Progress Toward Global Atmospheric CO2 and CH4 Flux Inventories

Atmospheric carbon dioxide (CO₂) and methane (CH₄) measurements complement bottom-up greenhouse gas (GHG) inventories by providing an integrated constraint on the exchanges of these gases between land and ocean surfaces and the atmosphere and their trends over time. While CO₂ and CH₄ fluxes inferred from atmospheric measurements are not as source-specific as the data sources typically used in inventories, they include contributions from sources that are often omitted or poorly characterized by bottom-up inventory methods.

At global scales, atmospheric concentrations of CO₂, CH₄ and other well-mixed GHGs are well characterized by precise, ground-based in situ measurements from surface and airborne systems. Recent advances in space-based remote sensing methods are providing new opportunities to augment the resolution and coverage of the ground and airborne measurements with estimates of the column-averaged CO₂ and CH₄ dry air mole fractions (XCO₂ and XCH₄). GHG satellites now operating in low Earth orbit include Japan’s Greenhouse gases Observing Satellite (GOSAT), and GOSAT-2, NASA’s Orbiting Carbon Observatory-2 (OCO-2) and OCO-3, and the Copernicus Sentinel 5 Precursor. A number of other GHG missions are currently under development, including the first geostationary mission, NASA’s Geostationary Carbon Observatory, GeoCarb, which is currently scheduled to be deployed over North and South America in the 2023 timeframe. The accuracy and precision of the space-based XCO₂ and XCH₄ estimates from these space-based sensors has improved substantially over the past few years. While they still lack the quality of the in situ measurements, they can provide near global coverage at spatial scales as fine as a few km. For example, since 2014, the Orbiting Carbon Observatory-2 has been returning about 85,000 XCO₂ observations over the sunlit hemispheric each day with single-sounding random errors near 0.5 parts per million (ppm), and accuracies near 1 ppm.

These ground-based, airborne, and space-based atmospheric CO₂ and CH₄ estimates are now being assimilated into atmospheric transport models to estimate CO₂ and CH₄ fluxes on scales spanning individual large power plants to continents. The long-term objective of these efforts is to develop top-down global inventories that (i) reduce uncertainty of national emission inventory reporting, (ii) identify additional emission reduction opportunities and provide nations with timely and quantified guidance on progress towards their emission reduction targets, and (iii) track changes in the natural carbon cycle caused by human activities and climate change. Here, we will review our progress toward these goals.