A new study, published earlier this year in Nature, shows that the Congolese rainforests store far more carbon than previously thought: over 60 billion tonnes, about half of which in the living biomass of the forest trees, and the other half as shallow peat – mainly in the ‘Cuvette Central’, a huge forested wetland in its centre:
If this terrestrial carbon store were cut down, burned, drained, or otherwise oxidised (for instance through increasing droughts in the Congo Basin) that would equate to emissions of approximately 220 billion tonnes of CO2. And that makes protection of Africa’s densest biodiversity hotspot of immediate significance for the stability of the global climate system.
The Congolese rainforests: ‘in between’ the Amazon (very large) and Borneo (very thick)
The Congolese rainforests are the world’s second largest remaining expanse of tropical rainforest – between the rainforests of the Amazon Basin in South America and the forest complex on the island of Borneo in Southeast Asia.
And in a way the Congolese forests can also be seen as a mix between those two. If we oversimplify [please never do that at home!] the respective characteristics of the three main regions of tropical rainforests, the Amazon has a very large surface but (in most places) only a shallow organic soil – whereas the rainforests of Borneo and Sumatra are known as peat forests, as on these South-East Asian islands conditions prevailed for many thousands of years in which dead biomass could not fully decompose, therefore accumulated.
This characteristic is of large importance to the global climate system, as peat-forming ecosystems (the Arctic tundra (Yedoma permafrost) and the boreal forests (taiga) are the largest examples) represent the most powerful and also most durable terrestrial carbon sinks – durable if we think outside forces like the globally expanding palm oil industry, an industry that first cuts the forest, then drains the peat, allowing all that carbon to oxidise to CO2.
The thick layers of peat is what makes global CO2 emissions spike when for instance Borneo suffers another wave of man-made forest fires, usually after a prolonged El Niño drought – as powerfully captured in satellite imagery of the end-2015 forest fires. And although the Amazon overcompensates due to its much larger surface size, the forests of Borneo (including those thick layers of peat) are a carbon store of comparable proportions – in the order of 60-70 gigatonnes (the Amazon carbon store being about twice that large).
‘Cuvette Central swamp forest peat complex five times larger, therefore total tropical peat carbon store 36% higher’
Thus far it was thought that the Congolese rainforests were a substantially smaller carbon store (about half that size, ±30Gt) – and that thought was wrong, a research group led by Greta Dargie and Simon Lewis of the University of Leeds showed last January.
In their publication in Nature, titled ‘Age, extent and carbon storage of the central Congo Basin peatland complex’ they present new soil estimates of the Cuvette Central, French for ‘central basin’, a flat landscape in the heart of the Congolese rainforest, where the Congo river and its many tributaries slowly drain westward, partially inundating the forest.
Based on field measurements and remote sensing these researchers suggest the ‘swamp forest’ grows on one of the world’s largest layers of peat. The average thickness (accumulated over the full course of the Holocene) is only about 2 metres, but the surface of the peat soil is now estimated at 145,500 square kilometres, more than five times larger than an earlier study from 2011 had assessed and making the Cuvette Central the world’s most extensive tropical peatland complex.
The new measurements double to total assessed carbon store of the entire Congo Basin, the researchers write – and thereby increase the total tropical peatland carbon store by 36 percent, to over 100 billion tonnes:
“We estimate that the peatlands store approximately 30.6 petagrams [that’s 30.6 billion tonnes] of carbon belowground – a quantity that is similar to the above-ground carbon stocks of the tropical forests of the entire Congo Basin.”
“Our result for the Cuvette Centrale increases the best estimate of global tropical peatland carbon stocks by 36 per cent, to 104.7 petagrams of carbon (minimum estimate of 69.6 petagrams of carbon; maximum estimate of 129.8 petagrams of carbon). This stored carbon is vulnerable to land-use change and any future reduction in precipitation.”
We’ll take a better look at climate trends for the region soon. For now again it’s shown that preserving biodiversity and preventing climate change go hand in hand, and that from both perspectives Earth’s rainforests simply cannot be missed. This goes for Asia, for South America – and equally so for Africa.
© Rolf Schuttenhelm | www.bitsofscience.org