Potential hidden impact of sea level rise on Indonesian coastal peatland

Inescapable and swift, climate-change-induced sea level rise (SLR) poses risks to low-lying tropical coastal ecosystems globally. At present, research still heavily focusses on the saline-side of the coasts, such as mangrove forests, seagrass beds and saltmarshes. Meanwhile, potential dire impacts of SLR to similarly valuable freshwater carbon-dense coastal ecosystems, like the Indonesian coastal peatlands, are less well known and severely understudied. Coastal peatlands in Indonesia are freshwater forested wetlands that play an important role in tackling the triple planetary crisis: climate change, biodiversity loss and pollution. Mainly located in low-lying coastal plains, these ecosystems are highly vulnerable to the impacts of SLR.

Due to the slow nature of SLR, its impacts on coastal ecosystems can be assessed only through long-term or historical records. Sea level changes in the Holocene are extensively studied and widely used as analogue for future SLR. Especially in western Indonesia, where large areas of coastal peatlands are located, the sea level was five meters above the present level in the mid-Holocene. Palaeoecological records obtained from coastal peatlands that cover SLR changes throughout the Holocene therefore provide an opportunity to assess their ecosystem response to SLR. However, as most coastal peatlands in Indonesia formed after sea level stabilization in the late Holocene, documentation of an SLR impact on “fully established” coastal peatlands is almost nonexistent. For that, we conducted a long-term ecological study on a coastal peatland in southern Borneo known to form around 17,000 years Before Present (yr BP), which thus likely experienced the Holocene-SLR.

The results show evidence of large fire occurrences in the mid-Holocene (9,000 to 4,000 yr BP), when sea level was higher than the present level and El Niño Southern Oscillation (ENSO) was moderately active. ENSO is known to cause severe drought leading to peat swamp forest mortality and higher frequency of natural combustion source (lightning). However, the mid-Holocene fires were much larger or more severe than during the past 3,000 years, when ENSO was more frequent and stronger. It is possible that the impact of ENSO-related drought in the mid-Holocene was exacerbated by SLR. SLR-induced peatland salinization, as evident by mangrove encroachment into the study area, likely resulted in widespread peat swamp forest mortality, thus promoted high fuel availability during that period. This study warns that future SLR, in combination with projected stronger ENSO, could exacerbate the impact of current large-scale peatland degradation and drainage in Indonesia, and potentially lead to more devastating peat fire events, haze seasons and enormous carbon release.