Tony Bromley

A 21st century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4

Getting a rise out of agriculture Methane, a powerful and important greenhouse gas, has been accumulating nearly uninterruptedly in the atmosphere for the past 200 years, with the exception of a mysterious plateau between 1999 and 2006. Schaefer et al. measured methanes carbon isotopic composition in samples collected over the past 35 years in order to constrain the cause of the pause. Lower thermogenic emissions or variations in the hydroxyldriven methane sink caused the plateau. Thermogenic emissions didnt resume to cause the subsequent rise. Instead, the ongoing rise is most likely due to biogenic sources, most notably agriculture. Science, this issue p. 80 The atmospheric methane level has resumed its increase after a plateau between 1999 and 2006. 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.

Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific

Measurements of the mixing ratio and δ 13 C in methane (δ 13 CH 4 ) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ 13 CH 4 was highly dependent on season and varied from 0.5‰ in November 1996 with an estimated annual mean of 0.2-0.3‰. The seasonal cycles in δ 13 CH 4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January-February for the extratropical Southern Hemisphere, in September-October for the tropics, and in June-July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ 13 CH 4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ 13 CH 4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH 4 sink of 12-15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH 4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.

Transects of atmospheric CO, CH4, and their isotopic composition across the Pacific: Shipboard measurements and validation of inverse models

Measurements of carbon monoxide, methane, and their isotopic composition are presented for samples collected on six ship voyages across the Pacific between New Zealand and the United States between 1996 and 1998. The data cover a latitude range from approximately 40°S to 40°N and clearly define significant latitudinal gradients in mixing ratios and isotopic composition and their seasonal variations. Observational data are compared with recent three-dimensional inverse modeling studies of the global CO and CH4 distributions (Bergamaschi et al., 2000a, b, c) constrained by CO and CH4 mixing ratio data from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory network and additional isotope measurements from a set of five globally distributed stations. Thus the shipboard measurements represent a unique opportunity for independent validation of the inverse models. In general, we see a very good agreement between measurements and model results, both for CO and CH4 mixing ratios and their stable isotopic composition (δ13CO, δC18O, δ13CH4). This is in particular true for the simulation of mean latitudinal gradients of these quantities. For some voyages, however, significant deviations in the “fine structure” of the latitudinal profiles are apparent and are attributed to synoptic scale variations that are not captured by the model (for which a standard meteorology is applied). In most cases, deviations between observations and model results exhibit similar patterns for CH4 and CO, and the deviations in mixing ratios are linked with corresponding deviations in the isotopic signature. Furthermore, a clear correlation between CO and ethane mixing ratios is visible. Observations of 14CO show a general minimum around the equator and cycles of opposite phase in each hemisphere. Virtually identical 14CO minima were observed in both hemispheres in 1997, indicating similar OH levels. We also show model results for δCH3D and the mass-independent isotope fraction in C17O (despite the absence of observational data for the ship voyages) in our discussion to illustrate the potential of these isotopic signatures for the understanding of the global cycles of CH4 and CO, respectively.

Seasonal cycles of mixing ratio and 13C in atmospheric methane at Suva, Fiji

[1]xa0A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08′S, 178°26′E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid-2002, have been analyzed for methane mixing ratio and δ13C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Nina conditions. A modeling study was carried out using a modified version of the UK Meteorological Offices Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its δ13C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Nina conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in δ13C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.

A New Method for Analyzing 14C of Methane in Ancient Air Extracted from Glacial Ice

We present a new method developed for measuring radiocarbon of methane (14CH4) in ancient air samples extracted from glacial ice and dating 11,000-15,000 calendar years before present. The small size (~20 µg CH4 carbon), low CH4 concentrations ((CH4), 400-800 parts per billion (ppb)), high carbon monoxide concentrations ((CO)), and low 14C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air sam- ples was quantitatively removed using the Sofnocat reagent. 14C procedural blanks were greatly reduced through the construc- tion of a new CH4 conversion line utilizing platinized quartz wool for CH4 combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and 14C activity of extraneous carbon added in the new CH4 conversion line were determined to be 0.23 ± 0.16 µg and 23.57 ± 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 µg. The overall procedural blank for all stages of sample handling was 0.75 ± 0.38 pMC for ~20-µg, 14C-free air samples with (CH4) of 500 ppb. Duration of the graphitization reactions for small (<25 µg C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. 14C corrections for each step of sample handling have been determined. The resulting overall 14CH4 uncertainties for the ancient air samples are ~1.0 pMC.

Shipboard measurements and modeling of the distribution of CH4 and 13CH4 in the western Pacific

[1]xa0We present observations of methane (CH4) mixing ratio and 13C/12C isotopic ratios in CH4 (δ13C) data from a collaborative shipboard project using bulk carrier ships sailing between Nelson, New Zealand, and Osaka, Japan, in the western Pacific Ocean. Measurements of the CH4 mixing ratio and δ13C in CH4were obtained from large clean-air samples collected in each 2.5° to 5° of latitude between 30°S and 30°N on eight voyages from 2004 to 2007. The data show large variations in CH4 mixing ratio in the tropical western Pacific, and data analysis suggests that these large variations are related to the positions and strengths of the South Pacific Convergence Zone and the Intertropical Convergence Zone, with variability in the sources playing a much smaller role. These measurements are compared with results from a modified version of the Unified Model (UMeth) general circulation model along two transects, one similar to the ship transects and another 18.75° to the east. Although UMeth was run to a steady state with the same sources and sinks each year, the gradient structures varied considerably from year to year, supporting our conclusion that variability in transport is a major driver for the observed variations in CH4. Simulations forced with an idealized representation of the El Nino-Southern Oscillation (ENSO) suggest that a large component of the observed variability in latitudinal gradients of CH4 and its δ13C arises from intrinsic variability in the climate system that does not occur on ENSO time scales.