Study by team including NASA scientists estimates carbon stocks of trees in semi-arid sub-Saharan Africa; calculates 840 million tonnes of carbon locked up
There are far more trees spread across semi-arid regions of Africa than previously thought, but the trees also store less carbon than some models have predicted. A new study has taken inventory of nearly 10 billion trees in semi-arid sub-Saharan Africa.
A team of international scientists, including researchers from United States National Aeronautics and Space Administration published Sub-continental-scale carbon stocks of individual trees in African drylands in Nature journal.
The study detailed estimates of the amount of carbon stored in the ecosystem and found roughly 0.84 petagrams or 840 million tonnes of carbon are locked up in African drylands.
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The team collected data from more than 300,000 satellite images to help improve the understanding of the carbon cycle. This may be a useful resource for scientists, policymakers, dryland restoration practitioners and farmers.
“Our team gathered and analysed carbon data down to the individual tree level across the vast semi-arid regions of Africa or elsewhere — something that had previously been done only on small, local scales,” Compton Tucker, lead scientist on the project and an Earth scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland said in a statement by NASA.
Previous satellite-based estimates of tree carbon in Africa’s drylands often mistook grasses and shrubs for trees. “That led to over-predictions of the carbon there,” Tucker added.
The distribution of dryland trees and their density, cover, size, mass and carbon content is unknown at sub-continental to continental scales. This information is important for ecological protection, carbon accounting, climate mitigation and restoration efforts of dryland ecosystems.
Having an accurate tree carbon estimate is essential for climate change projections, which are influenced by how long trees and other vegetation store carbon, said the NASA statement.
This “carbon residence time,” as scientists call it, is very short for grasses and bushes, which grow seasonally, but much longer for trees that grow for years. Knowing how much carbon a landscape stores is dependent on knowing exactly what is growing there.
Carbon is constantly cycling between the land, the atmosphere, the ocean, and back. Trees remove carbon dioxide — a greenhouse gas — from Earth’s atmosphere during photosynthesis and store it in their roots, trunks, branches, and leaves.
For this reason, increasing tree cover is often suggested as a way to offset ever-increasing carbon emissions.
The researchers assessed 9.9 billion trees within semi-arid sub-Saharan Africa north of the Equator, covering nearly 10,000,000 square kilometres of land.
Machine learning was used to scan 326,523 high-resolution satellite images to identify and map individual trees, providing an opportunity to estimate the amount of carbon stored in the foliage, wood and roots of each tree.
Still image-view of the study region in northern Africa with climate zones indicated. Image: NASA
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Values were divided into different rainfall zones: Hyper-arid (0-150 millimetre of rain per year), arid (150–300 mm per year), semi-arid (300–600 mm per year) and dry sub-humid (600–1,000 mm per year).
The average carbon stock for a single tree is 51 kilogrammes of carbon (kg C) in the hyper-arid, 63 kg C in the arid, 72 kg C in the semi-arid and 98 kg C in the sub-humid zone. Most attempts have overestimated the carbon stocks of trees, comparisons with previous models for the same area indicated.
The African tree carbon data are publicly available with a viewer app developed by the team. It allows people to view every tree in the study area and the amount of carbon it stores.
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Beyond the vast tropical forests spread across the middle of the continent, African landscapes range from dry grasslands with a few trees to savannahs with scattered trees to more humid areas with many scattered trees. Photo: iStock.png)
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