Supplementary material from "Climate change mitigation potential of wetlands and the cost-effectiveness of their restoration"

The cost-effective mitigation of climate change through nature-based carbon dioxide removal strategies has gained substantial policy attention. Inland and coastal wetlands (specifically boreal, temperate and tropical peatlands; tundra; floodplains; freshwater marshes; saltmarshes and mangroves) are among the most efficient natural long-term carbon sinks, yet they also release methane (CH₄) that can offset the carbon they sequester. Here, we conducted a meta-analysis on wetland carbon dynamics to (i) determine the role they play on climate and discuss their suitability for inclusion in greenhouse gas emissions (GHG) mitigation policies; (ii) investigate the cost-effectiveness of wetland restoration for climate change mitigation strategies. Depending on metrics, a wetland can simultaneously be a net carbon sink (i.e. boreal and temperate peatlands net ecosystem carbon budget (NECB) = −28.1 ± 19.13 gC m⁻² y⁻¹) but have a net warming effect on climate at the 100 years time-scale (i.e. boreal and temperate peatland sustained global warming potential (SGWP) = 298.2 ± 100.6 gCO₂ eq⁻¹ m⁻² y⁻¹). This ambivalence shows that the choice of metric can lead to misinterpretation on the effect of wetlands on global temperature. Moreover, our review report high heterogeneity and a still limited number of studies that document wetland carbon budgets. In this study, we demonstrate that most coastal and inland wetlands have a net cooling effect as of today. This is explained by the limited CH₄ emissions that undisturbed coastal wetlands produce and the short lifetime of CH₄ in the atmosphere, compared with long-term carbon sequestration for older inland wetlands. Analysis of wetland restoration costs relative to the amount of carbon they can sequester revealed that restoration is more cost-effective in coastal wetlands such as mangroves (US$1800 ton C⁻¹) compared with inland wetlands (US$ 4200–49 200 ton C⁻¹). For inland wetlands, priority should be given to conservation rather than restoration, while for coastal wetlands, both conservation and restoration may be effective techniques for climate change mitigation.