Constraining past global tropospheric methane budgets with carbon and hydrogen isotope ratios in ice

The impact of spatially varying wetland source signatures on the atmospheric variability of δD-CH4

We present the first spatially resolved distribution of the [Formula: see text] signature of wetland methane emissions and assess its impact on atmospheric [Formula: see text]. The [Formula: see text] signature map is derived by relating [Formula: see text] of precipitation to measured [Formula: see text] of methane wetland emissions at a variety of wetland types and locations. This results in strong latitudinal variation in the wetland [Formula: see text] source signature. When [Formula: see text] is simulated in a global atmospheric model, little difference is found in global mean, inter-hemispheric difference and seasonal cycle if the spatially varying [Formula: see text] source signature distribution is used instead of a globally uniform value. This is because atmospheric [Formula: see text] is largely controlled by OH fractionation. However, we show that despite these small differences, using atmospheric records of [Formula: see text] to infer changes in the wetland emissions distribution requires the use of the more accurate spatially varying [Formula: see text] source signature. We find that models will only be sensitive to changes in emissions distribution if spatial information can be exploited through the spatially resolved source signatures. In addition, we also find that on a regional scale, at sites measuring excursions of [Formula: see text] from background levels, substantial differences are simulated in atmospheric [Formula: see text] if using spatially varying or uniform source signatures. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Bayesian Analysis of the Glacial-Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling

The observed rise in atmospheric methane (CH4) from 375 ppbv during the Last Glacial Maximum (LGM: 21,000 years ago) to 680 ppbv during the late preindustrial era is not well understood. Atmospheric chemistry considerations implicate an increase in CH4 sources, but process-based estimates fail to reproduce the required amplitude. CH4 stable isotopes provide complementary information that can help constrain the underlying causes of the increase. We combine Earth System model simulations of the late preindustrial and LGM CH4 cycles, including process-based estimates of the isotopic discrimination of vegetation, in a box model of atmospheric CH4 and its isotopes. Using a Bayesian approach, we show how model-based constraints and ice core observations may be combined in a consistent probabilistic framework. The resultant posterior distributions point to a strong reduction in wetland and other biogenic CH4 emissions during the LGM, with a modest increase in the geological source, or potentially natural or anthropogenic fires, accounting for the observed enrichment of δ13CH4.

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records

Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ13CH4 and δD(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.

Role of atmospheric oxidation in recent methane growth

The growth in global methane (CH₄) concentration, which had been ongoing since the industrial revolution, stalled around the year 2000 before resuming globally in 2007. We evaluate the role of the hydroxyl radical (OH), the major CH₄ sink, in the recent CH₄ growth. We also examine the influence of systematic uncertainties in OH concentrations on CH₄ emissions inferred from atmospheric observations. We use observations of 1,1,1-trichloroethane (CH₃CCl₃), which is lost primarily through reaction with OH, to estimate OH levels as well as CH₃CC₃ emissions, which have uncertainty that previously limited the accuracy of OH estimates. We find a 64-70% probability that a decline in OH has contributed to the post-2007 methane rise. Our median solution suggests that CH₄ emissions increased relatively steadily during the late 1990s and early 2000s, after which growth was more modest. This solution obviates the need for a sudden statistically significant change in total CH₄ emissions around the year 2007 to explain the atmospheric observations and can explain some of the decline in the atmospheric ¹³CH₄/¹²CH₄ ratio and the recent growth in C₂H₆ Our approach indicates that significant OH-related uncertainties in the CH₄ budget remain, and we find that it is not possible to implicate, with a high degree of confidence, rapid global CH₄ emissions changes as the primary driver of recent trends when our inferred OH trends and these uncertainties are considered.

Exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget

Globally, large-bodied wild mammals are in peril. Because "megamammals" have a disproportionate influence on vegetation, trophic interactions, and ecosystem function, declining populations are of considerable conservation concern. However, this is not new; trophic downgrading occurred in the past, including the African rinderpest epizootic of the 1890s, the massive Great Plains bison kill-off in the 1860s, and the terminal Pleistocene extinction of megafauna. Examining the consequences of these earlier events yields insights into contemporary ecosystem function. Here, we focus on changes in methane emissions, produced as a byproduct of enteric fermentation by herbivores. Although methane is ∼ 200 times less abundant than carbon dioxide in the atmosphere, the greater efficiency of methane in trapping radiation leads to a significant role in radiative forcing of climate. Using global datasets of late Quaternary mammals, domestic livestock, and human population from the United Nations as well as literature sources, we develop a series of allometric regressions relating mammal body mass to population density and CH4 production, which allows estimation of methane production by wild and domestic herbivores for each historic or ancient time period. We find the extirpation of megaherbivores reduced global enteric emissions between 2.2-69.6 Tg CH4 y(-1) during the various time periods, representing a decrease of 0.8-34.8% of the overall inputs to tropospheric input. Our analyses suggest that large-bodied mammals have a greater influence on methane emissions than previously appreciated and, further, that changes in the source pool from herbivores can influence global biogeochemical cycles and, potentially, climate.

Integrating Source Apportionment Tracers into a Bottom-up Inventory of Methane Emissions in the Barnett Shale Hydraulic Fracturing Region

A growing dependence on natural gas for energy may exacerbate emissions of the greenhouse gas methane (CH4). Identifying fingerprints of these emissions is critical to our understanding of potential impacts. Here, we compare stable isotopic and alkane ratio tracers of natural gas, agricultural, and urban CH4 sources in the Barnett Shale hydraulic fracturing region near Fort Worth, Texas. Thermogenic and biogenic sources were compositionally distinct, and emissions from oil wells were enriched in alkanes and isotopically depleted relative to natural gas wells. Emissions from natural gas production varied in δ(13)C and alkane ratio composition, with δD-CH4 representing the most consistent tracer of natural gas sources. We integrated our data into a bottom-up inventory of CH4 for the region, resulting in an inventory of ethane (C2H6) sources for comparison to top-down estimates of CH4 and C2H6 emissions. Methane emissions in the Barnett are a complex mixture of urban, agricultural, and fossil fuel sources, which makes source apportionment challenging. For example, spatial heterogeneity in gas composition and high C2H6/CH4 ratios in emissions from conventional oil production add uncertainty to top-down models of source apportionment. Future top-down studies may benefit from the addition of δD-CH4 to distinguish thermogenic and biogenic sources.

Gas hydrates: past and future geohazard?

Gas hydrates are ice-like deposits containing a mixture of water and gas; the most common gas is methane. Gas hydrates are stable under high pressures and relatively low temperatures and are found underneath the oceans and in permafrost regions. Estimates range from 500 to 10,000 giga tonnes of carbon (best current estimate 1600-2000 GtC) stored in ocean sediments and 400 GtC in Arctic permafrost. Gas hydrates may pose a serious geohazard in the near future owing to the adverse effects of global warming on the stability of gas hydrate deposits both in ocean sediments and in permafrost. It is still unknown whether future ocean warming could lead to significant methane release, as thermal penetration of marine sediments to the clathrate-gas interface could be slow enough to allow a new equilibrium to occur without any gas escaping. Even if methane gas does escape, it is still unclear how much of this could be oxidized in the overlying ocean. Models of the global inventory of hydrates and trapped methane bubbles suggest that a global 3( degrees )C warming could release between 35 and 940 GtC, which could add up to an additional 0.5( degrees )C to global warming. The destabilization of gas hydrate reserves in permafrost areas is more certain as climate models predict that high-latitude regions will be disproportionately affected by global warming with temperature increases of over 12( degrees )C predicted for much of North America and Northern Asia. Our current estimates of gas hydrate storage in the Arctic region are, however, extremely poor and non-existent for Antarctica. The shrinking of both the Greenland and Antarctic ice sheets in response to regional warming may also lead to destabilization of gas hydrates. As ice sheets shrink, the weight removed allows the coastal region and adjacent continental slope to rise through isostacy. This removal of hydrostatic pressure could destabilize gas hydrates, leading to massive slope failure, and may increase the risk of tsunamis.

Critical issues in trace gas biogeochemistry and global change

The atmospheric composition of trace gases and aerosols is determined by the emission of compounds from the marine and terrestrial biospheres, anthropogenic sources and their chemistry and deposition processes. Biogenic emissions depend upon physiological processes and climate, and the atmospheric chemistry is governed by climate and feedbacks involving greenhouse gases themselves. Understanding and predicting the biogeochemistry of trace gases in past, present and future climates therefore demands an interdisciplinary approach integrating across physiology, atmospheric chemistry, physics and meteorology. Here, we highlight critical issues raised by recent findings in all of these key areas to provide a framework for better understanding the past and possible future evolution of the atmosphere. Incorporating recent experimental and observational findings, especially the influence of CO2 on trace gas emissions from marine algae and terrestrial plants, into earth system models remains a major research priority. As we move towards this goal, archives of the concentration and isotopes of N2O and CH4 from polar ice cores extending back over 650,000 years will provide a valuable benchmark for evaluating such models. In the Pre-Quaternary, synthesis of theoretical modelling with geochemical and palaeontological evidence is also uncovering the roles played by trace gases in episodes of abrupt climatic warming and ozone depletion. Finally, observations and palaeorecords across a range of timescales allow assessment of the Earth's climate sensitivity, a metric influencing our ability to decide what constitutes 'dangerous' climate change.