The United Nations Intergovernmental Panel on Climate Change is drastically undercounting carbon emissions from the draining of peatlands in Southeast Asia to make way for oil palm plantations, according to a new report. Peatlands are crucial in sequestering carbon and slowing climate change.
Researchers from the University of Minnesota's Institute on the Environment and from the Union of Concerned Scientists, who published their findings in Environmental Research Letters earlier this month, say they set out to discover whether water table depth could be used as a proxy for measuring soil-carbon loss in plantations built on drained peat – a dense, marshy material that forms over thousands of years and holds immense carbon stores.
Peat forests cover about 250,000 square kilometers of Southeast Asia, according to the report. Over the past 15 years, the authors write, peat forests have been increasingly cleared, drained and burned for new oil palm and pulpwood plantations. This exposes the upper peat layer to oxygen, which spurs decomposition and pumps carbon into the atmosphere, driving global warming. After the United States and China, Indonesia is the world's third-largest emitter, largely on account of its peatland conversion.
A previous study found that, although peatlands cover only 2–3 percent of the Earth's land surface, they store about a quarter of the world's soil carbon – an amount roughly equal to the entire atmospheric load of carbon. In an effort to keep as much of that carbon in the ground as possible, many companies that buy commodities sourced from peat forests have committed to lowering their own carbon footprints by keeping the most problematic palm oil out of their supply chains.
Climatic changes caused by global warming are also causing peatlands to dry on their own, which leads to a vicious circle in which burning of peatlands feeds climate change, which in turn fuels additional carbon loss as the peat dries.
A peat swamp in Indonesia's Central Kalimantan province. Photo Rhett A. Butler
The UM and UCS researchers identified a correlation between how low the water table drops and how high the rate of carbon loss climbs, suggesting that water table monitoring can help companies more accurately measure greenhouse gas emissions from the palm oil they're buying and using in their own products or operations.
To arrive at their estimate of emissions from peat forest destruction, the researchers compiled data from published research assessing water table depth and carbon balance in tropical plantations built on peatlands, then compared two measurements of carbon loss: subsidence and mass balance.
To find the subsidence rate, they looked at not just how much the land had sunk but also how much carbon was stored in the soil of the plantations studied. But that alone, the authors write, does not give the full picture of how much the draining of a particular peat forest might contribute to global warming.
Accordingly, the researchers also used a mass-balance model to estimate carbon emissions based on the balance of carbon gains, such as new plant matter being added to the peatland, and losses, such as soil-carbon emissions. That allowed them to factor into their estimates both carbon dioxide and methane. The researchers say this led to a more accurate assessment of the global warming potential of converting peat forests into oil palm plantations.
The researchers found that at drainage depths of 70 centimeters, the annual rate of carbon loss is about 20 tonnes of carbon per hectare – nearly twice the assumed 12 tons of carbon per hectare per year the UN's IPCC uses to calculate emissions from oil palm plantations.
But the university's Kimberly Carlson said additional research was needed to fully reconcile her team's findings and the climate change panel's estimates. "While our calculations take advantage of an exciting set of newly published data, a serious lack of research in tropical peatlands means that such estimates of peat-carbon loss from plantation systems remain uncertain, and are frequently based on assumptions rather than empirical measurements," Carlson said in a statement.
The UCS's Lael Goodman said in a statement that the only feasible way to reduce greenhouse gas emissions from peatlands is to prevent palm plantations from expanding into intact peat forests.
"Our findings lend weight to the idea that draining peat soils should be avoided at all costs, due to the impact on global climate," Goodman said.