Degradation increases peat greenhouse gas emissions in undrained tropical peat swamp forests

Tropical peat swamp degradation can modify net peat greenhouse gas (GHG) emissions even without drainage. However, current Intergovernmental Panel on Climate Change (IPCC) guidelines do not provide default emission factors (EF) for anthropogenically-degraded undrained organic soils. We reviewed published field measurements of peat GHG fluxes in undrained undegraded and degraded peat swamp forests in Southeast Asia (SEA) and Latin America and the Caribbean (LAC). Degradation without drainage shifted the peat from a net CO₂ sink to a source in both SEA (− 2.9 ± 1.8 to 4.1 ± 2.0 Mg CO₂–C ha⁻¹ yr⁻¹) and LAC (− 4.3 ± 1.8 to 1.4 ± 2.2 Mg CO₂–C ha⁻¹ yr⁻¹). It raised peat CH₄ emissions (kg C ha⁻¹ yr⁻¹) in SEA (22.1 ± 13.6 to 32.7 ± 7.8) but decreased them in LAC (218.3 ± 54.2 to 165.0 ± 4.5). Degradation increased peat N₂O emissions (kg N ha⁻¹ yr⁻¹) in SEA forests (0.9 ± 0.5 to 4.8 ± 2.3) (limited N₂O data). It shifted peat from a net GHG sink to a source in SEA (− 7.9 ± 6.9 to 20.7 ± 7.4 Mg CO₂-equivalent ha⁻¹ yr⁻¹) and increased peat GHG emissions in LAC (9.8 ± 9.0 to 24.3 ± 8.2 Mg CO₂-equivalent ha⁻¹ yr⁻¹). The large observed increase in net peat GHG emissions in undrained degraded forests compared to undegraded conditions calls for their inclusion as a new class in the IPCC guidelines. As current default IPCC EF for tropical organic soils are based only on data collected in SEA ombrotrophic peatlands, expanded geographic representation and refinement of peat GHG EF by nutrient status are also needed.