New perspectives on temperate inland wetlands as natural climate solutions under different CO₂-equivalent metrics

Meeting the Paris Agreement goal of limiting global temperature rise to well below 2 °C requires achieving net-zero greenhouse gas emissions by 2050. Natural climate solutions, including protecting, conserving, and restoring ecosystems to remove atmospheric CO₂, are central to many national strategies. Temperate inland mineral soil wetlands can sequester CO₂ but also emit CH₄, creating uncertainty about their net climate effect, especially under human-induced state changes such as drainage and restoration. Conventional CO₂-equivalent metrics such as GWP100 are widely used but have been criticized for assuming pulse emissions and for inadequately representing differences between long-lived (CO₂) and short-lived (CH₄) climate pollutants. Alternative metrics (SGWP, GWP*) attempt to better reflect radiative forcing and temperature impacts over time, particularly for CH₄. This study addresses two key debates: whether the cooling from CO₂ sequestration in intact wetlands is offset by CH₄ warming, and whether restoration provides short-term climate benefits compatible with mid-century targets. The authors compile CO₂ and CH₄ flux data for temperate North American inland mineral soil wetlands, categorize CH₄ emissions into low and high clusters, simulate radiative forcing and temperature responses to wetland state conversions over 500 years, estimate switchover times from warming to cooling, and compare cumulative CO₂-e.q. carbon budgets using GWP, SGWP, and GWP* to evaluate wetlands as natural climate solutions.

The paper reviews the contrasting climate roles of CO₂ and CH₄ in wetlands: CO₂ uptake can provide long-term cooling while CH₄ emissions, though more radiatively efficient, are short-lived (~12 years). Widely used GWP metrics (e.g., GWP100/500) have limitations for continuous ecosystem emissions and can overstate cumulative CH₄ warming by neglecting atmospheric removal dynamics, potentially mischaracterizing wetland climate roles. SGWP was proposed to better represent sustained emissions/influxes for ecosystems. More recently, GWP* relates changes in CH₄ emission rates to equivalent CO₂, better tracking temperature impacts of short-lived pollutants and aligning more closely with integrated radiative forcing. Prior work shows intact/restored wetlands can deliver net cooling over long timescales but uncertainty remains about near- to mid-term effects and metric choice for policy-relevant horizons.

• Study scope: Temperate North American inland mineral soil wetlands (inland marshes). Compiled CO₂ and CH₄ fluxes from published/unpublished sources; CO₂ from eddy covariance and sediment radioisotope dating (137Cs, 210Pb), CH₄ from eddy covariance and chambers. CO₂ fluxes were normally distributed; CH₄ fluxes were non-normal and binned via K-means into low (0.02–149 kg C-CH₄ ha⁻¹ yr⁻¹) and high (326–724 kg C-CH₄ ha⁻¹ yr⁻¹) clusters; 67% of intact wetlands were low CH₄; drained and restored wetlands showed only low CH₄.
• Wetland state conversion scenarios over 550 years total: 50-year pre-conversion and 500-year post-conversion. Scenario 1: intact→intact (baseline). Scenario 2: intact→drained at year 0 (baseline for restoration). Scenario 3: drained→restored at year 0. Assumed constant annual sequestration and emission factors within each state. Year 0 approximates present (circa 2020), with -50 aligning to ~1970.
• GHG perturbation model: Simulates atmospheric inventories of wetland-derived CO₂ and CH₄ over time, accounting for (1) radiative efficiencies and atmospheric lifetimes (CH₄ lifetime 12.4 y; CO₂ represented by four pools with perturbation lifetimes ranging 4.3–394 y plus a permanent fraction; radiative efficiencies 1.28×10⁻¹⁵ W m⁻² kg⁻¹ CH₄, scaled by 1.65 for indirect effects, and 1.75×10⁻¹⁵ W m⁻² kg⁻¹ CO₂), (2) oxidation of CH₄ to CO₂, and (3) exchanges of atmospheric CO₂ with external reservoirs (biological, hydrological, geological). Atmospheric CH₄ inventory updated each time step (Δt = 0.2 y) via emission minus exponential decay; CO₂ inventory aggregated across pools including contributions from direct fluxes and CH₄ oxidation. Instantaneous radiative forcing (RF) computed by inventory × radiative efficiency; cumulative RF is time integral. Temperature response approximated as 1 K per 1.23 W m⁻² RF for instantaneous and cumulative RF-derived mean temperature. Monte Carlo simulations (n=1000) propagated uncertainty from flux data.
• CO₂-eq metrics: Compared GWP (pulse-based) and SGWP (sustained flux) over predefined 500-year horizon with conversion factors GWP500 = 11 and SGWP500 = 14 for CH₄, versus GWP* (dynamic) that relates changes in CH₄ emission rates to fixed CO₂ quantities. GWP* implemented as E_CO2-we = (4 × E_SLCP(t) − 3.75 × E_SLCP(t−20)) × GWP_H, using r = 0.75, s = 0.25, Δt = 20 y. Cumulative CO₂-e.q. carbon budgets were generated over 500 years for each scenario and metric. Switchover time (when CH₄-induced warming is overtaken by CO₂ sequestration cooling) was estimated from modeled temperature profiles.

• Flux characterization: Median CO₂ fluxes (kg C-CO₂ ha⁻¹ yr⁻¹): intact −810 (IQR 490), restored −2420 (IQR 1415), drained +4898 (IQR 1223). CH₄ flux clusters: low 0.02–149 and high 326–724 kg C-CH₄ ha⁻¹ yr⁻¹; 67% of intact wetlands were in the low CH₄ cluster; only low CH₄ observed for drained and restored sites.
• Intact wetlands: Initiating intact wetlands with low CH₄ flux showed a brief, small warming then a sustained net cooling over 500 years. Intact with high CH₄ flux produced net warming over 500 years, with switchover to cooling at ~1000 years (instantaneous RF-derived temperature) and ~2000 years (cumulative RF-derived). Given most intact wetlands are millennia old, both low- and high-CH₄ intact wetlands are net cooling today.
• Drainage of intact wetlands: Draining low-CH₄ intact wetlands caused immediate and persistent net warming over 500 years. Draining high-CH₄ intact wetlands yielded a short-lived relative climate benefit (reduced warming vs. remaining intact-high) that reversed to a sustained climate detriment (greater warming) thereafter.
• Restoration of drained wetlands: Restoring to low-CH₄ wetlands provided immediate climate benefit (reduced warming) relative to remaining drained, followed by a net cooling effect. No high-CH₄ restored wetlands were observed in the dataset.
• CO₂-e.q. carbon budgets and metrics: GWP* best matched modeled temperature responses. For intact low-CH₄ wetlands, GWP* showed a small rise in net CO₂-e.q. source for ~30 years, a ~10-year decline, then transition to a net CO₂-e.q. sink. Intact high-CH₄ wetlands remained a net CO₂-e.q. source over 500 years with only minor cumulative increase. GWP500 and SGWP500 classified intact low-CH₄ wetlands as immediate sinks and high-CH₄ as rapidly increasing sources, overemphasizing CH₄.
• After drainage: Low-CH₄ intact→drained showed a ~5-year reduced source/sink effect due to CH₄ cessation, then a stronger net CO₂-e.q. source (GWP*, GWP500, SGWP500). High-CH₄ intact→drained showed a ~65-year CO₂-e.q. sink under GWP* before becoming a linearly increasing source; GWP/SGWP failed to capture this initial sink, overestimating long-term source strength.
• After restoration: Drained→restored (low CH₄) showed an immediate reduction in net CO₂-e.q. source and, over ~110 years, transitioned to increasing net sink (consistent across GWP*, GWP500, SGWP500). Overall, GWP* provided dynamic CO₂-e.q. budgets aligned with RF-derived temperature profiles and better represented the contrasting roles of CH₄ and CO₂.

The study demonstrates that physically based simulation of sustained wetland CO₂ sequestration and CH₄ emissions, combined with an appropriate CO₂-equivalent metric, is essential to infer warming versus cooling and source versus sink status relevant to policy timelines. GWP*, by accounting for changes in CH₄ emission rates and their short atmospheric lifetime, closely tracks modeled temperature responses and avoids the biases of predefined-horizon GWPs that can overstate cumulative CH₄ warming. Results indicate that conserving intact inland mineral wetlands—particularly the majority with low CH₄ emissions—provides immediate, sustained net cooling consistent with net-zero 2050 goals. Even intact wetlands with historically high CH₄ emissions are net cooling in contemporary times due to their long residence on the landscape. Wetland drainage should be avoided because it creates persistent CO₂-driven warming that outweighs any short-term CH₄ reductions. Restoration of drained wetlands yields immediate climate benefits (reduced warming) and transitions to net cooling on decadal-to-century timescales, making it effective for both mid-century and long-term mitigation. To maximize near-term benefits, management of restored wetlands should aim to minimize CH₄ emissions through hydrologic stabilization, limiting sulphate inputs, managing vegetation (e.g., Typha control), and establishing riparian buffers, acknowledging potential trade-offs with biodiversity and ecosystem function.

By integrating a greenhouse gas perturbation model with multiple CO₂-e.q. metrics, the study shows that GWP* most accurately reflects temperature impacts of wetland CH₄ and CO₂ fluxes. Intact temperate inland mineral wetlands generally act as net CO₂-e.q. carbon sinks delivering net climate cooling; protecting these systems is a cost-effective, immediate natural climate solution aligned with net-zero 2050. Restoration of drained wetlands offers immediate reductions in warming and evolves to net cooling, especially when CH₄ emissions are minimized through targeted management. The work supports prioritizing wetland conservation and prompt restoration within climate strategies and provides a framework to apply dynamic metrics and perturbation modeling to other regions and wetland types. Future research should expand empirical datasets, include additional GHGs and lateral carbon fluxes, and account for hydrologic and climate variability to refine predictions.

• Sparse empirical data and inconsistent measurement designs prevented standardization across sites (e.g., colocated CO₂, CH₄, and sequestration measurements; year-round coverage; uniform methods).
• Post-conversion GHG fluxes likely vary interannually in early years; assuming constant emission factors can bias budgets.
• Water table dynamics, a key driver of CO₂ and CH₄ fluxes, were not explicitly modeled due to data limitations.
• The simulations did not incorporate concurrent responses to climatic variability or long-term climate change.
• Lateral fluxes of dissolved organic and inorganic carbon were assumed negligible; extension to coastal/tidal systems requires accounting for lateral exchanges.
• N₂O was excluded; while often small in inland mineral wetlands, including it—especially for drained agricultural contexts—could increase the apparent climate benefit of restoration.
• The perturbation model (and CO₂-e.q. metrics) assumes constant fluxes within states, which may not hold under real-world variability and successional changes.