P<0.001) on day 76 for hoggets grazing willow fodder blocks, while a different trend was observed for SF6 blood concentration being higher (P<0.01) on day 36 in hoggets grazing willow fodder blocks, but similar in both groups on day 76. Repeatability of blood CH4 concentration was 75% in period versus 84% in period 2. Methane and SF6 Ostwald solubility coefficients in blood were similar in both periods for sheep grazing willow fodder blocks and the control pasture. Hoggets grazing willow fodder blocks had lower BW gain (65g/day), carcass weight (16.1kg) and carcass fatness (9.2mm) than hoggets grazing control pasture (102g; 18.3kg; 11mm). Hoggets dosed with SF6 capsules had lower (P<0.05) red blood cells, haemoglobin and haematocrit concentrations when grazing either willow fodder blocks or control pasture, while neutrophil (P=0.063), platelet (P=0.073) and monocyte (P=0.072), white blood cell and total lymphocyte counts (P<0.05) were higher for willow fodder block-fed hoggets than those fed the control pasture. Differences in the reduction in CH4 emission between periods from grazing willow fodder blocks may be due to more willow leaf being eaten during the CH4 measurement period in period 1 than in period 2.

Converging evidence from new top‐down and bottom‐up estimates of fossil “radiocarbon‐free” methane emissions indicates that natural geologic sources account for a substantial component of the atmospheric methane budget. Comparing emission estimates based on atmospheric 14 CH 4 (“radiomethane”) with geologic emissions from seepage, including terrestrial macroseeps, microseepage, marine seeps, and geothermal/volcanic emissions from the Earth's crust, shows that such “geo‐CH 4 ” sources can be conservatively estimated at 53 ± 11 Tg yr −1 globally. This makes geo‐CH 4 second in importance to wetlands as a natural methane source. Such a new appraisal can easily be accommodated within the uncertainty of the global methane budget as recently compiled, and recognizes the importance of geophysical out‐gassing of methane generated within the lithosphere. We propose a new coherent contemporary budget in which 30 ± 5% (based on atmospheric radiomethane measurements) of the global source of 582 ± 87 Tg yr −1 has fossil origin, both natural and anthropogenic.

The aim of this study was to evaluate the sulphur hexafluoride (SF6) tracer technique for methane (CH4) emission measurement in sheep. Ten cryptorchid Romney sheep were involved in two indoor trials (T1 and T2), where daily CH4 emissions were individually measured both by the SF6 tracer (‘tracer CH4’) and by the indirect calorimetry chamber (‘chamber CH4’) techniques while fed on lucerne hay at 1.2 times maintenance requirements. Separate sets of permeation tubes with pre-calibrated permeation rates (‘pre-calibrated PRs’) were used in the two trials (for tracer CH4) and at the time of T1 and T2 these tubes had been deployed in the rumen for 250 and 30 days, respectively. The tracer CH4 measurements were carried out for 2 (T1) and 5 (T2) days in digestibility crates housed within a building (T1) or a well-ventilated covered yard (T2). Sheep were transferred to calorimetry chambers for 3 days acclimatisation, followed by measurement of CH4 emission for 7 (T1) and 3 (T2) days. In T1 samples from the chamber, outflow and inflow (collected over ∼22 h) were analysed for CH4 and SF6 concentrations using the tracer protocol. Thus, PRs of SF6 at the time of the trials (‘calculated PRs’) could be inferred and the corresponding CH4 emissions are then calculated using either the pre-calibrated PR or calculated PR. Permeation tubes were recovered at the end of the animal trials and their ‘post-recovery PR’ determined. In trial T1, the tracer CH4 estimates (based on the pre-calibrated PR) were much higher and more variable than the chamber CH4 values. In this trial, the calculated PR and the post-recovery PR were similar from each other but smaller than the pre-calibrated PR, and when the calculated PR was used in place of the pre-calibrated PR the CH4 emission estimates agreed well with the chamber CH4 values. This suggested that the discrepancy was due to a declining PR during the long deployment time of the tubes in T1, an observation reported elsewhere. When the long intra-ruminal deployment was avoided in T2, good agreement between the techniques for CH4 emission measurement was observed.