The large role of declining atmospheric sulfate deposition and rising CO2 concentrations in stimulating future wetland CH4 emissions.

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Existing projections of wetland methane emissions usually neglect feedbacks from global biogeochemical cycles. Using data-driven approaches, we estimate wetland methane emissions from 2000 to 2100, considering effects of meteorological changes and biogeochemical feedbacks from atmospheric sulfate deposition and CO2 fertilization. In low-CO2 scenarios (1.5° and 2°C warming pathways), the suppressive effect of sulfate deposition on wetland methane emissions largely diminishes by 2100 due to clean air policies, with resulting emission increases (7 ± 2 Tg a-1) being 35 and 22% of total wetland emission changes. In mid-CO2 scenarios (2.4° to 3.6°C warming pathways), sulfate deposition changes modestly, and CO2 fertilization contributes >30% of wetland emission increases. Across all scenarios, biogeochemical feedbacks can stimulate 30 to 45% of future wetland emission rises. Under 1.5° and 2°C pathways, wetland methane emissions will likely increase by 20 to 34 Tg a-1 by 2100, representing 8 to 15% of the allowable space for anthropogenic methane emissions, a factor not yet considered by current assessments.