Web pages of earlier CarbonTracker CH₄ releases are available below. All pages of older version include a RED BANNER and a warning at the top of the page with a link to the current CarbonTracker CH₄

Version	Start Time	End Time
Most current products
CarbonTracker-CH4-2023	01-Jan-2000	31-Dec-2022
Older products
CarbonTracker-CH4-2012	01-Jan-2000	31-Dec-2009

Release Notes for CarbonTracker-CH₄ 2023

1. Product
   • Released on September 30, 2023
   • CH₄ flux estimates are available from 1997 through 2021
   • Comprehensive documentation
2. Observations
   • We use measurements of air samples of CH₄ and its stable isotope ratios (δ¹³C-CH₄) from NOAA/INSTAAR (Institute of Arctic and Alpine Research of University of Colorado Boulder) with another 29 research laboratories around the world. More information on the harmonized data that we used for this version of the CarbonTracker-CH₄ is available here: https://gml.noaa.gov/ccgg/arc/?id=166
   • Detailed information on setting the model-data mismatch (MDM) and selection of assimilated data is available on the Documentation page.
3. Prior Flux Modules
   • This release of CarbonTracker-CH₄ uses the emissions from EDGAR 4.3.2 as prior emission estimates for emissions from anthropogenic fossil fuel and microbial sources. We also use the wetland emissions and soil oxidation of CH₄ from a process-based model, a Terrestrial Ecosystem Model (TEM), that uses static wetland distribution (Liu et al. 2020). We use static natural fossil emissions reported by Etiope et al. 2019. For biomass and biofuel burning emissions of this CarbonTracker-CH₄ release, we use the Global Fire Emission Database (GFED) 4.1s for 1997–2021 (Van Der Werf et al. 2017). The stable carbon isotopic signatures of different source sectors are primarily from (Sherwood et al. 2021). More information on the prior setup can be found in Section A on the Documentation page.
   • We spun up our model for 14 years from 1984 to 1997, and selected 1997–2021 as the analysis period. To avoid the initial condition artifacts for δ¹³C-CH₄, we run a global mass balance, which can be found in Section A4 on the Documentation page.
4. Photochemical Loss Module
   • For the present version of CarbonTracker-CH₄, monthly climatological CH₄ loss rates in the stratosphere due to OH, Cl, and O(1D) are constructed from a run of the ECHAM5/MESSy1 chemistry transport model (Jöckel et al. 2006). Loss due to tropospheric Cl is simulated using a recent model-derived estimate (Hossaini et al. 2016). For tropospheric OH, we use the monthly OH climatology of (Spivakovsky et al. 2000) after scaling by 0.9 to match the declining atmospheric abundance of methyl chloroform in the early 2000s (Montzka et al. 2011; Basu et al. 2022). The result in a methane lifetime is about 9 years in our sink setup. The current version of CarbonTracker-CH₄ does not attempt to adjust the global photochemical data by assimilation of the methane observations.
5. Data Assimilation and Transport Modeling
   • For use in TM5, the ECMWF meteorological data are preprocessed into mass fluxes at coarser horizontal and vertical resolutions. In this release of CarbonTracker-CH₄, TM5 is run at a global 3°x 2° spatial resolution and 30-minute temporal resolution. The vertical resolution of TM5 in CarbonTracker-CH₄ is 25 hybrid sigma-pressure levels.
   • We use the TM5-4DVAR inversion framework (Meirink et al. 2008), and two tracers are simulated in our inversion: total CH₄ and the artificial tracer that multiplies total CH₄ and its stable isotope ratio (¹³CH₄). From the inversion, we optimize monthly surface fluxes for microbial, fossil, and pyrogenic sources at 3°x 2° spatial resolution.

Release Notes for CarbonTracker-CH₄ 2012

1. Product
   • Released on June 1, 2012
   • CH₄ flux estimates are available from 2000 through 2009
   • Comprehensive documentation
2. Observations
   • 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):
3. 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.
4. 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 9 1/2 years. The current version of CarbonTracker-CH4 does not attempt to adjust the global photochemical data by assimilation of the methane observations.
5. 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.