University of Bremen: Moore as climate tipping points

If peat bogs dry out, they can emit large amounts of greenhouse gases. Because they react so sensitively to climate change, they are important tipping points. A study published in Nature shows how sensitive carbon stored in peat is to environmental changes.
Not only seas and oceans bind carbon from the atmosphere, but also peatlands. They are considered to be the largest terrestrial carbon reservoirs. On the areas covered with water, plants, i.e. carbon, are decomposed and stored under oxygen-poor conditions – as long as the peat is covered with water. A moor as a carbon store only works if the moors do not dry out, for example due to climate change or human activities such as agriculture, peat extraction or road construction.

The Congo Basin is one of the largest river basins on earth. Tropical forests dominate in large parts here, but in the central basin, the so-called cuvette, swamp forests prevail. Until about the year 2000 it was assumed that there was only rainforest there. Only then did satellite images reveal that the land below the trees was under water. In 2017, mapping showed that the world’s largest tropical peatland complex is located here – in numbers over 167,600 square kilometers, which corresponds to over four times the area of ​​Baden-Württemberg. To promote the preservation of this unique ecosystem, 1.5 billion US dollars were pledged at the 26th United Nations Climate Change Conference in 2021, including by the European Union and Germany.

dr Enno Schefuss from MARUM – Center for Marine Environmental Sciences has been researching the Congo Basin and its importance for the global carbon cycle for a long time. In the spring of 2022, he led an expedition to the area to collect samples. The ongoing Franco-German joint project is co-financed by the German Research Foundation (DFG). Together with colleagues, he investigates the sensitivity of this unique ecosystem to climate change. “Almost nothing is known about the formation and history of this peat area and thus also its carbon dynamics. However, this understanding is important to determine the vulnerability of this ecosystem to climate change and to provide information on how deforestation, oil exploration and agriculture affect it,” says Enno Schefuss, one of the lead authors of theNature article.

By dating peat wells from the region, the researchers came across a pattern that was always the same. Between about 7,500 and 2,000 years ago there was a phase in which the peat was extremely condensed. Using geochemical analyses, they were able to determine that although peat was deposited during this period, it decomposed and lost most of the carbon. The current, condensed peat from this period is just the remnant of the original peat, several meters thick. At the same time, old carbon, i.e. undegraded degradation products of the old peat, was deposited in marine sediments off the mouth of the Congo. Such a river-to-ocean discharge of terrestrial carbon is an important component of the global carbon cycle,

What happened? “Using a so-called paleo-hydrological reconstruction, i.e. recording past rainfall conditions, we found that the moor dried up during this phase. We managed to get approximate information about how much it rained before, during and after the decomposition phase,” says Schefuss. It was interesting that the decomposition not only affected the peat that was formed during this time, but also the underlying peat layers. “So the decomposition ‘eaten’ its way into the peat.”

Using modern climate data, the exact peat distribution and the findings from the rain reconstructions, Schefuss and his colleagues were able to determine under what conditions the peat formed, under which it was broken down and what the situation is today. Before the decomposition phase, the rain conditions corresponded to those of today’s tropical bogs in North and South America, Asia and Oceania. During decomposition, it rained about a meter less per year. Only 2,000 years ago the situation stabilized again and the peat began to grow again. However, the peat bog in Central Africa is now in significantly drier climatic conditions than other tropical bogs. It is therefore – the authors of the current study come to this conclusion – dangerously close to the tipping point.

“Our task as scientists is to generate reliable data that allow policymakers to protect vulnerable ecosystems while enabling sustainable development. 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 can recover if the development is favorable,” adds Enno Schefuss. “I would strongly encourage further research, involving local colleagues, to better assess the vulnerability of this species- and carbon-rich ecosystem to 21st century climate change in order to predict its future evolution.”

The MARUM gains fundamental scientific knowledge about the role of the ocean and the sea floor in the entire earth system. The dynamics of the ocean and the sea floor significantly shape the entire earth system through interactions of geological, physical, biological and chemical processes. As a result, the climate and the global carbon cycle are influenced and unique biological systems are created. The MARUM stands for basic and open-ended research with responsibility to society, for the benefit of the marine environment and in line with the sustainability goals of the United Nations. It publishes its quality-controlled, scientific data and makes them freely accessible. The MARUM informs the public about new insights into the marine environment, and provides actionable knowledge in dialogue with society. MARUM’s cooperation with companies and industrial partners is carried out while maintaining its goal of protecting the marine environment.