Field study of the emissions of methyl chloride and other halocarbons from biomass burning in Western Africa

A field study of trace gas emissions from biomass burning in Equatorial Africa gave methyl chloride emission ratios of 4.3×10^–5±0.8×10^–5 mol CH₃Cl/mol CO₂. Based on the global emission rates for CO₂ from biomass burning we estimate a range of 226–904×10⁹ g/y as global emission rate with a best estimate of 515×10⁹ g/y. This is somewhat lower than a previous estimate which has been based on laboratory studies. Nevertheless, our emission rate estimates correspond to 10–40% of the global turnover of methyl chloride and thus support the importance of biomass burning as methyl chloride source. The emission ratios for other halocarbons (CH₂Cl₂, CHCl₃, CCl₄, CH₃CCl₃, C₂HCl₃, C₂Cl₄, F-113) are lower. In general there seems to be a substantial decrease with increasing complexity of the compounds and number of halogen atoms. For dichloromethane biomass burning still contributes significantly to the total global budget and in the Southern Hemisphere biomass burning is probably the most important source for atmospheric dichloromethane. For the global budgets of other halocarbons biomass burning is of very limited relevance.     

AGAGE Observations of Methyl Bromide and Methyl Chloride at Mace Head, Ireland, and Cape Grim, Tasmania, 1998–2001

In situ AGAGE GC-MS measurements of methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) at Mace Head, Ireland and Cape Grim, Tasmania (1998–2001) reveal a complex pattern of sources. At Mace Head both gases have well-defined seasonal cycles with similar average annual decreases of 3.0% yr⁻¹ (CH₃Br) and 2.6% yr⁻¹ (CH₃Cl), and mean northern hemisphere baseline mole fractions of 10.37 ± 0.05 ppt and 535.7 ± 2.2 ppt, respectively. We have used a Lagrangian dispersion model and local meteorological data to segregate the Mace Head observations into different source regions, and interpret the results in terms of the known sources and sinks of these two key halocarbons. At Cape Grim CH₃Br and CH₃Cl also show annual decreases in their baseline mixing ratios of 2.5% yr⁻¹ and 1.5% yr⁻¹, respectively. Mean baseline mole fractions were 7.94 ± 0.03 ppt (CH₃Br) and 541.3 ± 1.1 ppt (CH₃Cl). Although CH₃Cl has astrong seasonal cycle there is no well-defined seasonal cycle in the Cape Grim CH₃Br record. The fact that both gases are steadily decreasing in the atmosphere at both locations implies that a change has occurred which is affecting a common, major source of both gases (possibly biomass burning) and/or their major sink process (destruction by hydroxyl radical).     

Identification of Regional Sources of Methyl Bromide and Methyl Iodide from AGAGE Observations at Cape Grim,Tasmania

There are large uncertainties in identifying and quantifying the globally significant sources and sinks of methyl bromide (CH₃Br) and methyl iodide (CH₃I). Long-term, quasi-continuous observations can provide valuable information about their regional sources, which may be significant in the global context. We report 3 years of in situobservations of these trace gases from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41 °S, 145 °E). The average background levels of CH₃Br and CH₃I during March 1998–March 2001 were 8.05 and 1.39 ppt (dry air mole fractions expressed in parts per 10¹²), respectively. The CH₃Br background data showed little seasonal variability. Trajectory analyses reveal that air masses showing elevated CH₃Br levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater and/or urban source regions. The CH₃I background data showed a seasonal cycle with a 3-year average amplitude of 0.47 ppt and maximum concentrations in summer, suggesting that the Southern Ocean is a significant source.Trajectory analyses reveal that air masses showing highly elevated CH₃I levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater regions and/or the open-ocean regions of Bass Strait and the Tasman Sea.     

Atmospheric methane budget in Africa

This assessment of the atmospheric methane budget for the African Continent is based on a set of experimental data obtained in tropical Africa including methane emission from various biogenic sources, and biomass burning, and methane consumption in savanna and forest soils. Emission rates from the various sources, uptake rates of soils, and complementary data from the litterature allow calculation of regional methane fluxes by means of different data bases. Biomass burning, animals and natural wetlands are the three dominant sources of methane in Africa while rice paddy fields and termites appear as minor sources. The total methane emission is estimated to be in the range 20–40 MT of CH₄ per year, methane uptake by soils being less than 2 MT per year. Net methane emission from the African continent accounts for less than 10% of global emissions from terrestrial ecosystems.     

Isotopic Composition of Non-Methane Hydrocarbons in Emissions from Biomass Burning

The stable carbon isotope ratios of nonmethane hydrocarbons (NMHC) and methyl chloride emitted from biomass burning were determined by analyzing seven whole air samples collected during different phases of the burning process as part of a laboratory study of wood burning. The average of the stable carbon isotope ratios of emitted alkanes, alkenes and aromatic compounds is identical to that of the burnt fuel; more than 50% of the values are within a range of ±1.5 of thecomposition of the burnt fuel wood. Thus for the majority of NMHC emitted from biomass burning stable carbon isotope ratio of the burnt fuel a good first order approximation for the isotopic composition of the emissions. Of the more than twenty compounds we studied, only methyl chloride and ethyne differed in stable carbon isotope ratios by more than a few per mil from the composition of the fuel. Ethyne is enriched in ¹³C by approximately 20–30, and most of the variability can beexplained by a dependence on flame temperature. The ¹³C values decreaseby 0.019 /K (±0.0053/K) with increasing temperature. Methyl chloride is highly depleted in ¹³C, on average by25. However the results cover a wide range of nearly 30. Specifically, in two measurements with wood from Eucalyptus (Eucalyptus delegatensis) as fuel we observed the emission of extremely light methyl chloride (–68.5and–65.5). This coincides with higher than average emission ratiosfor methyl chloride (15.5 × 10^–5 and 18 ×10^–5 mol CH₃Cl/mol CO₂). These high emission ratios are consistent with the highchlorine content of the burnt fuel, although, due to the limited number of measurements, it would be premature to generalize these findings. The limited number of observations also prevents any conclusion on a systematic dependence between chlorine content of the fuel, emission ratios and stable carbon isotope ratio of methyl chloride emissions. However, our results show that a detailed understanding of the emissions of methyl chloride from chloride rich fuels is important for understanding its global budget. It is also evident that the usefulness of stable carbon isotope ratios to constrain the global budget of methyl chloride will be complicated by the very large variability of the stable carbon isotope ratio of biomass burning emissions. Nevertheless, ultimately the large fractionation may provide additional constraints for the contribution of biomass burning emissions to the atmospheric budget of methyl chloride.     

Automobile Emissions of Acetonitrile: Assessment of its Contribution to the Global Source

Based on an estimated global fuel consumption of 2.57 × 10¹⁵g(C) y^–1 and the assumption thatthe fossil fuel burned in Austria is globallyrepresentative, an upper limit of 0.021 (+150%, –50%)Tg y^–1 for global CH₃CN emission dueto fossil fuel burning was obtained from the relativeenhancement of the concentrations of toluene, benzene,and acetonitrile (methyl cyanide) during strong,short-term traffic pollution. This is less than 6% ofthe total global budget of CH₃CN, which is dominatedby an emission rate of 0.8 Tg y^–1 from biomassburning.     

Long-term effects of anthropogenic CO<Subscript>2</Subscript> emissions simulated with a complex earth system model

A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model, a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse of the deep water formation in the North Atlantic on the atmospheric CO₂ concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO₂ concentrations due to oceanic uptake and delayed response to increasing atmospheric CO₂ concentrations before.     

NUMERICAL SIMULATION OF ATMOSPHERIC METHANE TRENDS OVER THE LAST 150 YEARS*

A global two-dimensional chemistry model is developed to study long-term trends of CH₄ since industrial revolution.The sources of CH₄,CO and NO_x are parameterized as functions of latitude and time.With two long-term emission scenarios,long-term trends of CH₄ are simulated.The results have a good agreement with observation from ice cores.The modeled CH₄ increased from 760 ppbv in 1840 to 1611.9 ppbv in 1991, while the modeled number concentration of tropospheric OH decreased from 7.17×10⁵ cm^-3 in 1840 to 5.79×10⁵ cm^-3 in 1991.The increase of atmospheric CH₄ can be explained by the increase of emission of CH₄ and build-up because of decrease of OH radicals that remove CH₄ from the atmosphere.The model is also used to simulate the distribution of CH₄.Comparisons between the model results and observations show that the model can simulate both latitudinal distribution and seasonal variation of CH₄ well.     

Domestic Combustion of Biomass Fuels in Developing Countries: A Major Source of Atmospheric Pollutants

Biomass burning has important impacts on atmospheric chemistry and climate. Fires in tropical forests and savannas release large quantities of trace gases and particulate matter. Combustion of biofuels for cooking and heating constitutes a less spectacular but similarly widespread biomass burning activity. To provide the groundwork for a quantification of this source, we determined in rural Zimbabwe the emissions of CO₂, CO, and NO from more than 100 domestic fires fueled by wood, agricultural residues, and dung. The results indicate that, compared to open savanna fires, emissions from domestic fires are shifted towards products of incomplete combustion. A tentative global analysis shows that the source strength of domestic biomass burning is on the order of 1500 Tg CO₂–C yr^–1, 140 Tg CO–C yr^–1, and 2.5 Tg NO–N yr^–1. This represents contributions of about 7 to 20% to the global budget of these gases.     

Application of stable isotope analysis for improved understanding of the methane budget: comparison of TROICA measurements with TM3 model simulations

Presented is a detailed comparison of CH₄ and δ¹³C–CH₄ measurements with simulations of the global transport model TM3. Experimental data were obtained during campaigns along the Trans-Siberian railroad in the framework of the TROICA project. Two summer (1999 and 2001) and one spring (2003) expeditions are evaluated. Model simulations include sensitivity tests to further investigate the isotopic composition of natural gas and emissions from Siberian wetlands. Comparison of the average mixing ratio of methane and its isotopic composition (δ¹³C) has been performed for different geographic zones, including the European part of Russia, Western Siberia and Central Siberia. Simulations are in reasonable agreement with the measurements for the European part of Russia and confirm a high contribution of natural gas to the observed methane levels. An increase of emission from bogs shifts the simulated methane isotopic composition closer to the observations. The relative importance of the Western Siberia emissions in current inventories is underestimated in comparison with other wetland regions in the former USSR. Simulated average mixing ratios are in a good agreement with the observations in Central Siberia, while ¹³C(CH₄) values tend to be higher than measured in all considered scenarios. These results point to a bias in the modeled source mixture over Russia, which could be repaired by shifting emissions from isotopically heavy methane sources (e.g. coal, oil or biomass burning) to light sources (e.g. wetlands, ruminants, waste treatment). Alternatively, the average isotopic signature of Siberian wetlands may be lighter than expected.     

我国甲烷排放情景分析:IPAC模型结果

近期发布的IPCC第六次评估报告再次强调了短寿命期温室气体减排对温升减缓的作用。甲烷是最重要的短寿命期非CO₂温室气体。在各国提出各自新的减排目标之后,针对甲烷减排的行动方案也越来越多。甲烷减排正在成为下一阶段各国和全球合作的重点领域之一。本文在我国碳减排目标下的能源转型基础上,结合其他非能源活动的减排排放源的减排技术选择基础上,利用IPAC模型对未来甲烷的排放情景进行了分析。在模型设定的两个情景分析基础之上,研究发现,到2050年的能源转型可明显减少能源活动的甲烷排放,和2015年相比能源活动的排放可减少67％。和其他行业相比,能源部门的甲烷减排具有更好的协同性。如果考虑进一步减排甲烷,则需要在考虑其他大气污染物减排的基础上,可通过实现天然气的进一步减排来实现。同时其他部门的甲烷减排也具有很大潜力,低甲烷排放情景可以实现到2050年将甲烷排放减少到1 494万吨,和2015年相比全范围排放可减排58％。     

Impacts of future Indian greenhouse gas emission scenarios on projected climate change parameters deduced from MAGICC model

The MAGICC (Model for the Assessment of Greenhouse gas Induced Climate Change) model simulation has been carried out for the 2000–2100 period to investigate the impacts of future Indian greenhouse gas emission scenarios on the atmospheric concentrations of carbon dioxide, methane and nitrous oxide besides other parameters like radiative forcing and temperature. For this purpose, the default global GHG (Greenhouse Gases) inventory was modified by incorporation of Indian GHG emission inventories which have been developed using three different approaches namely (a) Business-As-Usual (BAU) approach, (b) Best Case Scenario (BCS) approach and (c) Economy approach (involving the country’s GDP). The model outputs obtained using these modified GHG inventories are compared with various default model scenarios such as A1B, A2, B1, B2 scenarios of AIM (Asia-Pacific Integrated Model) and P50 scenario (median of 35 scenarios given in MAGICC). The differences in the range of output values for the default case scenarios (i.e., using the GHG inventories built into the model) vis-à-vis modified approach which incorporated India-specific emission inventories for AIM and P50 are quite appreciable for most of the modeled parameters. A reduction of 7% and 9% in global carbon dioxide (CO₂) emissions has been observed respectively for the years 2050 and 2100. Global methane (CH₄) and global nitrous oxide (N₂O) emissions indicate a reduction of 13% and 15% respectively for 2100. Correspondingly, global concentrations of CO₂, CH₄ and N₂O are estimated to reduce by about 4%, 4% and 1% respectively. Radiative forcing of CO₂, CH₄ and N₂O indicate reductions of 6%, 14% and 4% respectively for the year 2100. Global annual mean temperature change (incorporating aerosol effects) gets reduced by 4% in 2100. Global annual mean temperature change reduces by 5% in 2100 when aerosol effects have been excluded. In addition to the above, the Indian contributions in global CO₂, CH₄ and N₂O emissions have also been assessed by India Excluded (IE) scenario. Indian contribution in global CO₂ emissions was observed in the range of 10%–26%, 6%–36% and 10%–38% respectively for BCS, Economy and BAU approaches, for the years 2020, 2050 and 2100 for P50, A1B-AIM, A2-AIM, B1-AIM & B2-AIM scenarios. CH₄ and N₂O emissions indicate about 4%–10% and 2%–3% contributions respectively in the global CH₄ and N₂O emissions for the years 2020, 2050 and 2100. These Indian GHG emissions have significant influence on global GHG concentrations and consequently on climate parameters like RF and ∆T. The study reflects not only the importance of Indian emissions in the global context but also underlines the need of incorporation of country specific GHG emissions in modeling to reduce uncertainties in simulation of climate change parameters.     

Carbon Sequestration in Arable Soils is Likely to Increase Nitrous Oxide Emissions,Offsetting Reductions in Climate Radiative Forcing

Strategies for mitigating the increasing concentration of carbon dioxide (CO₂) in the atmosphere include sequestering carbon (C) in soils and vegetation of terrestrial ecosystems. Carbon and nitrogen (N) move through terrestrial ecosystems in coupled biogeochemical cycles, and increasing C stocks in soils and vegetation will have an impact on the N cycle. We conducted simulations with a biogeochemical model to evaluate the impact of different cropland management strategies on the coupled cycles of C and N, with special emphasis on C-sequestration and emission of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O). Reduced tillage, enhanced crop residue incorporation, and farmyard manure application each increased soil C-sequestration, increased N₂O emissions, and had little effect on CH₄ uptake. Over 20 years, increases in N₂O emissions, which were converted into CO₂-equivalent emissions with 100-year global warming potential multipliers, offset 75–310% of the carbon sequestered, depending on the scenario. Quantification of these types of biogeochemical interactions must be incorporated into assessment frameworks and trading mechanisms to accurately evaluate the value of agricultural systems in strategies for climate protection.     

Boreal forest and tundra ecosystems as components of the climate system

The effects of terrestrial ecosystems on the climate system have received most attention in the tropics, where extensive deforestation and burning has altered atmospheric chemistry and land surface climatology. In this paper we examine the biophysical and biogeochemical effects of boreal forest and tundra ecosystems on atmospheric processes. Boreal forests and tundra have an important role in the global budgets of atmospheric CO₂ and CH₄. However, these biogeochemical interactions are climatically important only at long temporal scales, when terrestrial vegetation undergoes large geographic redistribution in response to climate change. In contrast, by masking the high albedo of snow and through the partitioning of net radiation into sensible and latent heat, boreal forests have a significant impact on the seasonal and annual climatology of much of the Northern Hemisphere. Experiments with the LSX land surface model and the GENESIS climate model show that the boreal forest decreases land surface albedo in the winter, warms surface air temperatures at all times of the year, and increases latent heat flux and atmospheric moisture at all times of the year compared to simulations in which the boreal forest is replaced with bare ground or tundra. These effects are greatest in arctic and sub-arctic regions, but extend to the tropics. This paper shows that land-atmosphere interactions are especially important in arctic and sub-arctic regions, resulting in a coupled system in which the geographic distribution of vegetation affects climate and vice versa. This coupling is most important over long time periods, when changes in the abundance and distribution of boreal forest and tundra ecosystems in response to climatic change influence climate through their carbon storage, albedo, and hydrologic feedbacks.     

Emission of aliphatic amines from animal husbandry and their reactions: Potential source of N2O and HCN

We measured the emissions of volatile aliphatic amines and ammonia produced by the manure of beef cattle, dairy cows, swine, laying hens and horses in livestock buildings. The amine emissions consisted almost exclusively of the three methylamines and correlated with those of ammonia. The molar emission ratios of the methylamines to ammonia, and data on NH₃ emissions from animal husbandry in Europe, together with global statistics on domestic animals, were used to estimate the global emissions of amines. Annual global methylamine-N input to the atmosphere from animal husbandry in 1988 was 0.15±0.06 TgN (Tg=10¹² g). Almost 3/4 of these emissions consisted of trimethylamine-N. This represents about half of all methylamine emissions to the atmosphere. Other sources are marine coastal waters and biomass burning.Possible reaction pathways for atmospheric methylamines are shown. Among various speculative but possible products N₂O and HCN are of interest because the emission of methylamines could contribute to the global budgets of these compounds. Maximum atmospheric N₂O production from methylamines are below 0.4 Tg N/year, which is less than 10% of the annual N₂O growth rate. Although we do not expect the methylamine emissions to contribute in a major way to the atmospheric N₂O budget, more studies are needed to establish this conclusion beyond doubt. Similar conclusions hold for HCN.     

Formulation and Evaluation of IMS, an Interactive Three-Dimensional Tropospheric Chemical Transport Model 2. Model Chemistry and Comparison of Modelled CH4, CO, and O3 with Surface Measurements

In part two of this series of papers on the IMS model, we present the chemistry reaction mechanism usedand compare modelled CH₄, CO, and O₃ witha dataset of annual surface measurements. The modelled monthly and 24-hour mean tropospheric OH concentrationsrange between 5–22 × 10⁵ moleculescm^–3, indicating an annualaveraged OH concentration of about 10 × 10⁵ moleculescm^–3. This valueis close to the estimated 9.7 ± 0.6 × 10⁵ moleculescm^–3 calculated fromthe reaction of CH₃CCl₃ with OH radicals.Comparison with CH₄ generally shows good agreementbetween model and measurements, except for the site at Barrow where modelledwetland emission in the summer could be a factor 3 too high.For CO, the pronounced seasonality shown in the measurements is generally reproduced by the model; however, the modelled concentrations are lower thanthe measurements. This discrepancy may due to lower the CO emission,especially from biomass burning,used in the model compared with other studies.For O₃, good agreement between the model and measurements is seenat locations which are away from industrial regions. The maximum discrepancies between modelled results and measurementsat tropical and remote marine sites is about 5–10 ppbv,while the discrepancies canexceed 30 ppbv in the industrial regions.Comparisons in rural areas at European and American continental sites arehighly influenced by the local photochemicalproduction, which is difficult to model with a coarse global CTM.The very large variations of O₃ at these locations vary from about15–25 ppbv in Januaryto 55–65 ppbv in July–August. The observed annual O₃amplitude isabout 40 ppbv compared with about 20 ppbv in the model. An overall comparison of modelled O₃ with measurements shows thatthe O₃seasonal surface cycle is generally governed bythe relative importance of two key mechanisms that drivea springtime ozone maximum and asummertime ozone maximum.     

Vegetation feedback under future global warming

It has been well documented that vegetation plays an important role in the climate system. However, vegetation is typically kept constant when climate models are used to project anthropogenic climate change under a range of emission scenarios in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios. Here, an atmospheric general circulation model, and an asynchronously coupled system of an atmospheric and an equilibrium terrestrial biosphere model are forced by monthly sea surface temperature and sea ice extent for the periods 2051?C2060 and 2090?C2098 as projected with 17 atmosphere?Cocean general circulation models participating in the IPCC Fourth Assessment Report, and by appropriate atmospheric carbon dioxide concentrations under the A2 emission scenario. The effects of vegetation feedback under future global warming are then investigated. It is found that the simulated composition and distribution of vegetation during 2051?C2060 (2090?C2098) differ greatly from the present, and global vegetation tends to become denser as expressed by a 21% (36%) increase in global mean leaf area index, which is most pronounced at the middle and high northern latitudes. Vegetation feedback has little effect on globally averaged surface temperature. On a regional scale, however, it induces statistically significant changes in surface temperature, in particular over most parts of continental Eurasia east of about 60°E where annual surface temperature is expected to increase by 0.1?C1.0?K, with an average of about 0.4?K for each future period. These changes can mostly be explained by changes in surface albedo resulting from vegetation changes in the context of future global warming.     

Production of N2O,CH4, and CO2 from soils in the tropical savanna during the dry season

Emissions of N₂O, CH₄, and CO₂ from soils at two sites in the tropical savanna of central Venezuela were determined during the dry season in February 1987. Measured arithmetic mean fluxes of N₂O, CH₄, and CO₂ from undisturbed soil plots to the atmosphere were 2.5×10⁹, 4.3×10¹⁰, and 3.0×10¹³ molecules cm^-2 s^-1, respectively. These fluxes were not significantly affected by burning the grass layer. Emissions of N₂O increased fourfold after simulated rainfall, suggesting that production of N₂O in savanna soils during the rainy season may be an important source for atmospheric N₂O. The CH₄ flux measurements indicate that these savanna soils were not a sink, but a small source, for atmospheric methane. Fluxes of CO₂ from savanna soils increased ninefold two hours after simulated rainfall, and remained three times higher than normal after 16 hours. More research is needed to clarify the significance of savannas in the global cycles of N₂O, CH₄, CO₂, and other trace gases, especially during the rainy season.     

Accounting for the missing carbon-sink with the CO2-fertilization effect

A terrestrial-biosphere carbon-sink has been included in global carbon-cycle models in order to reproduce past atmospheric CO₂, ¹³C and ¹⁴C concentrations. The sink is of large enough magnitude that its effect on projections of future CO₂ levels should not be ignored. However, the cause and mechanism of this sink are not well understood, contributing to uncertainty of projections. The estimated magnitude of the biospheric sink is examined with the aid of a global carbon-cycle model. For CO₂ emissions scenarios, model estimates are made of the resulting atmospheric CO₂ concentration. Next, the response of this model to CO₂-emission impulses is broken down to give the fractions of the impulse which reside in the atmosphere, oceans, and terrestrial biosphere - all as a perturbation to background atmospheric CO₂ concentration time-profiles that correspond to different emission scenarios. For a biospheric sink driven by the CO₂-fertilization effect, we find that the biospheric fraction reaches a maximum of roughly 30% about 50 years after the impulse, which is of the same size as the oceanic fraction at that time. The dependence of these results on emission scenario and the year of the impulse are reported.     

Methane,carbon monoxide and methylchloroform in the southern hemisphere

New observational data on CH₄, CO and CH₃CCl₃ in the southern hemisphere are reported. The data are analysed for long term trends and seasonal cycles. CH₃CCl₃ data are used to scale the OH fields incorporated in a two dimensional model, which in turn, is used to constrain the magnitude of a global CH₄ source function. The possible causes of observed seasonality of CH₃CCl₃, CH₄ and CO are identified, and several other aspects of observed CH₄ variability are discussed.Possible future research directions are also given.