After 6 years of observation of sub-Saharan Africa at 750 km altitude, we notice a decline in its vegetation cover, and the loss of carbon stocks associated with it. To obtain their results, the authors of the study used data from the SMOS satellite (Soil Moisture and Ocean Salinity), which is able to measure and monitor soil moisture from space for the first time. The satellite captures the radiation emitted by the Earth in the range of low frequency microwaves that are very sensitive to water content. In order to convert these data into information on the carbon content of the Earth's surface, the authors have developed a new index called L-VOD (L-band vegetation optical depth, the optical thickness of vegetation in L-band) to measure the variation of the annual quantity of biomass (plant material above the surface), and thus deduce the amount of carbon stocks associated with it.
The results of this study on sub-Saharan Africa indicate a trend of carbon loss between 2010 and 2016: a decrease in the amount of carbon stored in vegetation, and consequently an increase in carbon released into the atmosphere. The total carbon stock is estimated at about64.5 ± 13 Pg C (petagrammes of carbon, or one billion tonnes) but varies with time and tends to decrease: - 0.05 Pg C (petagrammes of carbon) per year on average, but with an increased loss during drought events (2015, net loss of - 0.69 Pg C that is fourteen times the annual average over the period studied). Conversely, semi-arid regions behave like carbon sinks in very wet years such as 2011 ((+ 0.15 Pg C net over the year) and 2013 (+ 0.17 Pg C).
Variation in carbon stocks between 2010 and 2016, with gains in green and losses in red. In "a", each pixel represents a statistically significant loss or increase. In "b", the net budget by subtracting gains to losses, transposed to latitudes in "c". d" and "e" separately represent losses and gains according to their intensity, then transposed according to their latitude in "f" in percentages of the average L-VOD.
Credits : Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands, Nature Ecology & Evolution volume 2, pages 827–835 (2018)
The cause is twofold. Africa is heavily deforested by human activity, which is aggravated by droughts and fires: when burning, forests release carbon dioxide into the atmosphere during combustion. Tracking the amount of carbon stored is now a necessity to be able to achieve the objectives set by international carbon agreements. And for that, reliable indicators are needed: before using L-VOD (1.4 GHz frequency) with SMOS, VOD (Vegetation Optical Depth) was obtained from data from other satellites that used more frequencies, above 5 GHz, which saturate over forest areas.
“The opacity of the vegetation depends on its density but the ability to penetrate within a canopy is related to the wavelength of observation, Yann Kerr explains. Low-frequency microwaves can probe the whole canopy layer, whereas the probing capabilities of VOD indices from high-frequency observations (5 GHz and beyond) are to the top of the canopy if the canopy layer is relatively dense. At 5 GHz, the depth of penetration is 5 times smaller than in the L-band, so only one surface layer of forest is being probed. As a result, the sensors of the other satellites saturate as soon as the thickness of the canopy exceeds a certain level depending on the wavelength. It did not make it an effective tool for calculating the amount of biomass and carbon globally and especially on dense forests. So all this progress has been made thanks to SMOS L-band radiometry."
Not only does the study demonstrate the feasibility of using the L-VOD index, its usefulness and its reliability for monitoring carbon budget in relation to climatic conditions since it brings a temporal dimension to global maps, but above all, it reveals the strong dynamics of dry savannahs, despite their low carbon content per unit area. Moreover, it is by knowing the amount and distribution of moisture and carbon in the African vegetation that vulnerable areas can be identified. "Net changes in semi-arid regions are clearly associated with drying trends," Yann Kerr says. “Depending on the year, there is a high variability in stocks with gains in very wet years and losses in very dry years. And the phenomenon could increase with global warming and its impact on dry or wet periods. If droughts become more frequent, carbon losses could be accelerated by climate change and help to intensify it."
- Publication : Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands, Nature Ecology & Evolution volume 2, pages827–835 (2018)
Email: yann.kerr at cesbio.cnes.fr
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Centre National d'Etudes Spatiales, 18 Avenue Edouard Belin, 31401 Toulouse Cedex 9, France
Solid Earth Programme Manager
Email: Selma.Cherchali at cnes.fr
Phone: 05 61 28 13 84 / +33 5 61 28 13 84
Centre National d'Études Spatiales, 18 Avenue Édouard Belin, 31401 Toulouse Cedex 9, France