2025-12-25 北海道大学,日本文理大学,国立環境研究所,京都大学
土地利用変化に伴う1ヘクタールあたりの年間排出量の変化（CO2：トンCO2、CH4：キログラムCH4、GHG：トンCO2換算 = CO2 + 45×CH4）
＜関連情報＞
- https://www.hokudai.ac.jp/news/2025/12/post-2155.html
- https://www.hokudai.ac.jp/news/pdf/251225_pr.pdf
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025AV001861
東南アジアの熱帯泥炭地における二酸化炭素とメタンの正味放出量に与える土地利用変化と干ばつの影響 Impact of Land Use Change and Drought on the Net Emissions of Carbon Dioxide and Methane From Tropical Peatlands in Southeast Asia
Takashi Hirano, Tomohiro Shiraishi, Ryuichi Hirata, Masato Hayashi, Chandra Shekhar Deshmukh, Lulie Melling, Bettycopa Amit, Masayuki Itoh, Tomomichi Kato, Frankie Kiew, Sofyan Kurnianto, Kitso Kusin, Nardi Nardi, Nurholis Nurholis, Tiara Nales Nyawai, Elisa Rumpang, Ayaka Sakabe, Ari Putra Susanto, Joseph Wenceslaus Waili, Guan Xhuan Wong
AGU Advances Published: 16 December 2025
DOI:https://doi.org/10.1029/2025AV001861
Abstract
Peat decomposition is progressing in Southeast Asia due to lowered groundwater levels (GWL) caused by drainage. Additionally, droughts during El Niño events significantly lower the GWL, the main environmental factor that controls greenhouse gas (GHG; carbon dioxide (CO2) and methane) emissions in peatlands. Consequently, tropical peatlands have been recognized as a significant source of carbon emissions, and these emissions have been estimated for the region using constant decomposition rates of peat for each land use (Tier 1 emission factors). However, these factors hardly reflect the spatiotemporal variation of the GWL. Furthermore, these estimates do not account for CO2 uptake through photosynthesis. To reduce uncertainty, we developed a method to estimate spatiotemporal GWL variation from satellite-derived antecedent precipitation. Using the estimated GWL, we calculated the monthly net ecosystem-scale GHG emissions from peat forests and managed peatlands using the observed relationship between eddy covariance GHG fluxes and GWL, though carbon losses from deforestation, fires, and fluvial export were not covered in this study. Spatiotemporal variations in GHG emissions across Sumatra, Borneo, and the Malay Peninsula over a decade revealed the following: (a) Peat forests are a net source of CO2-equivalent GHGs, even when undrained, (b) Decadal mean annual GHG emission rates increase 2.8-fold when forests are drained and 6.4-fold when undrained forests are converted to managed peatlands, (c) Droughts increase total annual GHG emissions by 16% across the study area. Additionally, climate models projected precipitation increase in the mid-21st century, suggesting an increase in GWL and a consequent reduction in peat decomposition.
Plain Language Summary
Peatlands in insular Southeast Asia have accumulated large amounts of carbon in the form of peat because of their consistently high groundwater level (GWL), coexisting with peat swamp forests (PSF). Recently, however, oxidative decomposition of the peat into carbon dioxide (CO2) has been progressing due to lowered GWL caused by drainage for land development. Additionally, droughts during El Niño events significantly lower the GWL. Therefore, tropical peatlands have attracted attention as a significant source of CO2 emissions, though there are still many uncertainties. We have successfully quantified the spatial distribution of GWL and greenhouse gas (GHG; CO2 and methane) emissions from peatlands across Sumatra, Borneo, and the Malay Peninsula using a newly developed method that employs monthly satellite-derived precipitation data. Monthly maps of GHG emissions from 2011 to 2020 showed that PSF was a net source of GHGs, even when undrained, and El Niño-induced droughts increase annual GHG emissions by 16% due to lowered GWL. Moreover, GHG emissions increased 2.8-fold when PSF was drained and 6.4-fold when converted to plantations or farmland. In addition, according to climate models, precipitation is projected to increase in the mid-21st century, suggesting a potential increase in GWL and a decrease in CO2 emissions.
Key Points
- Spatiotemporal variations in groundwater level and net emissions of CO2 and CH4 were quantified from satellite precipitation
- Forest drainage increases annual greenhouse gas emission rates 2.8-fold, and land conversion to managed peatlands increases them 6.4-fold
- El Niño droughts increase total annual greenhouse gas emissions by 16% across Sumatra, Borneo, and the Malay Peninsula
