A new study appearing today in Nature shows that thousands of years ago, climate change turned the world’s largest tropical peatland from a major carbon store to a source of harmful carbon dioxide emissions.

The climate of central Congo began to dry around the time Stonehenge was built, causing the peatlands to leak carbon dioxide.

A major international study led by the University of Leeds found that the peatlands only ceased releasing carbon and switched back to collecting carbon out of the atmosphere in the past 2,000 years when the climate became wetter once more.

The study’s researchers warn that if modern-day global warming causes droughts in the Congo region, history might repeat itself, dangerously speeding climate change.

If this occurs, up to 30 billion tonnes of carbon dioxide, a strong greenhouse gas, might be discharged from the peatlands into the atmosphere. That’s the same as the amount of pollution made by burning fossil fuels around the world for three years.

These findings present “a brutal warning from the past,” says senior author professor Simon Lewis, highlighting, “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.” 

Insights from the past

With a total area of 16.7 million hectares, larger than England and Wales put together, the Congo peatlands in central Africa are the largest tropical peatlands complex in the world.

Scientists from Congo and Europe took samples of peat from under the remote swamp forests in the center of Congo. The researchers were able to create a record of the vegetation and rainfall in the middle Congo basin during the previous 17,500 years, when the peat first started to form, by analyzing plant remnants.

Calculations of rainfall during the plant’s life were made using plant waxes that were retained in the peat.

The results show that central Africa has been experiencing drier weather for around 5,000 years.

At the peak of the drought, annual precipitation was reduced by at least 800 millimeters. As a result, the Congo peatlands’ water table dropped, exposing older layers of peat to the air and resulting in their oxidation and release of carbon dioxide.

“Ghost interval”

Peat layers either degraded or failed to accumulate between 7,500 and 2,000 years ago. This time period was referred to as the “ghost interval” by the researchers. This ghost interval was found in peat samples from the Democratic Republic of the Congo (DRC), which is hundreds of kilometers away. This shows that it happened all over the peatland region.

“The peat samples,” according to lead author Dr. Yannick Garcin, “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.” 

They are “vulnerable.”

While the peatlands are mostly undisturbed and are being sustainably maintained by locals, scientists warn that this might change.

Along with the risk of the peatlands becoming drier due to climate change, the area is also under strain from logging, oil prospecting, and draining the peatlands for industrial-scale agriculture. These pressures could harm the delicate peatland ecosystem.

 “This is another astonishing finding about the peatlands,” adds 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. 

“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.”

Image Credit: Greenpeace/Kevin McElvaney

Source: 10.1038/s41586-022-05389-3