Peatlands as climate tipping points
Researchers decipher the history and sensitivity of the largest tropical peatland in Congo
Peer-Reviewed PublicationIMAGE: DR. JOHANNA MENGES (MARUM, BREMEN) SAMPLING A PEAT CORE IN THE CUVETTE CONGOLAISE DURING THE 2022 EXPEDITION. PHOTO: MÉLANIE GUARDIOLA, CEREGE view more
CREDIT: MÉLANIE GUARDIOLA, CEREGE
Not only seas and oceans sequester carbon from the atmosphere, but also peatlands. They are considered to contain the largest terrestrial carbon stores. Plant remains, and thus carbon, that break down in areas covered with water are stored under oxygen-poor conditions as long as the peat remains covered with water. Peatlands, therefore, can only function as a carbon sink if the swamps do not dry out, for example, as a result of climate change or due to human activities such as agriculture, peat mining or road construction.
The Congo Basin is one of the largest river basins in the world. It is largely characterized by tropical forests, but in the central basin, known as the Cuvette, swamp forests predominate. Until the year 2000, it was believed that the area was only rain forest. Around that time, however, satellite observations revealed that the land under the trees is covered by water. Mapping in 2017 discovered that this area contains the world’s largest peatland complex, covering more than 167,600 square kilometers, which is more than four times the area of Baden-Württemberg. At the 26th United Nations Climate Change Conference in 2021, 1.5 billion US dollars were committed to promote the preservation of this unique ecosystem, in part by the European Union and Germany.
Dr. Enno Schefuß of MARUM – the Center for Marine Environmental Sciences, has long been studying the Congo Basin and its importance for the global carbon cycle. He led a sampling expedition to the area in the spring of 2022. The ongoing German-French cooperative project is financed in part by the German Research Foundation (Deutsche Forschungsgemeinschaft – DFG). He and his colleagues are now studying the sensitivity of this unique ecosystem in relation to climate change. “Almost nothing is known about the origin and history of this peatland area, or about its carbon dynamics,” says Enno Schefuß, one of the main authors of the Nature article. “But this knowledge is crucial for evaluating the susceptibility of the ecosystem to climate change and providing information about the impacts of logging, oil exploration and agriculture.”
Dating of the peat cores reveals a pattern that is consistently repeated in the region. Between around 7,500 and 2,000 years ago, there was a phase during which the peat was highly condensed. Geochemical analyses have shown that peat was being deposited during that time, but it decomposed and lost most of its carbon. The peat that now exists from that time interval is merely a remnant of the original peat, which was several meters thick. At the same time, in marine sediments off the coast of the Congo River, refractory, i.e. non-degraded parts of the older peat were deposited. This input of terrestrial organic material into the ocean by rivers is an important component of the global carbon cycle, which is a focus of research within the “Ocean Floor” Cluster of Excellence at MARUM.
What happened? “Using the technique of paleohydrological reconstruction, which allows the inference of precipitation conditions in the past, we concluded that the swamp dried out during this phase,” Schefuß reports. “We were able to obtain estimates of the amount of rainfall before, during and after the phase of decomposition.” It is interesting to note that the decomposition affected not only the peat formed during that time, but also older peat layers beneath it. “It could be said that the degradation ‘burned-down’ into the peat.”
Using modern climate data, the precise peat distribution, and the reconstruction of rain patterns, Schefuß and his colleagues were able to determine the conditions of peat formation, the decomposition conditions, and the present-day situation. Prior to the decomposition phase, the rainfall conditions were similar to today’s tropical swamps in North and South America, Asia and Oceania. During the decomposition, the rainfall averaged around one meter less each year. It was only about 2,000 years ago that the situation became sufficiently stabilized for the peat to start forming again. The peat swamps in tropical Africa today, however, exist under significantly drier climate conditions than are found in other tropical swamps. The authors of the study thus conclude that it is precariously close to a tipping point.
“As scientists, it is our task to produce robust data that will empower policy makers to protect vulnerable ecosystems while enabling sustainable development,” explains Schefuß. “Our results show that the peat in the tropical Congo Basin is close to the tipping point from being a carbon sink to becoming a carbon source, but also that it is resilient and can recover under favorable conditions. I would strongly emphasize the need for improving assessments of the vulnerability of these species- and carbon-rich ecosystems to climate change in the 21st century through continued research involving local colleagues, in order to predict their future development.”
MARUM produces fundamental scientific knowledge about the role of the ocean and the seafloor in the total Earth system. The dynamics of the oceans and the seabed significantly impact the entire Earth system through the interaction of geological, physical, biological and chemical processes. These influence both the climate and the global carbon cycle, resulting in the creation of unique biological systems. MARUM is committed to fundamental and unbiased research in the interests of society, the marine environment, and in accordance with the sustainability goals of the United Nations. It publishes its quality-assured scientific data to make it publicly available. MARUM informs the public about new discoveries in the marine environment and provides practical knowledge through its dialogue with society. MARUM cooperation with companies and industrial partners is carried out in accordance with its goal of protecting the marine environment.
JOURNAL
Nature
ARTICLE TITLE
Hydroclimatic vulnerability of peat carbon in the central Congo Basin
ARTICLE PUBLICATION DATE
2-Nov-2022
Climate change could trigger the Congo peatlands to release billions of tons of carbon
IMAGE: CONGO BASIN PEATLANDS EXPEDITION, FROM 2018 view more
CREDIT: GREENPEACE/KEVIN MCELVANEY
Climate change could trigger the Congo peatlands to release billions of tonnes of carbon
New research finds the peatlands are fragile and vulnerable to drought
As the peatlands dry, the peat decomposes and releases carbon dioxide which accelerates global warming
Study says this process has already happened once in the peatlands history - and could happen again
New research published in Nature today (Wed, Nov 2) reveals that the world’s largest tropical peatland turned 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.
Around the time that Stonehenge was built, 5,000 years ago, the climate of central Congo began to dry leading to the peatlands emitting carbon dioxide. The peatlands only stopped releasing carbon and reverted back to taking carbon out of the atmosphere when the climate got wetter again in the past 2,000 years, according to a major international study co-coordinated by the University of Leeds.
Scientists involved in the study are warning that if modern-day global heating produces droughts in the Congo region, history could repeat itself, dangerously accelerating climate change.
If that were to happen, up to 30 billion tonnes of carbon could be released from the peatlands into the atmosphere as carbon dioxide, a potent greenhouse gas. That is equivalent to the global emissions from fossil fuel burning over a three-year period.
Professor Simon Lewis, from the University of Leeds and University College London, a senior author of the study, said: “Our study brings a brutal warning from the past. If the peatlands dry beyond a certain threshold they will release colossal quantities of carbon to the atmosphere, further accelerating climate change.
“There is some evidence that dry seasons are lengthening in the Congo Basin, but it is unclear if these will continue. But evidence from our study shows that drier conditions have existed in the past and did trigger a breakdown of the peatlands as a store of carbon.
“This is an important message for world leaders gathering at the COP27 climate talks next week. 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.”
Warnings from the past
The Congo peatlands in central Africa are the world’s largest tropical peatlands complex, occupying an area of 16.7 million hectares, bigger than England and Wales combined.
Congolese and European scientists took peat samples from beneath the remote swamp forests of central Congo. By analysing plant remains, the researchers were able to build a record of the vegetation and rainfall in the central Congo basin over the last 17,500 years when the peat began to form.
Waxes from plant leaves, which were preserved in the peat, were used to calculate rainfall levels at the time the plant was living.
The findings - Hydroclimatic vulnerability of peat carbon in the central Congo Basin - paint a picture of a drier climate developing in central Africa, which began around 5,000 years ago.
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 carbon dioxide.
Ghost interval in the peat record
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.
Dr Yannick Garcin, from the National Research Institute for Sustainable Development of 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.
“The samples also reveal what the rainfall and vegetation was like when the peat was formed. Together they give a picture of a drying climate that got progressively drier until about 2,000 years ago.
“This drought led to a huge loss of peat, at least 2 metres. The drought flipped 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.”
Peatlands are ‘vulnerable’
The scientists warn that while the peatlands are currently largely intact and managed sustainably by local people, they are vulnerable.
Apart from the threat of the peatlands getting drier from climate change, the region is subject to additional pressures which could cause damage to the fragile peatland ecosystem, from draining the peatland for industrial-scale agriculture, logging, and oil exploration.
Professor Corneille Ewango, from the University of Kisangani in the Democratic Republic of the Congo 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, and 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.”
END
Congo basin peatlands expedition, from 2018
CREDIT
Greenpeace/Kevin McElvaney.
JOURNAL
Nature
METHOD OF RESEARCH
Case study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Hydroclimatic vulnerability of peat carbon in the central Congo Basin
ARTICLE PUBLICATION DATE
2-Nov-2022
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