Using Orbiting Carbon Observatory-2 (OCO-2) column CO₂ retrievals to rapidly detect and estimate biospheric surface carbon flux anomalies

Abstract

Abstract. The global carbon cycle is experiencing continued perturbations via increases in atmospheric carbon concentrations, which are partly reduced by terrestrial biosphere and ocean carbon uptake. Greenhouse gas satellites have been shown to be useful in retrieving atmospheric carbon concentrations and observing surface and atmospheric CO2 seasonal-to-interannual variations. However, limited attention has been placed on using satellite column CO2 retrievals to evaluate surface CO2 fluxes from the terrestrial biosphere without advanced inversion models at low latency. Such applications could be useful to monitor, in near real time, biosphere carbon fluxes during climatic anomalies like drought, heatwaves, and floods, before more complex terrestrial biosphere model outputs and/or advanced inversion modelling estimates become available. Here, we explore the ability of Orbiting Carbon Observatory-2 (OCO-2) column-averaged dry air CO2 (XCO2) retrievals to directly detect and estimate terrestrial biosphere CO2 flux anomalies using a simple mass-balance approach. An initial global analysis of surface–atmospheric CO2 coupling and transport conditions reveals that the western US, among a handful of other regions, is a feasible candidate for using XCO2 for detecting terrestrial biosphere CO2 flux anomalies. Using the CarbonTracker model reanalysis as a test bed, we first demonstrate that a well-established mass-balance approach can estimate monthly surface CO2 flux anomalies from XCO2 enhancements in the western United States. The method is optimal when the study domain is spatially extensive enough to account for atmospheric mixing and has favorable advection conditions with contributions primarily from one background region. We find that errors in individual soundings reduce the ability of OCO-2 XCO2 to estimate more frequent, smaller surface CO2 flux anomalies. However, we find that OCO-2 XCO2 can often detect and estimate large surface flux anomalies that leave an imprint on the atmospheric CO2 concentration anomalies beyond the retrieval error/uncertainty associated with the observations. OCO-2 can thus be useful for low-latency monitoring of the monthly timing and magnitude of extreme regional terrestrial biosphere carbon anomalies.

A multi‐satellite framework to rapidly evaluate extreme biosphere cascades: The Western US 2021 drought and heatwave

“…In our framework, Step I first includes detecting the total carbon anomaly (i.e., with XCO 2 ), indicating a large total CO 2 flux anomaly, and then detection of individual CO 2 flux components. However, given that some CO 2 flux anomalies may not be observed with XCO 2 from greenhouse gas satellites (Feldman et al, 2023), individual CO 2 component anomaly detection (i.e., vegetation indices) should be initiated, especially if there is a known climate anomaly. Furthermore, we demonstrate a rough estimation of surface carbon fluxes that can be used in this step, acknowledging its uncertainty and its usefulness only under specific spatiotemporal conditions (Feldman et al, 2023).…”

“…This approach was evaluated in detail in a recent study and we note that it is only feasible under select conditions, especially under simpler advection conditions as in the Western United States and for extreme fluxes (Feldman et al, 2023). We emphasize that XCO 2 anomalies serve as a complementary, independent form of detecting carbon‐climate feedbacks and thus inability of XCO 2 anomalies to detect and/or estimate an extreme flux does not hinder the procedure here.…”

“…This coupling gives credence to detection of a large CO 2 flux with an extreme XCO 2 anomaly. previously (Buchwitz et al, 2017;Feldman et al, 2023;Pandey et al, 2021). However, we emphasize limitations originating from atmospheric transport (Basu et al, 2018), simplistic assumptions of the mass balance approach (Varon et al, 2018), and OCO-2 instrument noise (Buchwitz et al, 2021).…”

“…Given strong influences of atmospheric transport on XCO 2 , we identified background regions that contribute air that mixes with XCO 2 within the Western US region. MERRA2 wind vectors reveal the Pacific Ocean as the consistent upwind region of the Western United States (Feldman et al, 2023). Therefore, concurrent analysis of background Pacific Ocean XCO 2 conditions offers insights into whether spatially integrated XCO 2 anomalies within the Western United States have contributions from a surface instead of upwind source.…”

“…Nevertheless, satellite XCO 2 observations are available at the low latency (~1–2 months) and global scale needed to monitor biosphere response to large‐scale climate extremes (Peters et al, 2007; Schimel et al, 2015). Among debate in how sensitive XCO 2 anomalies are to surface fluxes (Chevallier, 2018; Schuh et al, 2022), there is evidence that spatially concentrated, extreme surface CO 2 fluxes imprint on sub‐yearly variability of NASA's Orbiting Carbon Observatory‐2 (OCO‐2) XCO 2 retrievals (Chatterjee et al, 2017; Feldman et al, 2023; Liu et al, 2017; Weir et al, 2021).…”