Drying Congo peatlands threaten to accelerate climate change

Photo: Prof Simon Lewis, University of Leeds

University of Leicester researchers have contributed to a study which reveals how the Congo peatlands changed from being a major store of carbon to a source of damaging carbon dioxide emissions as a result of climate change thousands of years ago.

 Professor Susan Page and Dr Arnoud Boom were part of the CongoPeat team behind research published online by the scientific journal Nature this week. The study shows climate gradually got drier in central Africa, beginning 5,000 years ago, which led to peat decomposition and the release of carbon from long-term storage. The central Congo peatlands, the world’s largest tropical peatland, only returned to sequestering and storing carbon as the climate got wetter again in the past 2,000 years.

If modern day climate change results in droughts in the Congo basin, then the world’s largest tropical peatland could again release carbon to the environment, dangerously accelerating climate change.

The CongoPeat research team have previously established that the Congo peatlands store around 30 billion tonnes of carbon – equivalent to the amount of carbon released from global fossil fuel burning over the last three years.

Lessons from the past

Congolese and European scientists took peat samples from the remote swamp forests of central Congo and analysed the peat in the laboratory to reveal, for the first time, the history of peatland development.  

Dr Yannick Garcin, from the Aix Marseille University in France and lead author of the study, said: “The peat samples show us that there was a period of around 5,000 years when there was almost no build-up of peat, less than 0.1 mm per year. During this period, starting 5,000 years ago, the climate got progressively drier until about 2,000 years ago.”

“This drought led to a huge loss of peat, at least two metres. The drought transformed the peatland to a huge carbon source as the peat decomposed. This decomposition only stopped when the drought stopped allowing peat to start accumulating again.

Key to this was the research undertaken at the University of Leicester which focused on geochemistry and stable isotopes. Dr Boom, a co-author of the study along with Professor Page, said “The results showed that the peat organic matter during the drought was significantly altered. It was not the case that the peat production had simply slowed down, what we were looking at was the evidence for a very significant peat degradation event. These peatlands are a time bomb that went off a few thousand years ago and it could go off again.”

Professor Page added: “Peat is a legacy of plant activity – as plants grow they acquire carbon from the atmosphere. In a wet peatland, the soil is too wet to allow high levels of decomposition, so those dead remains, and the carbon they contain, accumulate over time. In tropical environments, this has led to the build up of vast carbon-rich peatlands. But peatland drainage, either brought about by a changing climate or land conversion for agriculture or forestry leads to rapid peat decomposition, and the release of carbon from long-term storage, exacerbating human-made climate change.”

“Our study results show that if the Congo peatlands dry beyond a certain threshold, that they could again release large amounts of carbon. Keeping the Congo peatlands wet is key – both to safeguarding their future and the security of our global climate.”

Professor Simon Lewis from the University of Leeds, who leads the CongoPeat project, added: “This is an important message for world leaders gathering at the COP27 climate talks. If greenhouse gas emissions drive the central Congo peatlands to become too dry, then the peatlands will contribute to the climate crisis rather than protect us.”

How researchers developed a picture of climate change thousands of years ago

The research team spent two weeks trekking into a remote area of swamp in the Likouala Department of the Republic of the Congo, an area with some of the deepest peatlands in the region, to take a six metre-long core sample of the peat.

By analysing plant remains in the sample, the researchers built a record of the vegetation and climate that existed in the Congo basin over the last 17,500 years. Waxes from plant leaves, which are preserved in the peat, indicate rainfall levels at the time the plant was living.

The study revealed that the climate began to get drier 5,000 years ago, a process that continued for about 3,000 years, most likely causing the peatlands to shift from a carbon store to a carbon source.

Peat sample revealed a ‘ghost interval’

At the most intense period of drought, rainfall was reduced by at least 800 mm a year. This caused the water table in the Congo peatlands to drop, exposing older layers of peat to the air, causing oxidation and release of CO2.

Between 7,500 and 2,000 years ago, the peat layers either decomposed or never accumulated. The researchers described this as the ‘ghost interval’. This same ghost interval was found in peat samples from hundreds of kilometres away in the Democratic Republic of the Congo (DRC) indicating it happened across the whole peatland region.

Peatlands are ‘vulnerable’

Professor Corneille Ewango, from the University of Kisangani in the DRC and who led the expeditions to collect the peat samples from the DRC, said: “This is another astonishing finding about the peatlands, they are more vulnerable than we thought. Everyone must play their role in protecting them.

“Polluting countries must cut their carbon emissions fast, to limit the possibility of droughts pushing the peatlands past their tipping point. The DRC will also need to strengthen protection of the peatlands. At stake is one of the most wildlife and carbon-rich ecosystems on Earth.”