Global Atmospheric Methane Synthesis (GAMS)

Inaugural Workshop Report
Bologna, Monday 13 September 1999

Workshop conveners: Martin Manning and David Etheridge

The evolution of atmospheric methane bears on the fundamental questions posed in the GAIM PLAN (1998). An inaugural workshop for a new GAIM activity involving methane was held in conjunction with the 6th IGAC Scientific Conference in Bologna on 13 September, 1999, and attended by 33 scientists. Five task areas were considered, each being introduced by one or more experts and then discussed for approximately an hour. Some participants were able to meet subsequently during the IGAC conference and develop more specific ideas for work within the activity.

The results are summarised below first in terms of identified key issues and then in terms of the types of interaction that the GAMS project should promote. These will now be used to develop a more specific action plan with identified participants for specific activities.

1. Key issues and directions for future research

1.1 Atmospheric observations.

Global coverage (contemporary): Most in situ mixing ratio and isotopic measurements have been located at marine or clean air sites. An increased emphasis on measurements closer to major sources is now justified, and will be necessary to improve regional source estimation using model based inversion techniques.

Campaign data (contemporary): The potential for use of ad hoc data — e.g. ship or aircraft samples on tracks which are not repeated regularly — to determine sources using chemical tracer models needs to be clarified.

Calibration: The NOAA/CMDL GLOBALVIEW CH4 co-operative project merges recent mixing ratio data from several groups that are able to convert their measurements to a common nominal concentration scale. Other groups making such measurements, but not yet able to convert to the CMDL scale, should be encouraged to do so, as this will extend the global coverage of consistent data. A well validated absolute mixing ratio scale is required to ensure accuracy in long term trends.

Isotopic measurements: An increase in the availability of both contemporary and ice-core or firn air 13CH4/12CH4 data is expected in the near future. Laboratories making such measurements are urged to set in place on-going inter-comparison projects to ensure that their data can be merged usefully.

Ice core data corrections: Past atmospheric methane mixing ratios can now be reliably found from ice core and firn air. However, recovery of accurate isotope ratio values from ice-core and firn air requires further testing and validation of gravitational and diffusion corrections in air-ice enclosure models.

Ice core data coverage: Valuable extensions of existing data can come from measurements of air from equatorial ice sheets and improved temporal resolution during the Holocene, particularly in the northern hemisphere.

1.2. Emissions

Focus where uncertainties are largest: The greatest uncertainties in paleo methane sources are associated with wetland emissions. For contemporary sources the greatest uncertainties are associated with wetlands, rice paddies, biomass burning, and landfill emissions. All these sources are spatially diffuse and influenced by a variety of environmental factors. Uncertainties in other source types may be important in specific circumstances.

Process models: Process models have generally been developed and validated for regional or local scale emissions. Only a few process-model based estimates of wetland or rice emissions are available at the global scale and further global studies are required. Process models should also be used to estimate diurnal as well as seasonal cycles in emission flux. Spatially and temporally resolved estimates of uncertainty in emissions are useful in some inverse model approaches.

Data for process models: Use of process models is often constrained by lack of appropriate input data. Compilation of appropriate data, e.g. on soils, vegetation, hydrological factors and rice management practices, is required.

Regional validation: For some source types, validation of emission process models can be based on well designed atmospheric measurements and use of mesoscale transport models to perform bottom-up vs top-down comparisons.

Reassessment of source estimates: A critical assessment of the literature on methane sources may lead to rejection of some early estimates and tighter ranges than provided by generic assessments such as those of the IPCC. A suitable process for such an assessment needs to be considered. This needs to reflect the fact that methane sources are continually changing, as is evident for the 1990’s.

Interface with IGAC/GEIA: Current emission inventory estimates compiled for the GEIA project (Global Emissions Inventory Activity) will be used in GAMS and new results from this project will feed back to GEIA.

1.3. Chemistry

CH4 chemistry: Although detailed CH4 and OH chemistry schemes are available in the literature, these may not be complete. More work is needed to validate current views of feedbacks between CH4, CO, NOx, NMHCs and OH. Better estimates of the emissions of these associated species are needed. Some isotopic observations suggest that fractionation effects in CH4 chemistry are not well understood and this may indicate as yet unidentified atmospheric chemistry pathways.

Chemistry — climate interactions: Detailed studies of the interactions between global climate change and CH4 chemistry are needed, taking into account the full range of past changes (pre-industrial and glacial to inter-glacial conditions) and likely future climates. Global average OH levels and the distribution of OH between tropics and extra-tropics are the key factors required.

OH history and trends: Further clarification of the role of anthropogenic emissions of CO, NOx and related species on CH4 chemistry is required in order to study the evolution of the CH4 budget from pre-industrial time to the present and into the future. The present ambiguity in the sign of the current trend in OH needs to be resolved. Measurements of CH4 mixing ratios and isotopic ratios may place some constraints on estimates of OH levels and trends.

Stratosphere — troposphere exchange: Improved estimates are needed of net CH4 loss rates to the stratosphere and isotopic fractionation in the stratosphere. Important couplings also occur through production of stratospheric H2O by CH4 oxidation and the impact of stratospheric O3 depletion and UV irradiance increases on tropospheric chemistry.

1.4. Contemporary budgets

Inversion studies: The Bayesian or synthesis inversion technique needs good a priori source distributions, while the adjoint technique needs comprehensive atmospheric data. Better comparisons of these techniques are needed, as are better understanding of their sensitivity to selection of observation sites, deficiencies in model transport, a priori estimates, and OH distributions.

Improved model transport: Study of recent inversion results suggests that better boundary layer meteorology is required, particularly to explain observations near source regions. Extension of observations to surface continental sites will place greater demands on the accuracy of model transport and use of model results appropriate to actual sampling conditions.

Regional scale inversion studies: Use of mesoscale models or nested regional/ global models may provide valuable inversion results in some circumstances — e.g. where regional observational networks provide high spatial resolution.

Time dependent budgets: Most inversion results are for sources which do not change from year to year and for an atmosphere in equilibrium with these sources. Additional studies need to consider time-dependent inversions and the potential for disequilibrium conditions to bias source estimates. Recent work suggests that disequilibrium effects are particularly important when interpreting isotopic data.

1.5. Paleo budgets

Interface with PAGES: Global scale models for ecosystem and land-cover changes appropriate to the Holocene and earlier periods are becoming available. These need to be used in the GAMS project for paleo-budget studies of CH4. Data on past distributions and numbers of animals also appears to be available in greater detail than used so far. Pre-industrial anthropogenic sources need further consideration.

Interface with GAIM/TRACES. A related new GAIM activity that will be interacting with, and ultimately envelop GAMS, is TRACES (Trace Gas and Aerosol Cycles in the Earth System), our PaleoTrace Gas and Aerosol Challenge. TRACES is directed toward understanding the decadal to multi-millennial-scale regulation, feedback and forcing between chemical components of the atmosphere (CO2, N2O, CH4, other reactive gases, SO42- aerosols, mineral aerosols) over the past 150,000 years. A better understanding methane resulting from GAMS is a key component of TRACES.

Climate — source relationships: The response of natural sources to climate change needs to be investigated further over the full range of glacial to inter-glacial conditions, and during the Holocene. This is particularly true for wetland emissions where both the total magnitude and latitudinal distribution are expected to be central to paleo-budgets.

Chemistry — climate interactions: (see above)

Holocene variations: Large and sometimes rapid variations in CH4 mixing ratios over the Holocene period are evident from ice-core data. These are still not adequately explained.

Synthesis with related species: Co-variation of CH4 with CO2 and N2O and known associations between sources of these different gases should be used to construct more robust paleo-budgets.


2. Specific inter-disciplinary interactions to be promoted by GAMS

2.1. Observations and Inversion modeling.

This will link those responsible for inversion models and for observational networks. The aims will be to consider potential use of campaign data, improvements in network design for source determination, and issues involving time-dependent inversions.

2.2. Process models and CTM / Climate models.

This will link those responsible for emission process models and for chemical tracer and climate models. The aims will include: coupling of both types of models to investigate climate feedbacks; the significance of diurnal cycles in some types of methane emissions; and the combined effects of external forcing (e.g.. the environmental and chemical effects of UV changes caused by, for example, ozone depletion or aerosol increases).

2.3. Evolution of the CH4 budget.

This will link PAGES, those compiling paleo-budgets for CH4 and related species (TRACES), and those compiling contemporary budgets. The aim will be to develop consistent histories of emissions by source and region based on all available information.


3. Participants

Participants’ in the GAMS inaugural workshop are listed below:


Bergamaschi, Peter
Brenninkmeijer, Carl
Butler, Tim
Chappellaz, Jerome
Collins, Bill
Cunnold, Derek
de Noblet, Nathalie
Denier van der Gon, Hugo
Etheridge, David
Granier, Claire
Harrison, Sandy
Hein, Ralf
Houweling, Sander
Isaksen, Ivar
Jagovkina, Svetlana
Janssen, Leon
Koide, Takashi
Lagun, Victor or
Lassey, Keith
Levin, Ingeborg
Manning, Martin
Marik, Thomas
Martinerie, Patricia
Nisbet, Euan
Olivier, Jos
Peterson, Jim
Plantevin, Paul-Henri
Stevenson, David
Taguchi, Shoichi
Tsuruta, Haruo
Tyler, Stan
Walter, Bernadette
Wuebbles, Don