National estimates of greenhouse gas emissions from oil palm plantations on tropical peatlands can be improved significantly with the help of computer modelling results from a recent study in Indonesia, according to scientists.
Countries reporting to the United Nations Framework Convention on Climate Change may be overestimating their carbon dioxide emissions from oil palm on converted peatlands while underestimating the release of nitrous oxide from decomposing peat. Inaccurate estimates can misrepresent the climate action of parties to the Paris Agreement and other international treaties.
These were among the findings of scientists at the Center for International Forestry Research and World Agroforestry (CIFOR-ICRAF) and the University of New Hampshire.
The authors based the study on simulated emissions from oil palm plantations in the Indonesian province of Central Kalimantan over a 30-year period. To begin with, field measurements of greenhouse gas emissions were collected over three years in order to capture the variability over time and extrapolate the results using computer modelling.
“In Indonesia, there is a lot of seasonal variation in rainfall, which can have a large impact on greenhouse gas emissions, so you really need field measurements that cover a full year, or several years to understand annual changes,” says Erin Swails, a research scientist at CIFOR-ICRAF and the study’s lead author.
Greenhouse gases
The scientists focused on carbon dioxide and nitrous oxide emissions, which together account for almost 100 percent of greenhouse gas emissions from oil palm plantations on peatlands. Methane – a potent gas that also contributes to global warming – was not considered in the study because these emissions are negligible in drained peatlands.
Oil palm plantations are responsible for much of the land-use change in Southeast Asia’s peat swamp forests, leading to drainage of soil, land-clearing fires and the use of fertilizers. This conversion process has a profound effect on the peat’s chemical composition and transforms the ecosystem from a potential carbon sink to a significant net source of greenhouse gas emissions, according to the CIFOR-ICRAF study.
Emission factors
The Intergovernmental Panel on Climate Change – the United Nations body for assessing the science relating to climate change – encourages the use of an internationally agreed framework to calculate and report national greenhouse gas emissions and removals, as outlined in its 2019 guidelines. These include default values of so-called emission factors, which are used to link the release of a greenhouse gas to the particular activity causing it.
The 2013 wetlands supplement provides “default” emission factors for oil palm plantations on organic soil to help quantify emissions of carbon dioxide, nitrous oxide and methane. An update of this supplement, based on research over the past decade, will be able to refine these factors.
Default data are often used when more specific figures are unavailable. However, this generic information may not reflect the real situation in any given country, highlighting the importance of fine-tuning emission factors. Currently, these are based on only a small number of observations from young plantations, resulting in uncertain estimates of greenhouse gas emissions, according to the study.
“The IPCC guidelines are based mostly on field measurements collected in Southeast Asia, primarily Malaysia and Indonesia. But, in general, very little data are available from peatlands in Latin America and Africa, so these regions would rely more on default emission factors,” Swails says.
The model
The authors used a “process-based model” that simulates physical and biological processes to describe an ecosystem’s behavior and is known as DNDC. It is particularly useful for investigating greenhouse gas emissions in tropical peatlands because it can replicate the impacts of common agricultural practices while incorporating wetland hydrology and biochemical processes in organic soil, the authors wrote.
Modelling supports the refinement of emission factors and can be very useful to countries developing their forest reference emission levels or reporting their national greenhouse gas inventories as it reduces uncertainty when estimating the release of such gases from peatlands, according to Swails.
“There are so many financial and logistical challenges involved in the collection of field measurements” she says. “We will always need field data, but models are another tool to help us refine emission factors.”
Improving estimates
The study showed that greenhouse gas emissions from oil palm plantations on tropical peatlands vary considerably over a 30-year period. The IPCC guidelines could be refined by reducing the default emission factors for carbon dioxide by 61 percent for the second decade and by 77 percent for the third decade, the authors recommended.
“If we are overestimating emissions from older plantations, it means we are inflating our baselines over the long term,” Swails says. “As a result, we might not be achieving the emission reductions that we think we are through policy changes and improved land management.”
Conversely, the emission factors for nitrous oxide could be increased by 34 percent in the latter two decades due to peat decomposition, and the increasing availability of ammonium – a form of nitrogen – for the formation of nitrous oxide.
However, the predicted rise in nitrous oxide emissions over time does not offset the corresponding decrease in carbon dioxide emissions, indicating that total greenhouse gas emissions decline over the 30-year rotation, according to the study.
“The implication is not that oil palm plantations become less of a problem the older they get, but that we need more accurate emission factors because these matter for understanding the impact of land-use change on greenhouse gas emissions and climate,” Swails says.
*This research received support from the U.S. and Norwegian governments, and was undertaken as part of the CGIAR research program on Climate Change, Agriculture and Food Security (CCAFS).
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