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CarboScope Release 4.1

The Methane Budget


Introduction

Atmospheric methane (CH4) is the third-most important present greenhouse gas (GHG) after water (H2O) and carbon dioxide (CO2). It is the second anthropogenic greenhouse gas after CO2 . As all greenhouse gas it traps longwave radiations in the atmosphere, contributing to make suitable conditions for life at the surface of our planet. CH4 variations played a major role in atmosphere history since its formation, possibly explaining the relative hot climate of the first 2 billion years of earth history and contributing the first large climate transition, called the Great Oxidation Event (GOE), approximately 2.4 billion years ago . Closer to present, atmospheric CH4 showed large variation during the glacial/interglacial cycles discovered in ice cores for the last 800000 years : CH4 is high in interglacial atmosphere (~700 ppb) and low in glacial atmosphere (~400 ppb), and varies in close relation to air temperature. The mechanisms explaining such variations are still discussed today ()

During the last 9000 years, methane oscillated between 550 and 700 ppb, and increased fastly since the middle of the XIXth century. Its mixing ratio was multiplied by a factor of 2.6 compared to preindustrial levels (Figure 1) , and reached almost 1800 ppb (1.8 ppm) today . While CH4 is 200 times less abundant than CO2, it is about 20 times more efficient than CO2 to trap outgoing longwave radiation, on a 50 years timescale . This makes the additional radiative forcing due to methane (~0.5 W/m2) around 1/3 of the one due to CO2 increase in the atmosphere. Furthermore, CH4 plays an important role in atmospheric chemistry, affecting the oxidizing capacity of the atmosphere and acting as a precursor of tropospheric ozone (O3). The mean atmospheric lifetime of CH4 is estimated to be 8.4 years on average [IPCC, 2001]. During le last 20 years, methane atmospheric growth rate, as measured by international atmospheric network, has dropped, but still presents large year to year variations, questioning our understanding of the methane budget and its evolution with time at different scales.





Figure 1: Global methane evolution in the atmosphere between 1984 and 2003.
Left : Global mixing ratio in ppb.
Right : Global growth rate in ppb/yr. Based on NOAA/ESRL observations, USA.



Methane sources and sinks

Methane is emitted at the earth surface by a variety of natural (~35%) and anthropogenic sources (~65%) . Three main paths exist to produce methane (Figure 2). The first process is a biogenic CH4 formation by methanogenic bacteria under anaerobic conditions. It occurs in natural wetlands, water-flooded rice paddies, landfills and stomachs of ruminant animals. In wetlands, CH4 is formed in anoxic waters and is prevented from being oxidised in CO2 either by rapid bubbling through water or by diffusion through rice aerenchyma (pipes). Estimates of the biogenic emissions are 92-237 TgCH4/yr for natural wetlands, 29-61 TgCH4/yr for rice cultivation, 100-135 TgCH4/yr for ruminant animals, termites and livestock manure management, 35-73 TgCH4/yr for landfills and waste waters (IPCC, 2001). The second path to produce CH4 is thermogenic formation, which is the main process for generation of natural gas deposits over geological time scales. Emissions related to fossil fuel extraction, processing, transportation, and distribution are estimated to be 75-110 TgCH4/yr. CH4 is also produced by incomplete burning of biomass with small emission factors of 3-4 g/kg on average (CO2=1600g/kg). Total biomass burning source of methane is estimated between 35 and 67 TgCH4/yr, including biofuel emissions. Finally, oceans also contribute the methane budget with a small source (10-25 TgCH4/yr) due to anaerobic digestion in marine zooplankton and fish, methanogenisis in sediments and drainage areas along coastal regions, and, possibly to release of trapped methane hydrates from ocean floor in continental margins. Recently (Keppler et al., 2006), a possible important source of methane due to aerobic emissions was found but is still controversial today (Dueck et al., 2007).



Figure 2: Annual methane emissions per category in TgCH4/yr


The different CH4 processes also present different compositions of the stable isotopes (?13C, ?D), providing isotopic fingerprints of the different source categories which can be utilized for further constraining the CH4 budget . According to inventories , spatial distribution of methane emissions show 2 maxima, one in the tropics and one at mid latitudes of the northern hemisphere (Figure 3). Overall, total emissions of methane are estimated to be 500-600 TgCH4/yr, with large uncertainties on individual processes and on their location and variability .

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Figure 3: Map of annual methane emissions.
Left: Zonal mean from inventories (thin line) and inverse results (solid line)
in TgCH4/deg latitude.
Right: Spatial distribution in TgCH4 per year and per model box.


The main sink of atmospheric methane is the reaction with hydroxyl radical (OH?) :
CH4 + OH? ---> CH3? + H2O
This reaction is the first of an oxidation chain leading to CO and CO2, and contributing northern hemisphere tropospheric ozone formation in presence of nitrogen oxids . The destruction of CH4 by OH in the troposphere represents about 90% of CH4 loss in the atmosphere . The rest of the sink (Figure 4) is due to an uptake of CH4 by soils, reaction with Cl in the marine boundary layer , and to destruction in the stratosphere by reactions with OH, Cl, and O(1D) .


Figure 4: Annual methane sinks per category in TgCH4/yr.
Global sinks represent 95% of methane emissions.


The production of OH radical in the troposphere is due to the reaction of excited atomic oxygen (O(1D)) produced by the photolysis of ozone (?<320nm) with water vapor. The global mean OH concentration is estimated to be around 10.105�molecules.cm-3 (8-12.105cm-3) , with large spatial, diurnal and seasonal variations depending mainly on available radiation, ozone and water vapor concentrations . As OH plays a major role in the removal of CH4, but also of other atmospheric trace gases such as CO, and NMHCs, quantification of annual global mean OH concentrations and their inter-annual to decadal changes are important. Such estimates are largely based on proxy methods, using trace gases (that react with OH) with relatively well known emissions. In particular methyl chloroform has been employed by several authors to infer OH fields based on different methodologies . There is no agreement yet on the variability of OH sink with time. Some authors find only small interannual variations of OH (Dentener et al.,2003) while some others infer large year-to-year variations of OH based on inversion of methyl-chloroform (MCF) observations (Prinn et al., 2001). Optimizing both MCF emissions and OH variability largely reduced OH variations (compared to previous studies), which appeared to be more compatible with CH4 cycle (Bousquet et al., 2006).




Estimating methane sources and sinks

A variety of approaches have been used to estimate surface emissions of CH4. They can be divided in ?bottom-up? and ?top-down? approaches. Bottom-up estimates generally use local measurements (e.g. wetlands, plants, landfills) to assess emissions factors (gCH4/m2/d), that are extrapolated or integrated in biogeochemical models in order to get global emissions for a given process . They can also be based on statistical and economical models (e.g. gas, coal). More recently, coupling satellite measurements and biogeochemical models has permitted to improve the estimates of location and strength of biomass burning emissions . Top-down estimates are based on measurements of atmospheric CH4 mixing ratios (including sometimes also measurements of its isotopic composition) and inverse atmospheric models. Mixing ratios of atmospheric CH4 have been directly monitored since 1983 by NOAA and other research organisations all over the world . More than 80 sites measuring methane on a weekly basis are available today. Some additional sites perform continuous measurements . Spatial CH4 gradients (and their changes with time) reflect regional sources and sinks of CH4, but also the action of atmospheric transport and chemistry. Optimizing surface emissions with atmospheric measurements using a chemical transport model (CTM) and prior estimates of sources and sinks is called a Bayesian inverse model. This top-down approach has been widely used to infer sources and sinks of CO2 and is increasingly used also for other trace gases. Several inverse modelling investigated global CH4 sources. These studies optimized emissions from larger global regions or from different source categories, and focussed on shorter timescales (1-2 years) or climatological averages . Some other studies focussed more on regional scales, such as European emissions (). A recent study by Bousquet et al. (2006) analysed 20 years of CH4 measurements to infer CH4 emissions amplitude and variations from 1984 to 2003.