What are the greenhouse gas observing system requirements for reducing fundamental biogeochemical process uncertainty? Amazon wetland CH<sub>4</sub> emissions as a case study

Abstract

<p><strong>Abstract.</strong> Understanding the processes controlling terrestrial carbon fluxes is one of the grand challenges of climate science. Carbon cycle process controls are readily studied at local scales, but integrating local knowledge across extremely heterogeneous biota, landforms and climate space has proven to be extraordinarily challenging. Consequently, top-down or integral flux constraints at process-relevant scales are essential to reducing process uncertainty. Future satellite-based estimates of greenhouse gas fluxes – such as CO<sub>2</sub> and CH<sub>4</sub> – could potentially provide the constraints needed to resolve biogeochemical process controls at the required scales. Our analysis is focused on Amazon wetland CH4 emissions, which amount to a scientifically crucial and methodologically challenging case study. We quantitatively derive the observing system requirements for testing wetland CH<sub>4</sub> emission hypotheses at a process-relevant scale. To capture the spatial and temporal patterns of the major hydrological and carbon controls over wetland CH<sub>4</sub> production, a satellite mission will need to resolve monthly CH4 fluxes at a 300 km resolution and with a 25 % flux precision. We simulate a range of low-earth orbit (LEO) and geostationary orbit (GEO) CH<sub>4</sub> observing system configurations to evaluate the ability of these approaches to meet the CH<sub>4</sub> flux requirements. Conventional LEO and GEO missions resolve monthly 300 km × 300 km Amazon wetland fluxes at a 186 % and 33 % median uncertainty level. Improving LEO CH<sub>4</sub> measurement precision by √2 would only reduce the median CH<sub>4</sub> flux uncertainty to 132 %. A GEO mission with targeted observing capability could resolve fluxes at a 21–27 % median precision by increasing the observation density in high cloud-cover regions at the expense of other parts of the domain. Process-driven greenhouse gas observing system simulations can enhance conventional uncertainty reduction assessments by providing the measurement needs for testing biogeochemical process hypotheses.</p>