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

The Intergovernmental Panel on Climate Change (IPCC) emphasizes achieving net-zero greenhouse gas (GHG) emissions by 2050 to limit global warming. Natural climate solutions, such as protecting and restoring wetlands, are gaining prominence. Wetlands, however, present a complex relationship with climate change; they sequester atmospheric CO₂, acting as carbon sinks, but also release CH₄, a potent greenhouse gas. This duality makes it crucial to accurately assess their net climate impact. Inland wetlands are categorized into peatlands (thick organic matter accumulation) and mineral soil wetlands (thinner organic matter layer). While mineral soil wetlands accumulate less organic matter, their anaerobic conditions still promote CO₂ sequestration. However, these same conditions also lead to significant CH₄ emissions. Human alterations of wetlands, such as drainage and restoration, introduce further uncertainty into their GHG fluxes. This research seeks to understand how wetland conservation and restoration affect CO₂ and CH₄ emissions, ultimately determining their effectiveness as natural climate solutions in achieving mid-century climate targets. Accurate assessment requires comparing CO₂ and CH₄ fluxes using appropriate CO₂-e.q. metrics. The commonly used Global Warming Potential (GWP) metric, while adopted in international climate policy, has limitations. It assumes pulse emissions and doesn't fully account for the differences in the climate impact mechanisms of long-lived (CO₂) and short-lived (CH₄) climate pollutants. Alternative metrics like sustained-flux GWP (SGWP) and GWP* have been developed to address these shortcomings.

Existing literature extensively documents the carbon sequestration capabilities of wetlands and their various ecosystem services beyond climate change mitigation. However, there's ongoing debate regarding the net climate impact of wetlands due to CH₄ emissions. While many studies highlight the net cooling effect of CO₂ sequestration in intact wetlands over centuries, uncertainty exists regarding the balance between this cooling effect and the warming effect of CH₄ emissions. Similarly, the short-term climate benefits of restoring drained wetlands, crucial for meeting mid-century net-zero targets, remain debated. Previous research has explored various CO₂-e.q. metrics to account for the different atmospheric lifetimes and radiative efficiencies of CO₂ and CH₄, including GWP, SGWP, and GWP*. These studies show that the choice of metric significantly affects the interpretation of wetland climate impacts, making it necessary to evaluate these metrics and their implications for climate change mitigation strategies.

This study uses three CO₂-e.q. metrics (GWP, SGWP, GWP*) to assess the climate impact of temperate North American inland mineral soil wetlands. The researchers compiled yearly GHG flux rates (CO₂ and CH₄) from various sources for intact, drained, and restored wetlands. These data were then used as input for a GHG perturbation model to simulate changes in atmospheric GHG concentrations, radiative forcing (instantaneous and cumulative), and mean temperature change over a 500-year period following wetland state conversion. The model simulates the atmospheric inventories of wetland-derived CO₂ and CH₄ over time, calculating instantaneous and cumulative radiative forcing, and estimating the changes in mean surface temperature. Three scenarios were simulated: (1) intact wetlands remaining intact; (2) intact wetlands being drained; and (3) drained wetlands being restored. A 50-year pre-conversion period was included in the model to allow for the establishment of steady-state GHG fluxes. The model output provides atmospheric concentrations of wetland-derived GHGs to calculate instantaneous and cumulative radiative forcing and the impact on mean surface temperature. The mean surface temperature switchover time (when cooling surpasses warming) was also calculated for each scenario. The study then calculates cumulative CO₂-e.q. carbon budget profiles over 500 years for each CO₂-e.q. metric, assessing the influence of the metric on interpreting wetlands as natural climate solutions. CH₄ flux data, which wasn't normally distributed, were analyzed using K-means cluster analysis to identify low and high CH₄ flux clusters. The model simplifies wetland carbon dynamics by assuming constant yearly carbon sequestration rates and GHG emission factors within each wetland state. Monte Carlo simulations (1000 iterations) were used to account for uncertainties in the compiled data.

The study found that CO₂ fluxes ranged from -810 (±490 kg C-CO₂ ha⁻¹ yr⁻¹) for intact wetlands to 4898 (±1223 kg C-CO₂ ha⁻¹ yr⁻¹) for drained wetlands. CH₄ fluxes were categorized into low (0.02-149 kg C-CH₄ ha⁻¹ yr⁻¹) and high (326-724 kg C-CH₄ ha⁻¹ yr⁻¹) clusters. Intact wetlands with low CH₄ fluxes showed a small, short-term net warming effect followed by a net cooling effect over 500 years. Intact wetlands with high CH₄ fluxes exhibited net warming over the entire 500-year period, although a switchover to net cooling was projected after approximately 1000-2000 years. Draining intact wetlands with low CH₄ fluxes led to sustained net warming, while draining those with high CH₄ fluxes resulted in a short initial climate benefit followed by sustained warming. Restoring drained wetlands with low CH₄ fluxes showed immediate climate benefits (reduced warming) followed by a net cooling effect. The GWP* metric most closely reflected the model's net cooling/warming effects. Based on GWP*, intact wetlands with low CH₄ fluxes acted as net CO₂-e.q. carbon sinks after an initial period, while those with high CH₄ fluxes remained net carbon sources. Draining wetlands resulted in a shift to net carbon sources, while restoration led to a long-term shift to net carbon sinks (after an initial period). The study highlights the importance of considering the temporal dynamics of GHG fluxes and using appropriate CO₂-e.q. metrics for accurate assessment of wetland climate impacts.

The findings demonstrate the significance of using a dynamic CO₂-e.q. metric like GWP* to accurately reflect the warming/cooling effects of wetlands. The GWP* metric aligns well with the GHG perturbation model's results, capturing the short- and long-term impacts of short-lived (CH₄) and long-lived (CO₂) climate pollutants. In contrast, predefined-period metrics like GWP and SGWP obscure these effects. The results support the crucial role of intact wetlands as net CO₂-e.q. carbon sinks and net cooling agents, regardless of CH₄ flux levels, especially considering their long existence on the landscape. Wetland restoration, although producing initial CH₄ emissions, provides substantial long-term climate benefits, demonstrating its effectiveness as a natural climate solution. The study underscores the importance of implementing management interventions to minimize CH₄ fluxes from restored wetlands.

This study emphasizes the critical role of temperate inland wetlands in climate change mitigation. Intact wetlands are effective carbon sinks and provide net cooling, while restoration, although initially associated with CH₄ emissions, offers significant long-term carbon benefits. The use of GWP* is crucial for accurate assessment of wetland climate impacts. Future research should focus on reducing uncertainties from data scarcity and incorporating water table level changes, climatic variability, and nitrous oxide emissions for a more comprehensive understanding of wetland climate effects. Protecting existing intact wetlands and restoring degraded ones, while implementing strategies to control CH₄ emissions from restored wetlands, are essential steps towards achieving net-zero emissions goals.

The study acknowledges limitations arising from sparse empirical data, preventing standardization of data across wetlands. The assumption of constant GHG emission factors may introduce biases. The model does not explicitly consider the impact of water table changes, climatic variability, or climate change on GHG fluxes. Furthermore, the study omits nitrous oxide (N₂O) emissions. Future work should address these limitations to improve accuracy and generalizability.