The huge peat bog areas in the African Congo Basin are still binding and storing enormous amounts of carbon from the atmosphere - but this could change in the course of climate change, a study gives reason to fear: Researchers were able to show that around 5000 to 2000 years ago, a dry climate in the central Congo Basin caused peat to decompose and release carbon. Today's warming is now threatening to dry out the peatland complex again, which could repeat history. The scientists say that the Congo Basin would mutate from a carbon sink to a source of greenhouse gases.
They are considered to be the largest terrestrial carbon reservoirs on earth: in moor areas, the biomass formed by plants is hardly ever decomposed, but is instead deposited in the form of peat. As a result, the carbon removed from the atmosphere is bound in the long term. But this only applies as long as the peat is covered with water and is therefore low in oxygen. On the other hand, when it dries up, microorganisms begin to decompose the organic matter, releasing the greenhouse gas carbon dioxide into the atmosphere. Great importance is therefore attached to the protection of moors and their exploration in the context of climate protection efforts.
In this context, an international research team is now focusing on the Congo Basin – one of the largest river systems on earth. Large parts of it are tropical forests, but swamp forests predominate in the central basin, the so-called cuvette. Through the analysis of satellite images, it became clear in previous studies that the largest tropical peatland complex in the world is located there. It extends over 167,600 square kilometers, which is more than four times the area of Baden-Württemberg. It is estimated that around 30 billion tons of carbon could be stored there - about 28 percent of the Earth's tropical peat carbon stock.
A gigantic carbon store in sight
“Almost nothing is known about the formation and history of this peat area and thus also its carbon dynamics. However, this understanding is important in order to determine the vulnerability of this ecosystem to climate change and to provide information on how deforestation, oil exploration and agriculture affect it," says co-author Enno Schefuss from MARUM - Center for Marine Environmental Sciences in Bremen. For their investigations, the scientists took peat samples from the subsoil of the remote swamp forests. By dating and analyzing the plant remains contained, they were able to draw conclusions as to when the formation of the layers began and how the peat deposition process developed over the past millennia. Through isotope analyzes and investigations of waxes from the leaves that were preserved in the peat, the researchers also gained information on the amounts of precipitation during the respective lifetimes of the plants.
The results indicated that peat accumulation began at least 17,500 years ago. As it became apparent, however, it has not progressed in a continuous manner until today: In the interval from 7500 to 2000 years ago, hardly any layers of peat formed, instead even older material was decomposed, according to the analyses. "So the decomposition has eaten its way into the peat," says Schefuss. Researchers refer to this as the "ghost interval." The same finding was seen at widely separated sampling sites, suggesting that the phenomenon affected the entire Congo Basin peatland region at the time.
Drought led to the “ghost interval”
According to the scientists, the cause was reflected in the examination of the plant remains: “The samples reveal what the precipitation and vegetation were like when the peat formed. Together they paint the picture of an arid climate,” says lead author Yannick Garcin from the University of Aix-Marseille. The detailed results showed that during the ghost interval it had rained about a meter less per year than before. The situation only stabilized again 2000 years ago. “This drought resulted in a huge loss of peat, at least two meters. This turned the bog into a huge source of carbon at the time as the material decomposed. This process only ended when the drought ended, allowing peat to accumulate again,” explains the researcher.
As the scientists emphasize, the peat bog area in Central Africa is now in significantly drier climatic conditions than other tropical peatlands. This means: The situation may be unstable: "Our results show that the peat in the tropical Congo Basin is close to the tipping point from a carbon sink to a source, but also that it is resilient, i.e. it changes again if it develops favorably can recover,” says Schefuss.
Co-author Simon Lewis from the University of Leeds concludes: "Our study thus provides a warning from the past: if the peatlands dry out beyond a certain threshold, they will release enormous amounts of carbon into the atmosphere and further accelerate climate change . There are already some signs that dry seasons in the Congo Basin are lengthening, but it is unclear how this will develop,” says the researcher. “There is also a message in our findings for leaders gathering next week for the COP27 climate talks. If greenhouse gas emissions cause the peatlands in central Congo to become too dry, then the peatlands will contribute to the climate crisis instead of protecting us,” Lewis said.
Source: University of Leeds, MARUM - Center for Marine Environmental Sciences, Article: Nature, doi: 10.1038/s41586-022-05389-3