- Research published yesterday in the journal Nature Ecology & Evolution finds that pressures from human activities and climate change caused the African continent to lose as much as 2.6 billion metric tons of carbon between 2010 and 2016.
- The study not only shows that there was an overall net carbon loss across sub-Saharan Africa, but also that substantial losses occurred in drylands — savannahs and woodlands that fall outside of humid zones — which lost approximately 5 percent of their total carbon stocks each year.
- In order to quantify changes in above-ground biomass-carbon in sub-Saharan Africa, an international team of scientists used a new remote sensing technique based on a satellite system that employs low-frequency, passive microwave signals as opposed to the more common high-resolution photography.
Research published yesterday in the journal Nature Ecology & Evolution finds that pressures from human activities and climate change caused the African continent to lose as much as 2.6 billion metric tons of carbon between 2010 and 2016.
The study not only shows that there was an overall net carbon loss across sub-Saharan Africa, but also that substantial losses occurred in drylands — savannahs and woodlands that fall outside of humid zones — which lost approximately 5 percent of their total carbon stocks each year (though those were gross losses, not let losses — in other words, those losses were offset by gross gains).
When most people think of how changes in above-ground biomass affect terrestrial carbon storage, they probably think first of tropical deforestation. But, in addition to ongoing deforestation, Africa is experiencing one of the driest periods recorded in the past few decades. In order to quantify how all of these factors have impacted annual changes in above-ground biomass-carbon in sub-Saharan Africa, an international team of scientists used a new remote sensing technique based on a satellite system that employs low-frequency, passive microwave signals as opposed to the more common high-resolution photography.
According to lead author Martin Brandt of the University of Copenhagen in Denmark, data from remote sensing techniques using optical imagery or high-frequency microwave signals are restricted to the upper canopy layer, which is especially problematic in areas where vegetation is dense. The new technique Brand and team developed, however, gave them the ability to look deeper into the vegetation canopy layer with less interference from green, non-woody plants.
“We find that with the new satellite data we have for the first time a tool that is not restricted to the monitoring of the upper canopy layer of vegetation, even when the vegetation is dense, like in woodlands or rainforests,” Brandt told Mongabay. “In contrast to optical satellite images and passive microwaves at high frequency, this signal is insensitive to clouds and almost insensitive to green vegetation, which allows the signal to sense deeper below the vegetation canopy and assess almost the entire vegetation layer including stems and branches.”
The satellite system they used — the Soil Moisture and Ocean Salinity (SMOS) satellite mission, launched in 2009 by the European Space Agency — operates at low frequencies known as the “L-band,” so the researchers call their new technique L-VOD, which stands for “L-band vegetation optical depth.” The SMOS satellite is the first of its kind, Brandt noted, though the U.S. National Aeronautics and Space Administration (NASA) has launched its Soil Moisture Active Passive (SMAP) satellite, which uses similar technology.
“We are just beginning to explore these new methods,” Brandt said. “We think it is a breakthrough in monitoring vegetation, similar as it was with infrared observations in the 1970s. It is the first time we can assess the full vegetation layer and not only the upper canopy.”
Brandt and team were able to measure the water content of all vegetation, no matter how dense, allowing them to estimate the biomass for both forests and savannas.
“We find that severe droughts (mainly 2015) have caused huge carbon losses which are not restricted to rainforests but also happened in savannahs,” Brandt said. He added that, even though they are relatively lower in overall carbon content, Africa’s drylands lost nearly as much carbon as its tropical forests. “Over the full period, losses in savannahs and woodlands were almost as high as in rainforests, which is mainly due to the large area they cover.”
While public opinion generally holds that rainforests store all the carbon, and drylands are often not even considered in calculations of carbon balances, Brandt said that “we show that drylands do play a role for storing carbon and especially in inter-annual carbon variations (gains and losses) at continental scale. Drylands cover large parts of Africa and large scale droughts can cause considerable carbon losses from drylands in spite of the rather low storage capacity per unit area.”
Study co-author Jean-Pierre Wigneron, a senior research scientist at France’s Institut National de Recherche Agronomiques (INRA), said the study challenges the general understanding that drylands serve as carbon sinks over the long term. In fact, during dry years, some drylands, especially those in southern Africa, turned from being a carbon sink into a source, demonstrating that the climate can affect short-term variations in carbon stocks across large landscapes.
Given this variability from year to year, the authors write in the study that “it is difficult to conclude from the 7 years of observation presented here whether the observed trends reflect quasi-decadal variations or whether it is a sign of longer-term dynamics.” But the authors argue that the carbon losses observed from 2010 to 2016 must at least force us to “reassess whether, in the long term, woody vegetation in African savannahs will indeed continue to be a carbon sink.”
If dry years become more common, large-scale carbon losses from drylands could exacerbate climate change, which is why it’s important that we now have the tools to monitor these types of changes.
“Droughts cannot be controlled, but deforestation can,” Brandt said. “Knowing and quantifying vegetation carbon dynamics by both drought and deforestation is a first step to control it.”
CITATION
• Brandt, M., Wigneron, J-P., … & Fensholt, R. (2018). Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands. Nature Ecology & Evolution. doi:10.1038/s41559-018-0530-6
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