New perspectives on the potential of temperate freshwater wetlands as natural climate solutions based on different CO2 equivalent metrics

doi:10.21203/rs.3.rs-4165480/v1

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New perspectives on the potential of temperate freshwater wetlands as natural climate solutions based on different CO2 equivalent metrics

https://doi.org/10.21203/rs.3.rs-4165480/v1

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published 28 Sep, 2024

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There is debate about the use of wetlands as natural climate solutions due to their ability to act as a “double-edge sword” with respect to climate impacts by both sequestering CO₂ while emitting significant amounts of CH₄. We used CO₂-equivalent metrics to assess the net flux of greenhouse gases from wetlands on a comparable basis. Three CO₂-equivalent metrics were used to describe the relative radiative impact of CO₂ and CH₄—the conventional global warming potential (GWP) that looks at pulse GHG emissions over a fixed timeframe, the sustained-flux GWP (SGWP) that looks the sustained GHG emissions over a fixed timeframe, and GWP* that explicitly accounts for changes in the radiative forcing of CH₄ over time (initially more potent but then diminishes after a specific period)—against model-derived mean temperature profiles. GWP* most closely estimated the mean temperature profiles associated with net wetland GHG emissions. Using the GWP*, intact wetlands serve as climate solutions. However, restored wetlands only serve as natural climate solutions for mid-century net-zero carbon emission initiatives if effective strategies are implemented to control CH₄ fluxes.

The most recent Intergovernmental Panel on Climate Change (IPCC) assessment report states that a reduction to net-zero global greenhouse gas (GHG) emissions by 2050 is necessary to hold global average temperature rise to below a 2°C increase above preindustrial levels¹. To achieve net-zero GHG emissions by 2050, several countries (e.g., USA and Canada) have emphasized the potential of implementing natural climate solutions, which involves protecting, conserving, and restoring natural ecosystems to remove carbon dioxide (CO₂) from the atmosphere^2,3,4. The use of wetlands as natural climate solutions is gaining popularity given their ability to sequester atmospheric CO₂ while simultaneously delivering multiple co-benefits beyond climate mitigation^2,3,4,5. Wetlands promote the long-term removal of CO₂ by sequestering this GHG into organic matter that accumulates in these productive but anaerobic systems. However, the same conditions that promote the long-term accumulation of carbon are also the conditions that result in wetlands being a considerable source of methane (CH₄) globally^6,7,8,9,10.

To effectively use wetlands as natural climate solutions to achieve mid-century climate targets, it is essential to understand how protecting, restoring, and draining wetlands affect CO₂ and CH₄ emissions. Restoring drained wetlands can inhibit soil carbon oxidation and effectively reduce CO₂ emissions; however, this often comes at the cost of increased CH₄ emissions^{7, 8,10,11}. Conversely, draining and converting wetlands to other land uses can result in a substantial release of CO₂ to the atmosphere while reducing CH₄ emissions^12,13. Despite extensive research on how intact and restored wetlands can deliver a net cooling effect on climate at the timescale of centuries^3,11,14, scientific debate continues on (1) whether the cooling effect of CO₂ sequestration in intact wetlands can be offset by the warming effect of CH₄ emissions, and (2) whether restored wetlands deliver short-term natural climate solutions for countries aiming to achieve mid-century net-zero GHG emissions targets^2,3,4,15.

To better understand the climate footprint of wetlands and their capacity as natural climate solutions for mid-century climate targets, the atmospheric lifetime of wetland GHGs (i.e., CO₂ and CH₄) and the relative potential of these GHGs to absorb infrared radiation in the atmosphere (i.e., radiative efficiency) need to be assessed on a comparable basis^{16,17,18,19,20}. To facilitate this comparison, wetland GHG fluxes need to be normalized to CO₂-equivalent (e.q.) measures^18,21,22. The 100-year variant of the Global Warming Potential (GWP₁₀₀) has been formally adopted in international climate policy (e.g., Paris Agreement) and is the standard CO₂-equivalent metric for expressing emissions in the scientific literature and general media²³. Despite being broadly used, GWP₁₀₀ or any GWP variant have been criticized^14,20,24 as they make the incorrect assumptions that wetland GHG emissions occur as a single pulse¹⁸ and that wetland carbon based GHGs have the same climate impact mechanism, ignoring the differences in climate warming associated with long-lived climate pollutants (LLCPs, e.g., CO₂) and short-lived climate pollutants (SLCPs, e.g., CH4)^20,24. CO₂ in the atmospheric reservoir persists for millennia in the absence of active CO₂ removal efforts²¹. As a result, atmospheric temperatures increase continuously if CO₂ emissions are maintained^20,21,24. Conversely, CH₄ in the atmospheric reservoir persists for a much shorter time because of natural removal mechanisms (e.g., CH₄ oxidation)^15,20,21,25. The shortcoming of GWPs is that they overstate the cumulative effect of wetland CH₄ on total warming given that natural removal mechanisms of atmospheric CH₄ are not captured, thereby resulting in misleading conclusions when assessing how wetland ecosystems may serve as natural climate solutions^15,20,24.

Several CO₂-e.q. metrics have been used to explore the effects of wetland GHG fluxes (expressed as CO₂-e.q.) on radiative forcing over different timeframes. Neubauer and Megonigal¹⁸ developed two static CO₂-e.q. metrics, known as the sustained-flux global warming potential (SGWP) and the sustained-flux global cooling potential (SGCP), accounting for GHG efflux and influx, respectively. SGWP and SGCP have been broadly adopted within the wetland research community and are frequently used to infer wetland climate impacts and/or role in mitigation strategies^3,11,15,26. Recently, Allen et al.^19,20 and Cain et al.²⁴ introduced an alternative way of estimating CO₂-e.q. (i.e., GWP*) by relating a change in CH₄ emissions rate to a fixed quantity of CO₂. GWP* has been found to reflect the impact of anthropogenic CH₄ emissions more accurately on average global temperature as compared to the conventional GWP metrics^21,24. Despite progress towards identifying a physically based CO₂-e.q. approach to assessing wetland climate footprints on a comparable basis, debate continues on what is the most appropriate way for simple yet effective CO₂-e.q. comparison of GHG emissions under different timeframes being considered ^{15, 21, 24}.

Here, we explore the potential of freshwater mineral wetlands (hereafter wetlands) as natural climate solutions using different CO₂-e.q. metrics (GWP, SGWP, GWP*). These wetlands make up most of the wetland area in temperate regions, where human settlements are largest and wetland losses greatest, and where restoration of these wetlands hold great promise in terms of serving as effective natural climate solutions^{3, 4, 27}. To test the various CO₂-e.q. metrics for mid-century natural climate solutions targets we: compiled yearly (snow free season) GHG flux rates for freshwater wetlands (Fig. 1); sorted these GHG flux rates into three scenarios (i.e., wetlands that remained intact, wetlands that were drained, and wetlands that were drained and then restored); used these GHG flux rates by wetland scenario input to a GHG perturbation model¹⁷ to simulate the changes in atmospheric concentration of wetland GHGs and the instantaneous radiative forcing, cumulative radiative forcing, and the impact on average temperature associated with changes in wetland GHG fluxes following a change in wetland state. Furthermore, we calculated the global mean surface temperature switchover time (i.e., the length of time after which the warming effect due to CH₄ emissions is overtaken by the cooling effect of CO₂ sequestration) associated with the change in wetland state^{13, 28}. Finally, we created cumulative CO₂-e.q. carbon budget profiles over 500 years for each of the CO₂-e.q. metrics (i.e., GWP, SGWP, GWP*), assessing the influence of the CO₂-e.q. metrics on interpretation of wetlands as natural climate solutions.

Wetland CO₂ and CH₄ fluxes

The CO₂ fluxes were normally distributed and ranged from − 1429 (± 761 kg C-CO₂ ha^− 1 yr^− 1) for intact wetlands, -3104 (± 1755 kg C-CO₂ ha^− 1 yr^− 1) for restored wetlands, to 4617 (± 1554 kg C-CO₂ ha^− 1 yr^− 1) for drained wetlands. The CH₄ fluxes were not normally distributed; therefore, a K means cluster analysis was conducted on all CH₄ flux data and three clusters of CH₄ fluxes were identified: low (-0.51 to 220 kg C-CH₄ ha^− 1 yr^− 1), moderate (262 to 600 kg C-CH₄ ha^− 1 yr^− 1), and high (940 to 1270 kg C-CH₄ ha^− 1 yr^− 1). Low, moderate, and high CH₄ were observed in intact wetlands, but only low and moderate CH₄ fluxes were observed in restored wetlands, and only low CH₄ fluxes in drained wetlands. The majority of the CH₄ flux data within each of the intact (73%) and restored wetlands (60%) fell within the low cluster.

Net cooling vs. warming effect of wetlands

Here, we focused on the effect of wetland state conversion on radiative forcing and changes in global mean surface temperature. The GHG perturbation model was used to simulate the atmosphere inventories of wetland derived CO₂ and CH₄ at any given time and estimate the pattern of radiative forcing (instantaneous and cumulative) and global mean surface temperature change (K) following the conversion of wetland state (from intact to intact, intact to drained, and drained to restored). The model input was the different categories of wetland GHG flux data for each wetland state. The model output was the atmospheric concentration of wetland derived carbon-based GHGs at any given time that was then used to calculate the instantaneous radiative forcing (W/m²) and cumulative radiative forcing (W/m²) of the wetland carbon-based GHG fluxes. Meanwhile, the impact of instantaneous and cumulative radiative forcings on the global mean surface temperature was calculated by multiplying the temperature conversion factor (~ 1 K per 1.23 W/m² radiative forcing). For each of the modelled scenarios of conversion of wetland state, a 50-year pre-conversion and a 500-year post-conversion time-period was used, since 500-year allows for successional steady-state to occur in many terrestrial ecosystems, including wetland ecosystems^{16, 18}. For each conversion scenario, the baseline was what would happen if the wetland conversion did not take place (i.e., for draining of intact wetland with low, moderate, or high CH₄ flux, the baseline was maintaining intact wetland with low, moderate, or high CH₄ flux; and for restoring of drained wetland, the baseline was remaining drained wetland). We simplified the modeling of the effect of wetland carbon dynamics on the climate by assuming yearly wetland carbon sequestration rates and GHG emission factors are constant within each wetland state.

Establishing intact wetland with low CH₄ fluxes (at year − 50) resulted in initial small and short period of net warming effect on the climate for approximately 30 years (instantaneous) and 50 years (cumulative), while maintaining intact wetlands with low CH₄ fluxes (at year 0) delivered a net cooling effect based on both instantaneous and cumulative radiative forcing-derived global mean temperature over the 500-year timeframe (Fig. 2A and D). Maintaining intact wetlands with moderate CH₄ fluxes (at year 0) deliver a net warming effect during the modelled 500 years and finally switch to deliver a net cooling effect after 600 years (instantaneous) and 1200 years (cumulative) (Fig. 4). Similarly, maintaining intact wetlands with high CH₄ fluxes (at year 0) delivered net warming effect during the modelled 500 years. The system had a switchover time to a net cooling effect after 1500 years (instantaneous) and 3000 years (cumulative) (Fig. 4). Given that most intact wetlands (73%) have low CH₄ fluxes, and that all intact wetlands (including those with moderate or high CH₄ fluxes) have been on the landscape since the retreat of the glaciers (about 10,000 years ago)²⁹, all intact wetlands would likely be currently contributing to a net cooling effect.

Draining intact wetlands with low CH₄ fluxes resulted in immediate net warming effect based on both instantaneous and cumulative radiative forcing derived mean temperature profile, which was sustained into the future (Fig. 2B and E). Further, draining intact wetlands with moderate CH₄ fluxes resulted in a small and short climate benefit (reduced warming, instantaneous (70 years) and cumulative (140 years)) (Fig. 2B and 2E), followed by a climate detriment which continued for an indefinite period (Fig. 2B and 2E), compared to remaining as an intact wetland with moderate CH₄ fluxes. Finally, draining intact wetlands with high CH₄ fluxes resulted in a stronger and longer climate benefit (i.e. reduced warming, instantaneous (230 years) and cumulative (410 years)) compared to remaining as an intact wetland with high CH₄ fluxes (Fig. 2B and 2E), followed by climate detriment (increased warming) which was sustained far into the future. Given many intact wetlands emit low levels of CH₄, draining intact wetlands resulted in immediate net warming effect. Despite draining intact wetlands with moderate and high CH₄ fluxes resulted in a short-term climate benefit, this was quickly overtaken by a long term increased warming effect and this heightened warming effect was sustained into the future.

Converting drained wetlands to restored wetlands with low CH₄ fluxes resulted in climate benefit (reduced warming) based on instantaneous (immediate, 0-year) (Fig. 2C) and cumulative (20-year lag period) (Fig. 2F) radiative forcing-derived global mean temperature compared to remaining a drained wetland, followed by a net cooling effect (after 70 years for instantaneous, Fig. 2C; and after 160 years for cumulative, Fig. 2F). Converting drained wetlands to restored wetlands with moderate CH₄ fluxes resulted in a larger magnitude, longer lasting net warming, followed by a net cooling effect (after 220 years for instantaneous, Fig. 2C; and after 480 years for cumulative, Fig. 2F). There were no restored wetlands with high CH₄ fluxes, and therefore this conversion was not modelled. Given that most restored wetlands (~ 60%) have low CH₄ fluxes, most wetland restorations will result in a reduced warming effect with a switchover to a net cooling effect after approximately 70 years (instantaneous) or 160 years (cumulative).

Carbon sinks vs. source status

A CO₂-e.q. metric is needed to assess wetland carbon sinks vs. source status on a comparable basis, thereby directly relating change in net cumulative CO₂-e.q. GHG emissions to their temperature responses. Here, we focused on comparing the cumulative carbon sink vs. source status of conversion of wetland states (intact-to-intact, intact-to-drained, drained-to-restored) using GWP*, the most recently proposed CO₂-e.q. metric, versus the earlier GWP and SGWP CO₂-e.q. metrics.

GWP*-derived net CO₂-e.q. carbon budgets (i.e., sink vs. source status) most closely aligned with the GHG perturbation model-derived mean temperature (i.e., the GWP* net carbon sink vs. source most closely aligned with the GHG perturbation model net cooling vs. warming effect). In the first scenario, establishing an intact wetland at year − 50 with low CH₄ fluxes resulted in a slight warming effect for about 30 years followed by a switch to a cooling effect for 500 years (Fig. 2A and D), which aligned with the increase in net carbon source for the initial 20 years followed by reduced net carbon source and subsequently net carbon sink effect calculated by GWP* (Fig. 3A). Establishing an intact wetland at year − 50 with moderate CH₄ fluxes resulted in a rapid warming effect for approximately 25 years followed by a gradual decline in the warming effect for the next 500 years, approaching a switchover point at 500 years (Fig. 2A), a pattern that aligned with the net carbon source effect calculated by GWP* which indicated an initial rapid increase (~ 20 years) and then stabilized carbon source effect (Fig. 3B). Establishing an intact wetland at year − 50 with high CH₄ fluxes resulted in an even greater magnitude of warming effect for approximately 30 years followed by a gradual decline in the warming effect for the following 500 years (Fig. 2A), a pattern that was similarly aligned with the net carbon source effect calculated by GWP* which indicated an even faster increase in carbon source effect for the initial 20 years after wetland establishment (Fig. 3C). For most intact wetlands (73% have low CH₄ fluxes), using GWP*, intact wetlands function as net carbon sinks once the suddenly introduced CH₄ associated with wetland establishment was neutralized by sustained CO₂ uptake, while intact wetlands with moderate and high CH₄ fluxes function as relative stable net carbon sources, with minor increase in their cumulative carbon source strength over the 500-year period. In contrast, using GWP₅₀₀ and SGWP₅₀₀, intact wetlands with low CH₄ fluxes function as net carbon sinks immediately, while intact wetlands with moderate to high CH₄ fluxes function as unstable net carbon sources, with their cumulative carbon source strength continuing to increase rapidly over the 500-year period (Fig. 3B and C).

In the second scenario, draining intact wetlands with low CH₄ fluxes resulted in a net warming effect for the entire 500 years post conversion period, which was then continue into the future (Fig. 2B and E), a pattern aligned with short-lived (approximately 10 years) reduced carbon sink effect due to the cessation of CH₄ emissions associated with wetland drainage followed by enhanced carbon source effects using GWP*, GWP₅₀₀, and SWGP₅₀₀ metrics (Fig. 3D). Draining intact wetlands with moderate and high CH₄ fluxes resulted in a period of reduced warming effect [Moderate: 70 years (instantaneous) and 140 years (cumulative); High: 230 years (instantaneous) and 410 years (cumulative)] given the cessation of CH₄ emission, patterns aligned with a period of net carbon sink effect for 80 years (moderate CH₄ fluxes) and 180 years (high CH₄ fluxes) calculated by GWP*, after which drained wetlands exhibited linearly increasing carbon source effects (Fig. 3E and F). GWP₅₀₀ and SGWP₅₀₀ derived net carbon budgets in drained wetlands with moderate or high CH₄ fluxes did not align with the mean temperature profiles (Fig. 2E and F); they failed to capture the initial post-drainage carbon sink effect as expressed by a reduction in temperature and thus overestimated the cumulative carbon source effect over the 500-year period.

In the third scenario, restoring drained wetlands to wetlands with low CH₄ fluxes caused a reduced warming effect for 70 years (instantaneous) and 160 years (cumulative) compared to baseline scenario (Fig. 2C and F) followed by net cooling effect, which was aligned with a 80-year reduced net carbon source effect followed by (an increasing) net carbon sink effect calculated GWP*, GWP₅₀₀, and SWGP₅₀₀ metrics (Fig. 3G). Restoring drained wetlands to wetlands with moderate CH₄ fluxes resulted in a approximately 50-year (instantaneous) and 90-year (cumulative) enhanced warming effect (Fig. 2C and F) followed by a reduced warming effect compared to baseline scenario (i.e., remaining a drained wetland), which aligned with a short term (~ 60 years) stronger carbon source effect informed by GWP* (Fig. SI-5I) (reminder, restored wetlands with high CH₄ fluxes were not observed). However, GWP₅₀₀ and SGWP₅₀₀ failed to capture the initial 60-year period of carbon source enhancement (Fig. SI-5G and H) and indicated that restoring drained wetlands to wetlands with moderate CH₄ fluxes immediately reduced carbon source strength. As a result, GWP* showed a longer net carbon source period for a duration of approximately 320 years in restored wetlands with moderate CH₄ fluxes as compared to GWP (~ 160 years) and SGWP (~ 240 years) over the 500-year period (Fig. 3H).

Our findings demonstrate that simulating the sustained CO₂ and CH₄ fluxes as they are emitted from or sequestered by wetlands and their atmospheric behavior is essential for predicting their warming/cooling effect. Meanwhile, using a suitable CO₂-e.q. metric that reflects the warming/cooling effect is essential for predicting their carbon sink/source status (Fig. 5). The goal of the Paris Agreement is to keep the rise in the Earth's average temperature well below 2°C above pre-industrial levels and to actively work towards restricting the temperature increase to 1.5°C. To achieve this initiative, CO₂ equivalent carbon emissions need to reach net zero (i.e., the balance of carbon sources and sinks) by 2050^{23, 24}. To assess the efficacy of wetlands as natural climate solutions for achieving mid-century net zero carbon emission targets, it is essential to have a method for calculating, reporting, and comparing CO₂-e.q. carbon budgets that can establish a reliable connection between wetland carbon sinks vs. source status and their corresponding warming vs. cooling effects for various wetland management scenarios.

GWP* offers a straightforward approach to calculate wetland CO₂-e.q. fluxes that correspond to their warming/cooling effects (Fig. 5)^{21, 24}. The dynamic CO₂-e.q. metric (GWP*) provided the most reliable indicator of warming/cooling and established a connection between warming/cooling and the cumulative CO₂-e.q. carbon budgets (Fig. 5); other static CO₂-e.q. metrics (i.e., GWP, SGWP) obscured these effects^{20, 21}. Unlike static CO₂-e.q. metrics that directly equate wetland CH₄ emissions by a single conversion factor and only represent one particular impact at or over a fixed timeframe (e.g., 100 and 500 years), GWP* shares the same characteristics as the GHG perturbation model, generating ‘equivalent’ CO₂ to describe dynamics over any timeframe of interest following wetland state conversion, which is essential for the detection of climate benefit/deficit periods^{21, 24}. Using GWP* is not merely a slight adjustment to the shape of the carbon budget and warming profile, or a more accurate scaling of cumulative wetland GHG emissions with temperature. It captures the distinct climate impact mechanisms associated with short- and long-lived climate pollutants, which is crucial to provide a broad indication of the potential of freshwater wetlands in terms of serving as effective solutions for mid-century climate targets.

Based on GWP* and the GHG perturbation model, our findings show that intact wetlands with low CH₄ fluxes (73% of the intact wetlands in our database) consistently served as cumulative net CO₂-e.q. carbon sinks and delivered net cooling effect on climate. Further, the short atmospheric lifetime of CH₄ means that intact wetlands with moderate and high CH₄ fluxes will eventually cause no additional warming once the effluxes and removes (CH₄ oxidation) of the atmospheric concentrations of CH₄ are approximately balanced^{19, 20, 21, 24}. The GHG perturbation model captured the short atmospheric lifetime of CH₄ and showed that maintaining intact wetlands characterized by moderate and high CH₄ fluxes delivered an initial net warming effect with a switchover time to a net cooling effect after 600–1500 years (instantaneous) and 1200–3000 years (cumulative) (Fig. 4). Given most intact wetlands in North America have been on the landscape for about 10,000 years^{29, 30}; intact wetlands with moderate and high CH₄ fluxes are effective net CO₂-equivalent carbon sinks and deliver net cooling effects on climate in contemporary times. Thus, protecting existing intact wetlands is an effective means of promoting wetlands as natural climate solutions and helping countries to meet their climate change mitigation and adaption targets by 2050^2,3,4.

Restoring drained wetlands has been widely promoted as an effective means of delivering natural climate solutions^{4, 8, 11, 12}. There are concerns that the overall climate regulation benefit may be offset by production and release of CH₄, since wetland restoration favors environmental conditions (i.e., prolonged hydroperiod) that can promote CH₄ emissions^{6, 8, 9, 10, 31}. With our simple model of wetland radiative forcing of climate and GWP*, we suggest that despite the increase in CH₄ emissions, restoring wetlands can deliver a net cooling effect on climate. Restoring wetlands with low CH₄ fluxes can deliver climate benefits within approximately 20 years (Fig. 2F), and therefore can contribute to mid-century climate targets. Based on our dataset, almost 60% of restored wetlands fall within the low CH₄ emission category (Fig. 1C), suggesting that restored wetlands in temperate North America are likely to provide climate cooling benefits and helping countries to meet their climate targets by 2050. However, restoring wetlands with higher CH₄ fluxes will take over 50 years to achieve the climate benefits (Fig. 2C and F). The lag period associated with moderate CH₄ fluxes will unfortunately limit the efficacy of restored wetlands to deliver climate solutions by mid-century. Thus, restoring wetlands will only deliver a net cooling effect for mid-century climate targets if appropriate restoration strategies are implemented. To take a precautionary approach for restoring wetlands to achieve mid-century targets, wetland restoration must be implemented as early as possible to halt the continued CO₂ emissions from drained wetlands. Meanwhile, it is also important to take restoration interventions that can minimize CH₄ fluxes from restored wetlands, since effective implementation of wetlands as natural climate solutions is likely to benefit most from simultaneous efforts to reduce both CO₂ and CH₄ emissions^{21, 28}.

Restoration interventions should be designed to modify physical, chemical, and biological characteristics of restored wetlands to inhibit CH₄ production and emission. First, water levels in restored wetlands should be maintained stable, since fluctuating water levels have been found to promote wetland CH₄ production and emission^12,31. Second, the ionic concentrations of restored wetland should be monitored, which has been found to regulate dominant anaerobic metabolism pathway. For instance, high abundance of sulfate associated with wetland salinization could suppress CH₄ production by altering the dominant anaerobic metabolism pathway from methanogenesis towards pathways that yield higher energy (e.g., SO₄²⁻ reduction)^{7, 32}. Third, plant community composition of restored wetland should be optimized, since cutting and/or grazing Typha within restored wetland could benefit the reduction of CH₄ production and emission³³. Finally, the riparian buffers should be established surrounding wetlands and/or at upland areas to restrict erosion and runoff induced carbon and nutrients from entering the restored wetlands, thereby constraining activities of CH₄-producing microbes, and reducing wetland CH₄ emissions³⁴.

Draining of intact wetlands is simply not a feasible climate solution, despite the existence of a short-term climate benefit period associated with cessation of CH₄ emissions. The continued increase in carbon source strength, due to consistent CO₂ emissions from drained wetlands and the accumulation of CO₂ in the atmosphere, imposes a long-term warming effect on the climate that will persist for an indefinite period. Meanwhile, given the higher amount of CO₂ effluxes associated with the loss of soil organic carbon because of wetland drainage^{3, 12}, the highest rate of increase in cumulative net CO₂-e.q. carbon source strength was observed in the initial 20 years after draining wetlands, which may cause significant adverse effects on zero carbon emission initiative by 2050.

Wetlands deliver other co-benefits, such as habitat preservation, water infrastructure enhancement, refuges for biodiversity and microclimate impact in addition to potential biogeochemical benefits^{5, 35, 36}. Wetland protection and restoration measures have focused on other co-benefits. However, our research demonstrates that most intact wetlands in temperate North America serve as natural climate solutions for mid-century net-zero carbon emission initiatives. To achieve the initiative of net zero carbon emission by 2050 and ultimately the climate targets, it is crucial to protect intact wetlands and promptly restore drained ones, while simultaneously implementing effective interventions to control CH₄ fluxes from restored wetlands.

Wetland conversion scenarios

Three wetland scenarios (Table. SI-1) were established to assess the net warming vs. cooling effect, and the carbon sink vs. source status (and the switchover time from carbon source to sink) associated with wetland conversion. For scenario 1, the pre- and post-conversion wetland states were intact. We assumed wetlands were created at year − 50 and remained intact from year 0 to 500 years. For scenario 2, the pre-conversion wetland state was intact, and the post-conversion wetland state was drained. We assumed wetlands were created at year − 50 and remained intact for 50 years before being drained at year 0, remaining drained for the next 500 years. Scenario 1 was used as a baseline scenario to compare against the conversion of intact wetlands to drained wetlands. For scenario 3, the pre-conversion wetland state was drained, and the post-conversion wetland state was restored. We assumed drained wetlands were drained at year − 50 and remained drained for 50 years before being hydrologically restored at year 0, remaining restored for the next 500 years. Scenario 2 was used as a baseline scenario to compare against the conversion of drained wetlands to restored wetlands.

We compiled CO₂ and CH₄ flux data as inputs for the model, simulated the net warming/cooling effect using the GHG perturbation model for each scenario, and calculated the net carbon/sink status of the wetland conversion using CO₂ e.q. metrics for each scenario. For each wetland scenario, Monte Carlo simulations (n = 1000 iterations) of the GHG perturbation models and CO₂-e.q. carbon budget calculations were used to capture the variability in switchover times due to the interannual variability in wetland carbon-based GHG fluxes and uncertainty in wetland carbon sequestration rates¹³.

Wetland CO₂ and CH₄ fluxes

Wetland carbon-based GHG fluxes data were compiled from published and unpublished (held by investigators) sources from the temperate region of North America (Fig. 1). Carbon sequestration data were estimated from eddy covariance and carbon analyzer-based measurements of organic carbon stock and sediment records. The compiled CO₂ and CH₄ flux data were from eddy covariance- and chamber-based measurements (c.f. Supplementary Information - Input Dataset). Efforts were made to compile wetland CH₄ flux data from different ecoregions throughout temperate North America to generate representative modeling results. However, the geographic distribution of CH₄ data clustered in some regions (i.e., the Prairie Pothole Region and US coastal areas) (Fig. 1a, SI-4). Greater spatial coverage of CH₄ data that would benefit from extensive flux tower establishments and satellite measurements (e.g., the MethaneSAT mission), will further improve our understanding on the warming versus cooling effects and the carbon sink versus source effects of wetlands under different physical conditions.

Statistical analysis (i.e., Shapiro-Wilk test, significance level p < 0.01) was performed on the compiled wetland carbon-based GHG flux data to determine the characteristics of the GHG flux distributions. Compiled CO₂ data for the various wetland states were normally distributed. However, the compiled CH₄ flux data for different wetland states exhibited a large range of variability and were not normally distributed. A K-means cluster analysis was conducted to establish three CH₄ flux ranges (low, moderate, and high). We used these three CH₄ flux range limits to bin CH₄ flux data from different wetland states into low, moderate, and high CH₄ fluxes (Fig. 1).

Net cooling vs. warming effect of wetlands

The GHG perturbation model simulates the atmosphere inventories of wetland carbon based GHGs and estimates CO₂ and CH₄ induced radiative forcing by considering the following three factors¹⁷.

First, the radiative efficiencies and atmospheric residence times of CO₂ and CH₄. The radiative efficiency of CO₂ is 1.75 x 10^− 15 Wm^− 2 per kg CO₂ and the radiative efficiency of CH₄ is 1.28 x 10^− 13 Wm^− 2 per kg CH₄²³. CH₄ is a short-lived climate pollutant and has the atmospheric lifetime of 12 years, while there is no single lifetime can be defined for CO₂ (generally from 3.4 to 10⁸ years)^14,17 because of the different rates of uptake by different non-atmospheric reservoirs (i.e., external biological, hydrological, and geological reservoirs) and removal processes³⁷.

Second, the oxidation of CH₄ to CO₂. The oxidation of CH₄ in the atmosphere involves reaction with the hydroxyl free radical (OH), producing CO₂ and H₂O as the primary products²⁵.

Third, the atmospheric CO₂ feedback among various non-atmospheric reservoirs. Atmospheric CO₂ equilibrates with three non-atmospheric reservoirs (i.e., external biological, hydrological, and geological reservoirs) over a variety of time scales, resulting in an exchange of CO₂ between the atmosphere and the non-atmospheric reservoirs. For instance, atmospheric CO₂ will have short-term exchange with the surface ocean, and long term (geological scales) exchange with continental rocks. Therefore, atmospheric CO₂ fixed by wetland essentially comes proportionally from each non-atmospheric reservoir.

The atmospheric inventory of CH₄ at any given time (t) therefore depends on the rate of wetland CH₄ fluxes and the loss rate of antecedent atmospheric CH₄ due to oxidation to CO₂. Meanwhile, wetland-derived atmospheric CO₂ inventory at any given time (t) depends on the rate of wetland CO₂ fluxes, and the gain rate of CO₂ due to the oxidation of CH₄ emitted from the wetland. Further, from a mathematical perspective, the interaction of atmospheric CO₂ with non-atmospheric reservoirs is modeled by considering the atmosphere as comprising four independent reservoirs of CO₂, each has its own reservoir fraction and a first-order CO₂ decay determined by the atmospheric perturbation lifetime^{18, 37}. Wetland-derived atmospheric CH₄ and CO₂ inventories at any given time (t) are therefore estimated using the following mathematical equations:

\({M}_{CH4-C\left(t\right)}={F}_{CH4-C}dt+\left[{M}_{CH4-C\left(t-1\right)}*{e}^{(-dt/\tau CH4)}\right]\) (Eq. 1)

where M_CH4−C is the mass of atmospheric CH₄ (g C m^− 2), F_CH4−C is the emission factor (g C m^− 2 y^− 1), dt is the time step (0.2 y), and \(\tau CH4\) is the atmospheric perturbation lifetime of CH₄ (12.4 years).

\({M}_{CO2-C\left(t\right)}=\sum _{i=1}^{4}{f}_{i}({F}_{CO2-c}dt+{M}_{CH4-ox})+\left[{M}_{CO2-C\_i\left(t-1\right)}*{e}^{(-dt/\tau CO2\_i)}\right]\) (Eq. 2)

where \({F}_{CO2-c}\) is the CO₂ flux factor (g C m^− 2 y^− 1), with CO₂ fluxes having a negative sign representing carbon sequestration, dt is the time step (0.2 y), \(\tau CO2\_i\) is the atmospheric perturbation lifetime for each of the four CO₂ pools (ranging from 4.3 to 394 years and one pool stays permanently in the atmosphere), and \({f}_{i}\) is the relative fractional size of pool i (range from 0.217 to 0.282)³⁷. \({M}_{CH4-ox}\) is the \(\left[{M}_{CH4-C\left(t-1\right)}*{e}^{(-dt/\tau CH4)}\right]\) term from Eq. 1, accounting for CH₄ oxidation to CO₂.

The atmospheric inventory of the wetland carbon-based GHG fluxes derived from the GHG perturbation model were subsequently converted into instantaneous radiative forcing of the wetland carbon-based GHG fluxes by multiplying the appropriate radiative efficiency value (i.e., 1.75 x 10^− 15 Wm^− 2 per kg CO₂ and 1.28 x 10^− 13 Wm^− 2 per kg CH₄)²³. The radiative efficiencies for CH₄ were then multiplied by a factor of 1.65 to account for the indirect effects of CH₄ on the global radiation balance (e.g., the impact of CH₄ on the concentrations of ozone in the troposphere and water vapor in the stratosphere)²³. The cumulative radiative forcing of the wetland carbon-based GHG fluxes was calculated by summing up the instantaneous radiative forcing values over the model period. Meanwhile, the impact of radiative forcing on the global mean temperature was estimated to be about 1 K for every 1.23 Wm^− 2 of radiative forcing^{28, 38}.

Wetland carbon sinks vs. source status

GWP is defined as the time-integrated change in radiative forcing due to a pulse emission of a given climate pollutant relative to a pulse emission of the same quantity of CO₂²³.

SGWP is defined as the time-integrated change in radiative forcing due to the sustained emission of a given climate pollutant relative to a sustained emission of the same quantity of CO₂¹⁸. SGWP is like GWP, as both are calculated based on cumulative radiative forcing of each gas, but SGWP is considered superior to the GWP as it explicitly considers the sustained behavior of wetland carbon-based GHG fluxes by treating wetland GHG emissions as persistent rather than pulse (one-time) events¹⁸. The emission of a given climate pollutant (e.g., CH₄) was converted into a CO₂-e.q. by multiplying the appropriate GWP or SGWP conversion factor (i.e., GWP₅₀₀ = 11; SGWP₅₀₀ = 14) for the specific time horizon using the following equation:

\({E}_{CO2-eq}={E}_{GHG}*{GWP}_{H} or {SGWP}_{H}\) (Eq. 3)

where E_CO2−eq is the CO₂-e.q. emission; E_GHG is the emission rate of the GHG (i.e., CH₄); GWP_H or SGWP_H is the time specific conversion factor; and H is the selected time horizon for GWP indices (e.g., 20, 100, 500 years).

GWP* is defined based on the differences in the atmospheric behavior between CO₂ and CH₄. Essentially, it considers a greater impact of new CH₄ emissions on temperature and recognizes that this impact diminishes after a specific period (i.e., 12 years) by treating (equalizing) sustained emissions of CH₄ as one-off release of a fixed amount of CO₂, since they both lead to a relatively stable increase in radiative forcing and mean surface temperature^{21, 24}.

Accordingly, GWP* establishes means of CO₂ equivalence by relating a change in the rate of CH₄ emissions to a fixed quantity of CO₂. The wetland CO₂-e.q. CH₄ budget was therefore calculated by accounting for changes in wetland CH₄ emission rate (ΔCH₄) instead of the magnitude of CH₄^{19, 20}. Cain et al.²⁴ advanced Allen et al.’s^{19, 20} work by accounting for the delayed temperature response associated with thermal equilibration to past increases in CH₄ emissions using the following equation:

\({E}_{CO2-w.e.}=\left(r*\frac{{\Delta }{E}_{SLCP}}{{\Delta }\text{t}}*H+s*{E}_{SLCP}\right)*{GWP}_{H}\) (Eq. 4)

where E_CO2−w.e. is CO₂-e.q. emission derived from the GWP*; Δ𝐸_{𝑆𝐿𝐶𝑃} is the change in wetland CH₄ emission rate over the time interval Δ𝑡 (years); E_SLCP is the current CH₄ emission rate; r and s are the weighting factors assigned to the climate impacts of the change in CH₄ emission rate and the current wetland CH₄ emissions, respectively (r + s = 1).

The parameters in Eq. 4 (i.e., Δ𝑡, r, s) were estimated from published literature. We adopted the same r (= 0.75), s(= 0.25) and Δ𝑡 (= 20 years) values used by Cain et al.²⁴. With these suggested parameters, the GWP* equation can be further simplified to:

\({E}_{CO2-w.e.}=\left(4*{E}_{SLCP\left(t\right)}-3.75*{E}_{SLCP\left(t-20\right)}\right)*{GWP}_{H}\) (Eq. 5)

where E_SLCP(t) is the current CH₄ emission rate, and E_{SLCP (t − 20)} is the rate of CH₄ emissions 20 years ago.

Contributions:

SM complied the data and performed the analysis with the support of IFC, PB and wrote the first draft. IFC assisted in study design and manuscript preparation. PB assisted in study design and manuscript preparation. All authors jointly revised the manuscript.

Contributions:

Acknowledgements:

This work was funded by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (05265-2019), an NSERC Strategic Partnership Grant (STPGP 506809), and an Environment and Climate Change (ECCC) Climate Action and Awareness Fund (CAAF) Grant (EDF-CA-2021i023) to IFC. Additional funding includes a Global Water Futures PhD Excellence Scholarship awarded to SM. We thank David Aldred for assistance with figures.

IPCC, 2023: Sections. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 35-115 doi: 10.59327/IPCC/AR6-9789291691647
Griscom, B. W., Adams, J., Ellis, P. W., Houghton, R. A., Lomax, G., Miteva, D. A., ... & Fargione, J. Natural climate solutions. Proceedings of the National Academy of Sciences 114(44), 11645-11650 (2017).
Drever, C. R., Cook-Patton, S. C., Akhter, F., Badiou, P. H., Chmura, G. L., Davidson, S. J., ... & Kurz, W. A. Natural climate solutions for Canada. Science Advances 7(23), eabd6034 (2021).
Creed, I. F., Enanga, E., Lobb, D. A., Badiou, P., Pattison-Williams, J. K., & Lloyd-Smith, P. Can restoration of freshwater mineral soil wetlands deliver nature-based climate solutions to agricultural landscapes? Frontiers in Ecology and Evolution, 622 (2022).
Thorslund, J., Jarsjo, J., Jaramillo, F., Jawitz, J. W., Manzoni, S., Basu, N. B., ... & Destouni, G. Wetlands as large-scale nature-based solutions: Status and challenges for research, engineering and management. Ecological Engineering 108, 489-497 (2017).
Bridgham, S. D., Megonigal, J. P., Keller, J. K., Bliss, N. B., & Trettin, C. The carbon balance of North American wetlands. Wetlands 26(4), 889-916 (2006).
Poffenbarger, H. J., Needelman, B. A., & Megonigal, J. P. Salinity influence on methane emissions from tidal marshes. Wetlands 31(5), 831-842 (2011).
Li, T., Canadell, J. G., Yang, X. Q., Zhai, P., Chao, Q., Lu, Y., ... & Qin, Z. Methane emissions from wetlands in China and their climate feedbacks in the 21st century. Environmental Science & Technology 56(17), 12024-12035 (2022).
Bao, T., Jia, G., & Xu, X. Weakening greenhouse gas sink of pristine wetlands under warming. Nature Climate Change 13(5), 462-469 (2023).
Zhang, Z., Poulter, B., Feldman, A. F., Ying, Q., Ciais, P., Peng, S., & Li, X. Recent intensification of wetland methane feedback. Nature Climate Change 13(5), 430-433 (2023).
Hemes, K. S., Chamberlain, S. D., Eichelmann, E., Anthony, T., Valach, A., Kasak, K., ... & Baldocchi, D. D. Assessing the carbon and climate benefit of restoring degraded agricultural peat soils to managed wetlands. Agricultural and Forest Meteorology 268, 202-214 (2019).
Badiou, P., McDougal, R., Pennock, D., & Clark, B. Greenhouse gas emissions and carbon sequestration potential in restored wetlands of the Canadian prairie pothole region. Wetlands Ecology and Management 19(3), 237-256 (2011).
Arias‐Ortiz, A., Oikawa, P. Y., Carlin, J., Masqué, P., Shahan, J., Kanneg, S., ... & Baldocchi, D. D. Tidal and nontidal marsh restoration: A trade‐off between carbon sequestration, methane emissions, and soil accretion. Journal of Geophysical Research: Biogeosciences 126(12), e2021JG006573 (2021).
Neubauer, S. C. On the challenges of modeling the net radiative forcing of wetlands: reconsidering Mitsch et al. 2013. Landscape Ecology 29(4), 571-577 (2014).
Bansal, S., Creed, I. F., Tangen, B. A., Bridgham, S. D., Desai, A. R., Krauss, K. W., ... & Zhu, X. Practical guide to measuring wetland carbon pools and fluxes. Wetlands 43(8), 105 (2023).
Whiting, G. J., & Chanton, J. P. Greenhouse carbon balance of wetlands: methane emission versus carbon sequestration. Tellus B 53(5), 521-528 (2001).
Frolking, S., Roulet, N., & Fuglestvedt, J. How northern peatlands influence the Earth's radiative budget: Sustained methane emission versus sustained carbon sequestration. Journal of Geophysical Research: Biogeosciences 111(G1) (2006).
Neubauer, S. C., & Megonigal, J. P. Moving beyond global warming potentials to quantify the climatic role of ecosystems. Ecosystems 18(6), 1000-1013 (2015).
Allen, M. R., Fuglestvedt, J. S., Shine, K. P., Reisinger, A., Pierrehumbert, R. T., & Forster, P. M. New use of global warming potentials to compare cumulative and short-lived climate pollutants. Nature Climate Change 6(8), 773-776 (2016).
Allen, M. R., Shine, K. P., Fuglestvedt, J. S., Millar, R. J., Cain, M., Frame, D. J., & Macey, A. H. A solution to the misrepresentations of CO₂-equivalent emissions of short-lived climate pollutants under ambitious mitigation. NPJ Climate and Atmospheric Science 1(1), 1-8 (2018).
Lynch, J., Cain, M., Pierrehumbert, R., & Allen, M. Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short-and long-lived climate pollutants. Environmental Research Letters 15(4), 044023 (2020).
Neubauer, S. C. Global warming potential is not an ecosystem property. Ecosystems 1-11 (2021).
Myhre, G., Shindell, D., Breon, F-M., Collins, W., Fuglestvedt, J.S., Huang, J., Koch, D., Lamarque, J.F., Lee, D., Mendoza, B., Nakajima, T., Robock, A., Stephens, G., Takemura, T., Zhang, H. Anthropogenic and natural radiative forcing. In: Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM, editors. Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. p. 659–740 (2013).
Cain, M., Lynch, J., Allen, M. R., Fuglestvedt, J. S., Frame, D. J., & Macey, A. H. Improved calculation of warming-equivalent emissions for short-lived climate pollutants. NPJ climate and atmospheric science 2(1), 1-7 (2019).
Dean, J. F., Middelburg, J. J., Röckmann, T., Aerts, R., Blauw, L. G., Egger, M., ... & Dolman, A. J. Methane feedbacks to the global climate system in a warmer world. Reviews of Geophysics 56(1), 207-250 (2018).
Kroeger, K. D., Crooks, S., Moseman-Valtierra, S., & Tang, J. Restoring tides to reduce methane emissions in impounded wetlands: A new and potent Blue Carbon climate change intervention. Scientific Reports 7(1), 1-12 (2017).
Nahlik, A. M., & Fennessy, M. Carbon storage in US wetlands. Nature Communications 7(1), 1-9 (2016).
Günther, A., Barthelmes, A., Huth, V., Joosten, H., Jurasinski, G., Koebsch, F., & Couwenberg, J. Prompt rewetting of drained peatlands reduces climate warming despite methane emissions. Nature communications 11(1), 1-5 (2020).
Eisenlohr Jr, W. S. Measuring evapotranspiration from vegetation-filled prairie potholes in North Dakota. Journal of the American Water Resources Association 3(1), 59-65 (1967).
Loder, A. L., & Finkelstein, S. A. Carbon accumulation in freshwater marsh soils: A synthesis for temperate North America. Wetlands 40(5), 1173-1187 (2020).
Holgerson, M. A., & Raymond, P. A. Large contribution to inland water CO2 and CH4 emissions from very small ponds. Nature Geoscience 9(3), 222-226 (2016).
Soued, C., Bogard, M. J., Finlay, K., Bortolotti, L. E., Leavitt, P. R., Badiou, P., ... & Kowal, P. Salinity causes widespread restriction of methane emissions from small inland waters. Nature Communications 15(1), 717 (2024).
Bansal S, Lishawa SC, Newman S, Tangen BA, Wilcox D, Albert D, Anteau MJ, Chimney MJ, Cressey RL, DeKeyser E, Elgersma KJ. Typha (cattail) invasion in North American wetlands: biology, regional problems, impacts, ecosystem services, and management. Wetlands 39:645-84 (2019).
Pasut, C., Tang, F. H., Hamilton, D., Riley, W. J., & Maggi, F. Spatiotemporal assessment of GHG emissions and nutrient sequestration linked to agronutrient runoff in global wetlands. Global Biogeochemical Cycles 35(4), e2020GB006816 (2021).
Bansal, S., Post van der Burg, M., Fern, R. R., Jones, J. W., Lo, R., McKenna, O. P., ... & Gleason, R. A. Large increases in methane emissions expected from North America’s largest wetland complex. Science Advances 9(9), eade1112 (2023).
Fluet-Chouinard, E., Stocker, B. D., Zhang, Z., Malhotra, A., Melton, J. R., Poulter, B., ... & McIntyre, P. B. Extensive global wetland loss over the past three centuries. Nature 614(7947), 281-286 (2023).
Joos, F., Roth, R., Fuglestvedt, J. S., Peters, G. P., Enting, I. G., Von Bloh, W., ... & Weaver, A. J. Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: a multi-model analysis. Atmospheric Chemistry and Physics 13(5), 2793-2825 (2013).
Stocker, T. F. et al. in Climate Change 2013 (eds Stocker, T. F. et al.) 33–118 (Cambridge University Press, Cambridge, UK and New York, USA, 2013).

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New perspectives on the potential of temperate freshwater wetlands as natural climate solutions based on different CO2 equivalent metrics

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

Wetland CO₂ and CH₄ fluxes

Net cooling vs. warming effect of wetlands

Carbon sinks vs. source status

Discussion

Methods

Wetland conversion scenarios

Wetland CO₂ and CH₄ fluxes

Net cooling vs. warming effect of wetlands

Wetland carbon sinks vs. source status

Contributions:

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

New perspectives on the potential of temperate freshwater wetlands as natural climate solutions based on different CO2 equivalent metrics

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

Wetland CO2 and CH4 fluxes

Net cooling vs. warming effect of wetlands

Carbon sinks vs. source status

Discussion

Methods

Wetland conversion scenarios

Wetland CO2 and CH4 fluxes

Net cooling vs. warming effect of wetlands

Wetland carbon sinks vs. source status

Contributions:

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Wetland CO₂ and CH₄ fluxes

Wetland CO₂ and CH₄ fluxes