Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands

Abstract

The African continent is facing one of the driest periods in the past three decades as well as continued deforestation. These disturbances threaten vegetation carbon (C) stocks and highlight the need for improved capabilities of monitoring large-scale aboveground carbon stock dynamics. Here we use a satellite dataset based on vegetation optical depth derived from low-frequency passive microwaves (L-VOD) to quantify annual aboveground biomass-carbon changes in sub-Saharan Africa between 2010 and 2016. L-VOD is shown not to saturate over densely vegetated areas. The overall net change in drylands (53% of the land area) was -0.05 petagrams of C per year (Pg C yr) associated with drying trends, and a net change of -0.02 Pg C yr was observed in humid areas. These trends reflect a high inter-annual variability with a very dry year in 2015 (net change, -0.69 Pg C) with about half of the gross losses occurring in drylands. This study demonstrates, first, the applicability of L-VOD to monitor the dynamics of carbon loss and gain due to weather variations, and second, the importance of the highly dynamic and vulnerable carbon pool of dryland savannahs for the global carbon balance, despite the relatively low carbon stock per unit area.

Sources of Uncertainty in Regional and Global Terrestrial CO2 Exchange Estimates

“…Satellite‐based inversions might help to reduce uncertainty in the flux partitioning between land, ocean, and different continents, and regional inversions might provide better constraints of regional CO fluxes. Above‐ground biomass products at annual time steps and moderate resolution are currently available at least for the tropics (Brandt et al, ), providing a truly independent estimate of losses and gains of carbon in ecosystems due to both natural and anthropogenic processes. Considerable work is being put into improving tropical phenology, vegetation‐water interactions, mortality, and nutrient limitations by several modeling teams.…”

Ecological engineering projects increased vegetation cover, production, and biomass in semiarid and subhumid Northern China

“…VOD, derived from passive microwaves, is sensitive to the water content of all photosynthetic and nonphotosynthetic parts of vegetation (Tian et al, ), less prone to saturation, less sensitive to interannual variations in climate, and almost insensitive to clouds. Thus, it has been widely used as an indicator to estimate AGB (Brandt et al, ; Liu et al, ). The monthly long‐term VOD (1992–2012) data set we used was provided in a spatial resolution of 0.25° × 0.25° and values ranged from 0 to ~1.3 (Liu, de Jeu, McCabe, Evans, & van Dijk, ).…”

Constraining estimates of terrestrial carbon uptake: new opportunities using long‐term satellite observations and data assimilation

“…Long‐term satellite VOD observations most directly track changes in the water content of above‐ground vegetation, but they can also be applied to estimate changes in above‐ground biomass due to the tight relationship between water content and biomass (Liu et al ., ; Brandt et al ., ; Konings et al ., ). Long‐term VOD datasets have been previously used to quantify multidecadal trends in above‐ground biomass at the global scale (Liu et al ., ) (Figs , ).…”

“…Long‐term VOD datasets have been previously used to quantify multidecadal trends in above‐ground biomass at the global scale (Liu et al ., ) (Figs , ). Recent work reinforces the near‐linear relationship between VOD and above‐ground biomass across a range of ecosystem types (Brandt et al ., , ). Thus, together satellite LAI and/or fAPAR estimates and VOD‐based above‐ground biomass estimates capture changes in above‐ground biomass per unit light absorbed, a proxy for LUE (LUE VOD ) that could be used to constrain this direct CFE pathway (Fig.…”