Methane Growth Rate Research Articles

Enhanced methane emissions from tropical wetlands during the 2011 La Ni\xf1a

Year-to-year variations in the atmospheric methane (CH4) growth rate show significant correlation with climatic drivers. The second half of 2010 and the first half of 2011 experienced the strongest La Niña since the early 1980s, when global surface networks started monitoring atmospheric CH4 mole fractions. We use these surface measurements, retrievals of column-averaged CH4 mole fractions from GOSAT, new wetland inundation estimates, and atmospheric δ13C-CH4 measurements to estimate the impact of this strong La Niña on the global atmospheric CH4 budget. By performing atmospheric inversions, we find evidence of an increase in tropical CH4 emissions of ∼6–9 TgCH4 yr−1 during this event. Stable isotope data suggest that biogenic sources are the cause of this emission increase. We find a simultaneous expansion of wetland area, driven by the excess precipitation over the Tropical continents during the La Niña. Two process-based wetland models predict increases in wetland area consistent with observationally-constrained values, but substantially smaller per-area CH4 emissions, highlighting the need for improvements in such models. Overall, tropical wetland emissions during the strong La Niña were at least by 5% larger than the long-term mean.

Comparative analysis of land, marine, and satellite observations of methane in the lower Atmosphere in the Russian Arctic under conditions of climate change

Land, marine, and satellite observations have been used to study changes in methane concentrations in the lower atmosphere during the warm months of the year (July through October) in Arctic regions having different potentials for methane production. The Atmospheric Infrared Sounder (AIRS) data for 2002–2013 are used to explore the interplay between local methane sources in the terrestrial region of the Eurasian Arctic and on the Arctic shelf over the warm period of the year. Linear trends in atmospheric methane concentrations over different Arctic regions are calculated, and a hypothesis of the relation of concentration variations to climatic parameters is tested. The combination of land, marine, and satellite observation is used to develop a conceptual model of the atmospheric methane field in the terrestrial part of the Russian Arctic and on the Arctic shelf. It is shown that the modern methane growth rate in the Arctic does not exceed the Northern Hemisphere mean. It is concluded that the methane emission in the Arctic has little effect on global climate compared to other factors.

Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons

Abstract. The short-chain non-methane hydrocarbons (NMHC) are mostly emitted into the atmosphere by anthropogenic processes. Recent studies have pointed out a tight linkage between the atmospheric mole fractions of the NMHC ethane and the atmospheric growth rate of methane. Consequently, atmospheric NMHC are valuable indicators for tracking changes in anthropogenic emissions, photochemical ozone production, and greenhouse gases. This study investigates the 1950–2010 Northern Hemisphere atmospheric C2–C5 NMHC ethane, propane, i-butane, n-butane, i-pentane, and n-pentane by (a) reconstructing atmospheric mole fractions of these trace gases using firn air extracted from three boreholes in 2008 and 2009 at the North Greenland Eemian Ice Drilling (NEEM) site and applying state-of-the-art models of trace gas transport in firn, and by (b) considering eight years of ambient NMHC monitoring data from five Arctic sites within the NOAA Global Monitoring Division (GMD) Cooperative Air Sampling Network. Results indicate that these NMHC increased by ~40–120% after 1950, peaked around 1980 (with the exception of ethane, which peaked approximately 10 yr earlier), and have since dramatically decreased to be now back close to 1950 levels. The earlier peak time of ethane vs. the C3–C5 NMHC suggests that different processes and emissions mitigation measures contributed to the decline in these NMHC. The 60 yr record also illustrates notable increases in the ratios of the isomeric iso-/n-butane and iso-/n-pentane ratios. Comparison of the reconstructed NMHC histories with 1950–2000 volatile organic compounds (VOC) emissions data and with other recently published ethane trend analyses from ambient air Pacific transect data showed (a) better agreement with North America and Western Europe emissions than with total Northern Hemisphere emissions data, and (b) better agreement with other Greenland firn air data NMHC history reconstructions than with the Pacific region trends. These analyses emphasize that for NMHC, having atmospheric lifetimes on the order of < 2 months, the Greenland firn air records are primarily a representation of Western Europe and North America emission histories.

Simulation of Spherically Expanding Turbulent Premixed Flames

Statistically spherical expanding turbulent premixed flames are computed using an unsteady Reynolds-averaged Navier–Stokes (URANS) approach. Mean reaction rate is closed using strained and unstrained flamelet models and an algebraic model. The flamelets are parametrized using the scalar dissipation rate in the strained flamelet model. It is shown that this model is able to capture the measured growth rate of methane–air turbulent flame ball, which is free of thermo–diffusive instability. The spherical flames are observed to accelerate continuously. The flame brush thickness grows in time and the role of turbulent diffusion on this growth seems secondary compared to the convection due to the fluid velocity induced by the chemical reaction. The spherical flames have larger turbulent flame speed, the leading-edge displacement speed st, compared to the planar flames for a given turbulence and thermochemical condition. The computational results suggest with 0.57 ≤ n ≤ 0.58, where Re t is the turbulence Reynolds number and is the unstrained planar laminar flame speed, for both spherical and planar flames.

Methane–Propane Mixed Gas Hydrate Film Growth on the Surface of Water and Luvicap EG Solutions

We studied the growth rate of methane–propane mixed gas hydrate films on the surface of water and up to 3 wt % aqueous solutions of a kinetic hydrate inhibitor, Luvicap EG. The greater the subcooling, the faster the growth rate of the hydrate film on water. Using Mori’s heat-transfer-limited growth model, we estimated the initial thickness of the hydrate films on water to be between 1.3 and 25 μm. The addition of Luvicap EG inhibited both the nucleation and the growth of the hydrate films, because we observed that the films on Luvicap EG solutions grew slower and their nucleation required greater subcoolings than those on water. Notably, the hydrate films that grew on Luvicap EG solutions were more transparent and, thus, thinner than those on water. The thinner and yet more slowly growing hydrate films on Luvicap EG solutions suggest that the growth mechanisms had changed from heat-transfer-limited growth to mass-transfer-limited growth.

Analysis of global methane changes after the 1991 Pinatubo volcanic eruption

Abstract. The global methane (CH4) growth rate showed large variations after the eruption of Mount Pinatubo in June 1991. Both sources and sinks of tropospheric CH4 were altered following the eruption, by feedback processes between climate and tropospheric photochemistry. Such processes include Ultra Violet (UV) radiative changes due to the presence of volcanic sulfur dioxide (SO2) and sulphate aerosols in the stratosphere, and due to stratospheric ozone depletion. Changes in temperature and water vapour in the following years caused changes in tropospheric chemistry, as well as in natural emissions. We present a sensitivity study that investigates the relative effects that these processes had on tropospheric CH4 concentrations, using a simple one-dimensional chemistry model representative for the global tropospheric column. To infer the changes in UV radiative fluxes, the chemistry model is coupled to a radiative transfer model. We find that the overall effect of natural processes after the eruption on the CH4 growth rate is dominated by the reduction in CH4 lifetime due to stratospheric ozone depletion. However, all the other processes are found to have non-negligible effects, and should therefore be taken into account in order to obtain a good estimate of CH4 concentrations after Pinatubo. We find that the overall effect was a small initial increase in the CH4 growth rate after the eruption, followed by a decrease of about 7 ppb yr−1 by mid-1993. When changes in anthropogenic emissions are employed according to emission inventories, an additional decrease of about 5 ppb yr−1 in the CH4 growth rate is obtained between the years 1991 and 1993. The results using the simplified single column model are in good qualitative agreement with observed changes in the CH4 growth rate. Further analysis, taking into account changes in the dynamics of the atmosphere, variations in emissions from biomass burning, and in biogenic emissions of non-methane volatile organic compounds (NMVOC), requires the use of a full three-dimensional model.

The Impact of Uncertainties in African Biomass Burning Emission Estimates on Modeling Global Air Quality, Long Range Transport and Tropospheric Chemical Lifetimes

The chemical composition of the troposphere in the tropics and Southern Hemisphere (SH) is significantly influenced by gaseous emissions released from African biomass burning (BB). Here we investigate how various emission estimates given in bottom-up BB inventories (GFEDv2, GFEDv3, AMMABB) affect simulations of global tropospheric composition using the TM4 chemistry transport model. The application of various model parameterizations for introducing such emissions is also investigated. There are perturbations in near-surface ozone (O3) and carbon monoxide (CO) of ~60–90% in the tropics and ~5–10% in the SH between different inventories. Increasing the update frequency of the temporal distribution to eight days generally results in decreases of between ~5 and 10% in near-surface mixing ratios throughout the tropics, which is larger than the influence of increasing the injection heights at which BB emissions are introduced. There are also associated differences in the long range transport of pollutants throughout the SH, where the composition of the free troposphere in the SH is sensitive to the chosen BB inventory. Analysis of the chemical budget terms reveals that the influence of increasing the tropospheric CO burden due to BB on oxidative capacity of the troposphere is mitigated by the associated increase in NOx emissions (and thus O3) with the variations in the CO/N ratio between inventories being low. For all inventories there is a decrease in the tropospheric chemical lifetime of methane of between 0.4 and 0.8% regardless of the CO emitted from African BB. This has implications for assessing the effect of inter-annual variability in BB on the annual growth rate of methane.

Interpreting methane variations in the past two decades using measurements of CH<sub>4</sub> mixing ratio and isotopic composition

Abstract. The availability δ13C-CH4 measurements from atmospheric samples has significantly improved in recent years, which allows the construction of time series spanning up to about 2 decades. We have used these measurements to investigate the cause of the methane growth rate decline since 1980, with a special focus on the period 1998–2006 when the methane growth came to a halt. The constraints provided by the CH4 and δ13C-CH4 measurements are used to construct hypothetical source and sink scenarios, which are translated into corresponding atmospheric concentrations using the atmospheric transport model TM3 for evaluation against the measurements. The base scenario, composed of anthropogenic emissions according to EDGAR 4.0, constant emissions from natural sources, and a constant atmospheric lifetime, overestimates the observed global growth rates of CH4 and δ13C-CH4 by, respectively, 10 ppb yr−1 and 0.02‰ yr−1 after the year 2000. It proves difficult to repair this inconsistency by modifying trends in emissions only, notably because a temporary reduction of isotopically light sources, such as natural wetlands, leads to a further increase of δ13C-CH4. Furthermore, our results are difficult to reconcile with the estimated increase of 5 Tg CH4 yr−1 in emissions from fossil fuel use in the period 2000–2005. On the other hand, we find that a moderate (less than 5% per decade) increase in the global OH concentration can bring the model in agreement with the measurements for plausible emission scenarios. This study demonstrates the value of global monitoring of methane isotopes, and calls for further investigation into the role OH and anthropogenic emissions to further improve our understanding of methane variations in recent years.

Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air

Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH(4)) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C(2)H(6)) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20 years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14-16 teragrams per year (1 Tg = 10(12) g) and dropped to 8-10 Tg yr(-1) by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane.

Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources

Atmospheric methane (CH(4)) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium, after which levels increased once more. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors. Here we show that the late-twentieth-century changes in the CH(4) growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH(4) mixing and (13)C/(12)C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of (13)C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The (13)C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH(4) emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application and reductions in water use.

Effects of Climate-induced Changes in Isoprene Emissions after the eruption of Mount Pinatubo

The eruption of Mt. Pinatubo in June 1991 was the largest in the twentieth century. One of its effects was to produce cooler and drier conditions in the years following the eruption. We present the results of an integrated model study of the effect of these climatic changes on the emissions of isoprene from the biosphere. Our emissions model simulations showed that global isoprene emissions were reduced by 9% from 1990 to 1992. When incorporated into our model of global atmospheric chemistry this reduction of isoprene emissions led to an increase in the tropospheric OH burden of 2%. This caused an increase in the removal of methane via oxidation by OH of up to 5 Tg per year. This could have contributed to the observed changes in methane growth rate at this time.

Growth Rate, Seasonal, Synoptic, Diurnal Variations and Budget of Methane in the Lower Atmosphere

We have used an AGCM (atmospheric general circulation model)-based Chemistry Transport Model (ACTM) for the simulation of methane (CH4) in the height range of earth’s surface to about 90 km. The model simulations are compared with measurements at hourly, daily, monthly and interannual time scales by filtering or averaging all the timeseries appropriately. From this model-observation comparison, we conclude that the recent (1990-2006) trends in growth rate and seasonal cycle at most measurement sites can be fairly successfully modeled by using existing knowledge of CH4 flux trends and seasonality. A large part of the interannual variability (IAV) in CH4 growth rate is apparently controlled by IAV in atmospheric dynamics at the tropical sites and forest fires in the high latitude sites. The flux amplitudes are optimized with respect to the available hydroxyl radical (OH) distribution and model transport for successful reproduction of latitudinal and longitudinal distribution of observed CH4 mixing ratio at the earth’s surface. Estimated atmospheric CH4 lifetime in this setup is 8.6 years. We found a small impact (less than 0.5 ppb integrated over 1 year) of OH diurnal variation, due to temperature dependence of reaction rate coefficient, on CH4 simulation compared to the transport related variability (order of ±15 ppb at interannual timescales). Model-observation comparisons of seasonal cycles, synoptic variations and diurnal cycles are shown to be useful for validating regional flux distribution patterns and strengths. Our results, based on two emission scenarios, suggest reduced emissions from temperate and tropical Asia region (by 13, 5, 3 Tg-CH4 for India, China and Indonesia, respectively), and compensating increase (by 9, 9, 3 Tg-CH4 for Russia, United States and Canada, respectively) in the boreal Northern Hemisphere (NH) are required for improved model-observation agreement.

Atmospheric methane in the Mediterranean: Analysis of measurements at the island of Lampedusa during 1995–2005

Measurements of atmospheric methane and other greenhouse gases are routinely carried out at the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean Sea. The CH 4 mixing ratio record obtained during the period June 1995–September 2005 shows a distinct annual cycle characterized by a maximum in March and a minimum in late summer. The cycle peak-to-peak amplitude is about 30 ppb. In the period of investigation the CH 4 growth rate (GR) shows two positive peaks, the first in 1998 with 19 ppb yr −1, and the second at the end of 2001 with 14 ppb yr −1. A sharp minimum of −13 ppb yr −1 occurs in April 2000. The GR remains close to zero after mid-2002. The evolution of methane GR and global temperature anomaly appears well correlated ( R=0.71). The methane removal by reaction with OH, acting both on a global and on smaller scales, seems to play a large role. Significant differences exist among the GR at stations located within or close to the Mediterranean basin, indicating that regional and smaller scale processes must be taken into account to explain the methane evolution. The weekly measurements have been combined with backward airmass trajectories to study the relationship between CH 4 mixing ratio and transport. On average, the methane mixing ratio in airmasses from Eastern Europe is about 8 ppb higher than in airmasses from Western Europe, 15 ppb higher than in airmasses from oceanic regions, and about 20 ppb higher than in airmasses from Africa. Among the airmasses from Africa, those from Northern Algeria display the largest CH 4 mixing ratio, possibly because of emissions from gas and oil production and leakage from the gas pipelines. A North-to-South latitude gradient in CH 4 mixing ratio is also apparent.

Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

During the past 15 years the annual growth rate of tropospheric methane (CH4) has shown striking changes over 2–3 year periods, varying from +1% yr−1 to slightly negative values (−0.2% yr−1). These fluctuations are superimposed on an overall slowdown of the CH4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH4 pulses in 1998 and 2002–2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large‐scale biomass burning. We also report the seventh year of near‐zero growth of global CH4 levels (Dec. 1998–Dec. 2005). The global CH4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH4 increases of 118–376 ppbv between 2000–2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely.

Contribution of anthropogenic and natural sources to atmospheric methane variability

Methane is an important greenhouse gas, and its atmospheric concentration has nearly tripled since pre-industrial times. The growth rate of atmospheric methane is determined by the balance between surface emissions and photochemical destruction by the hydroxyl radical, the major atmospheric oxidant. Remarkably, this growth rate has decreased markedly since the early 1990s, and the level of methane has remained relatively constant since 1999, leading to a downward revision of its projected influence on global temperatures. Large fluctuations in the growth rate of atmospheric methane are also observed from one year to the next, but their causes remain uncertain. Here we quantify the processes that controlled variations in methane emissions between 1984 and 2003 using an inversion model of atmospheric transport and chemistry. Our results indicate that wetland emissions dominated the inter-annual variability of methane sources, whereas fire emissions played a smaller role, except during the 1997-1998 El Niño event. These top-down estimates of changes in wetland and fire emissions are in good agreement with independent estimates based on remote sensing information and biogeochemical models. On longer timescales, our results show that the decrease in atmospheric methane growth during the 1990s was caused by a decline in anthropogenic emissions. Since 1999, however, they indicate that anthropogenic emissions of methane have risen again. The effect of this increase on the growth rate of atmospheric methane has been masked by a coincident decrease in wetland emissions, but atmospheric methane levels may increase in the near future if wetland emissions return to their mean 1990s levels.

A 3‐D model analysis of the slowdown and interannual variability in the methane growth rate from 1988 to 1997

Methane has exhibited significant interannual variability with a slowdown in its growth rate beginning in the 1980s. We use a 3‐D chemical transport model accounting for interannually varying emissions, transport, and sinks to analyze trends in CH4 from 1988 to 1997. Variations in CH4 sources were based on meteorological and country‐level socioeconomic data. An inverse method was used to optimize the strengths of sources and sinks for a base year, 1994. We present a best‐guess budget along with sensitivity tests. The analysis suggests that the sum of emissions from animals, fossil fuels, landfills, and wastewater estimated using Intergovernmental Panel on Climate Change default methodology is too high. Recent bottom‐up estimates of the source from rice paddies appear to be too low. Previous top‐down estimates of emissions from wetlands may be a factor of 2 higher than bottom‐up estimates because of possible overestimates of OH. The model captures the general decrease in the CH4 growth rate observed from 1988 to 1997 and the anomalously low growth rates during 1992–1993. The slowdown in the growth rate is attributed to a combination of slower growth of sources and increases in OH. The economic downturn in the former Soviet Union and Eastern Europe made a significant contribution to the decrease in the growth rate of emissions. The 1992–1993 anomaly can be explained by fluctuations in wetland emissions and OH after the eruption of Mount Pinatubo. The results suggest that the recent slowdown of CH4 may be temporary.

The impact of meteorology on the interannual growth rate of atmospheric methane

The impact of interannual changes in meteorology on the local and global growth rates of atmospheric methane is assessed in a nineteen year simulation using a tropospheric chemical transport model forced by ECMWF meteorological analyses from 1980 to 1998. A very simple CH4 chemistry scheme has been implemented, using prescribed OH fields. There are no interannual variations in modeled methane emissions or in the OH fields, so any changes in the modeled growth rate arise from changes in meteorology. The methane simulation shows significant interannual variability at both local and global scales. The local scale variability is comparable in magnitude to the interannual variability found in surface observations and shows some clear correlation with observed changes in growth rates. This suggests that, even over interannual timescales, meteorology could be important in driving the interannual fluctuations of atmospheric methane at the surface.

Interannual variability in methane growth rate simulated with a coupled Ocean‐Atmosphere‐Chemistry model

We assess the contribution made to the interannual variability of the global methane accumulation rate from its atmospheric sink using the STOCHEM tropospheric chemistry model coupled to the HadCM3 climate model. For both control and climate change scenarios, the standard deviation of the detrended accumulation rate was 1.4 ppbv/yr for the period 1990–2009, compared with the measured standard deviation of 3.1 ppbv/yr for the period 1984–1999. As the model emissions have no variability, the methane sink processes in the model are responsible for all the simulated variability of the methane accumulation rate. This appears to explain a significant fraction of the observed variability and was well correlated with simulated water vapour. The largest component of the model interannual variability is derived from the El‐Niño Southern Oscillation cycle in the coupled Ocean‐Atmosphere model, and this mode of variation is shown to be present in the methane accumulation rate.

Implications of the recent fluctuations in the growth rate of tropospheric methane

Global measurements show that the mixing ratio of tropospheric methane (CH4) increased by 1.1% (19.5 ± 1.7 ppbv) over the five‐year period 1996–2000, with striking fluctuations in its annual growth rate. Whereas the global CH4 growth rate reached 15.9 ± 0.7 ppbv yr−1 in 1998, the growth rate was −2.1 ± 0.8 ppbv yr−1 in 2000. This is the first time in our 23‐year global monitoring program that we have measured a negative annual CH4 growth rate. The CH4 growth rate fluctuates in an unpredictable fashion, and we reemphasize that global CH4 concentrations cannot be extrapolated into the future based on past trends. As a result, we suggest that the slowing of the CH4 growth rate during much of the 1980s and 1990s cannot be used to imply that CH4 will no longer be of concern in greenhouse gas studies during this century.

Modeling modern methane emissions from natural wetlands: 2. Interannual variations 1982–1993

A global run of a process‐based methane model [Walter et al., this issue] is performed using high‐frequency atmospheric forcing fields from the European Centre for Medium‐Range Weather Forecasts (ECMWF) reanalyses of the period from 1982 to 1993. Modeled methane emissions show high regional, seasonal, and interannual variability. Seasonal cycles of methane emissions are dominated by temperature in high‐latitude wetlands, and by changes in the water table in tropical wetlands. Sensitivity tests show that globally, ±1°C changes in temperature lead to ±20% changes in methane emissions from wetlands. Uniform changes of ±20% in precipitation alter methane emissions by about ±8%. Limitations in the model are analyzed and the effects of sub‐grid‐scale variations in model parameters and errors in the input data are examined. Simulated interannual variations in methane emissions from wetlands are compared to observed atmospheric growth rate anomalies. Our model simulation results suggest that contributions from sources other than wetlands and/or the sinks are more important in the tropics than north of 30°N. In high northern latitudes it seems that a large part of the observed interannual variations can be explained by variations in wetland emissions. Our results also suggest that reduced wetland emissions played an important role in the observed negative methane growth rate anomaly in 1992.