Nature Geoscience Manuscript NGS-2020-12-02844

Tropical peatlands are threatened by climate and land cover changes but there remain substantial uncertainties about their present and future role in the global carbon cycle due to limited quantitative measurements. Between mid-2017 and mid-2020, measurements of carbon dioxide exchange at two contrasting sites in a coastal peatland in Sumatra, Indonesia indicate that the degraded peat swamp forest lost two-and a half times more carbon (40.1 ± 4.2 tCO₂ ha⁻¹ yr⁻¹, mean ± standard deviation) than the intact site (15.6 ± 3.8 tCO₂ ha⁻¹ yr⁻¹). Carbon losses increased as groundwater level declined, regardless of land cover. The significant carbon loss from the intact site, during an extreme drought caused by a positive Indian Ocean Dipole phase combined with El Niño, indicates the potential importance of climate regime in determining the future carbon balance of tropical peatlands. Avoiding carbon loss from these globally important ecosystems is of increasing importance in the context of global climate change mitigation and regional land use planning. Here, we show that protecting the remaining intact peatland in Southeast Asia from deforestation and fire offers a viable way to avoid significant carbon loss, which for our study in Sumatra was 25 ± 4 tCO₂ ha⁻¹ yr⁻¹.