CH4 flux estimates are available from 2000 through 2009
We use measurements of air samples collected at surface sites in the NOAA ESRL Cooperative Global Air Sampling Network except those identified as having analysis or sampling problems, or those thought to be influenced by local sources.
We use in situ quasi-continuous CH4 time series from 4 towers and 4 surface sites operated by Environment Canada (EC):
Prior Flux Modules
This release of CarbonTracker-CH4 uses the 1x1 degree gridded emissions from the EDGAR 3.2FT2000 as prior emission estimates for fugitive emissions from coal, oil and gas production as well as anthropogenic emissions from agriculture and waste. We have not extrapolated this data over the period covered by CarbonTracker, and have instead kept prior emission estimates constant at 2000 levels. This will allow us to test whether the assimilation is able to recover, for example, the large increase in emissions from coal production in Asia.
The current version of CarbonTracker-CH4 used the wetland prior flux estimates of Bergamaschi et al. (2007). The global total of the wetland prior flux estimate is 175 TgCH4/yr and we assume a prior flux uncertainty of 75%.
The soil sink of methane is based on the study of Ridgwell et al. (1999) and the termite and wild animal sources are from Sanderson (1996) and Houweling et al. (1999).
The fire module currently used in CarbonTracker is based on the Global Fire Emissions Database (GFED), which uses the CASA biogeochemical model as described in the documentation to estimate the carbon fuel in various biomass pools.
For the ocean fluxes in this version of CarbonTracker-CH4 we have used the estimates of Houweling et al., (1999) and Lambert and Schmidt (1993) as prior flux estimates.
Prior flux uncertainties are all assumed to be 75% of the value of the prior flux.
Photochemical Loss Module
For the present version of CarbonTracker-CH4 we use pre-calculated OH fields from a global photochemical model that have been optimized against global observations of methyl chloroform. The photochemical loss fields consist of a single, repeating seasonal cycle, and result in a methane lifetime of about 91/2 years. The current version of CarbonTracker-CH4 does not attempt to adjust the global photochemical data by assimilation of the methane observations.
Data Assimilation and Transport Modeling
CarbonTracker-CH4 estimates fluxes for 12 land regions and 1 global ocean region. Scaling parameters for 10 terrestrial source processes are estimated for each land region. These processes include fugitive emissions from coal, and oil and gas production, enteric fermentation (animals), waste, rice agriculture, wetlands, termites, wild animals, uptake in dry soils, and emissions from wildfires.
Source regions were based on CarbonTracker-CO2 and TransCom 3 source regions with the addition of an equatorial African region. Ocean fluxes are relatively small for CH4, so for this release we used one global ocean region was used.
500 ensemble members were found to give the best results for these simulations.
At the end of 2005, ECMWF changed the vertical resolution in its model from 60 to 91 layers. For use in the TM5 offline transport model, the 60-layer model results are reduced to 25 layers. Similarly, the 91-layer results are reduced to 34. As a result, the transport model has a 25-to-34 level discontinuity at the end of 2005. We have integrated over this transition by interpolating the CH4 fields to the higher resolution, using a mass-conserving algorithm.
For this first release of CarbonTracker-CH4, 6x4 deg. transport fields were used.