Understanding Tropical Methane Emissions: Uniting Top-Down Inversions of Satellite Observations with Bottom-Up Inventories

Abstract

We use 2021 TROPOMI and GOSAT satellite observations of atmospheric methane in an analytical inversion to quantify national methane emissions from South America at up to 25 km × 25 km resolution. From the inversion, we derive optimal posterior estimates of methane emissions, adjusting a combination of national anthropogenic emission inventories reported by individual countries to the United Nations Framework Convention on Climate Change (UNFCCC), the UNFCCC-based Global Fuel Exploitation Inventory (GFEIv2), and the Emissions Database for Global Atmospheric Research (EDGARv7) as prior estimates. We also evaluate two alternative wetland emission inventories (WetCHARTs and LPJ-wsl) as prior estimates. Our best posterior estimates for wetland emissions are consistent with previous inventories for the Amazon but lower for the Pantanal and higher for the Paraná. Our best posterior estimate of South American anthropogenic emissions is 48 (41--56) Tg a$^{-1}$, where numbers in parentheses are the range from our inversion ensemble. This is 55% higher than our prior estimate and is dominated by livestock (65% of anthropogenic total). We find that TROPOMI and GOSAT observations can effectively optimize and separate national emissions by sector for 10 of the 13 countries and territories in the region, 7 of which account for 93% of continental anthropogenic emissions: Brazil (19 (16--23) Tg a$^{-1}$), Argentina (9.2 (7.9--11) Tg a$^{-1}$), Venezuela (7.0 (5.5--9.9) Tg a$^{-1}$), Colombia (5.0 (4.4--6.7) Tg a$^{-1}$), Peru (2.4 (1.6--3.9) Tg a$^{-1}$), Bolivia (0.96 (0.66--1.2) Tg a$^{-1}$), and Paraguay (0.93 (0.88--1.0) Tg a$^{-1}$). Our estimates align with the prior estimates for Brazil, Bolivia, and Paraguay but are significantly higher for other countries. Emissions in all countries are dominated by livestock (mainly enteric fermentation) except for oil--gas in Venezuela and landfills in Peru. Methane intensities from the oil--gas industry are high in Venezuela (33%), Colombia (6.5%), and Argentina (5.9%). The livestock sector shows the largest difference between our top-down estimate and the UNFCCC prior estimates, and even countries using complex bottom-up methods report UNFCCC emissions significantly lower than our posterior estimate. These discrepancies could stem from underestimations in IPCC-recommended bottom-up calculations or uncertainties in the inversion from aggregation error and the prior spatial distribution of emissions (chapter 1). We combine inventory and atmospheric data to quantify 2023 national methane emissions in Colombia using a high-resolution analytical inversion framework. We use satellite observations from TROPOMI, GOSAT, and point source imagers (GHGSat, EMIT, aircraft AVIRIS-NG) in an analytical Bayesian inversion at ≈12$\times$12 km$^2$ resolution with the Integrated Methane Inversion v2.0 framework. We construct a spatially resolved version of the national bottom-up emission inventory from the Biennial Update Report (BUR) to the UNFCCC for use as prior estimate in the inversion, and combine it with high-resolution wetland extent data (GLWDv2) to separate anthropogenic from wetland emissions. Total posterior methane emissions are 8.9 (8.7--9.1) Tg a$^{-1}$, contributed by wetlands (5.7 (5.5--6.0) Tg a$^{-1}$) and anthropogenic emissions (3.2 (3.1--3.2) Tg a$^{-1}$), mainly livestock (2.0 (1.9--2.1) Tg a$^{-1}$) and waste (0.76 (0.74--0.76) Tg a$^{-1}$). Adjustments relative to the BUR are +18% for livestock, +19% for waste, +60% for oil/gas, and +92% for coal. We provide recommendations to improve the BUR, offering a blueprint for combining satellite observations with national inventory data to produce sectorally and spatially resolved emissions estimates that can inform policy (chapter 2). Colombia has been identified as a hotspot of increasing methane emissions over the 2019--2024 period by coarse-resolution global inversion of TROPOMI satellite observations. Here we reproduce this result with a high-resolution (25 $\times$ 25 km$^2$) monthly analytical inversion of TROPOMI observations for the same period conducted within the Integrated Methane Inversion (IMI) framework and evaluated independently with GOSAT and GHGSat satellite observations. Prior estimates for the inversion include spatially resolved national inventory reports and a measurement-based oil and gas sector inventory for anthropogenic emissions and high-resolution monthly CYGNSS satellite inundation data. We exploit the seasonality in the monthly inversion results to separate wetlands from anthropogenic sources. Total emissions from Colombia optimized by the inversion increase at a best estimated rate (and uncertainty) of 0.23 (0.025--0.46) Tg a$^{-2}$ (2% a$^{-1}$) during 2019--2024. Most of this increase is attributed to wetland emissions, peaking in 2021--2022 due to high precipitation driven by La Niña but reflecting interannual variability rather than a sustained trend. Anthropogenic sources account for 35% of the increase driven by cattle population growth, concentrated waste sources near Bogotá, increasing depth of surface coal mines, and fugitive emissions in oil and gas fields. The large seasonal and interannual variability of wetland emissions closely aligns with total water storage anomalies measured by the GRACE satellite instrument, consistent with the importance of water table dynamics in controlling emissions. This work demonstrates that high-resolution satellite inversions can resolve sectoral drivers of national methane trends and monitor changes in emissions at policy-relevant scales (chapter 3).