Freshwater wetland restoration and conservation are long-term natural climate solutions

1. Introduction

Despite only covering an estimated 5–8 % of the Earth’s surface (Mitsch et al., 2013), wetlands (such as freshwater marshes, peatlands, and swamps) have a disproportionately large influence on the global carbon cycle. Wetlands emit 20–25 % of global methane emissions, yet they have the potential to serve as long-term carbon sinks (Mitsch et al., 2013; Mitsch and Mander, 2018; Rosentreter et al., 2021). At the global scale, freshwater wetlands are estimated to sequester between 0.7 and 1.75 Pg C year⁻¹ (0.7 to 1.75 × 10⁹ t C year⁻¹) (Lal et al., 2018). Importantly, teal carbon ecosystems like freshwater wetlands retain large parts of the sequestered carbon in their soils for centuries to millennia as anoxic soil conditions slow microbial decomposition (Richardson and Vepraskas, 2001). Overall, the carbon stored within freshwater wetland soils has one of the highest carbon densities among all terrestrial ecosystems, constituting one-third of the world’s total soil carbon stocks (Cole et al., 2007; Kayranli et al., 2010).

In recent centuries, competition for space due to increased urbanisation and agriculture has caused a decline in the extent and the condition of freshwater wetlands. An estimated 21 % of the global freshwater wetland area (3.4 million km²) has been lost due to drainage and land-use conversion between 1700 and 2020 (Fluet-Chouinard et al., 2023). Crucially, the degradation of natural wetlands is causing the release of significant quantities of previously stored soil carbon as carbon dioxide (CO₂) and, to a lesser extent, methane (CH₄), effectively turning natural wetlands from carbon sinks into carbon sources (Lal and Pimentel, 2008; Lane et al., 2016).

With global warming intensifying, the preservation of natural wetlands and the restoration of degraded ones have become increasingly popular natural climate solutions to help mitigate climate change (Bossio et al., 2020). Unlike other terrestrial ecosystems, freshwater wetlands (particularly peatlands) play a key role in reducing atmospheric carbon as their carbon storage potential does not reach saturation (Vepraskas and Craft, 2016). Beyond their role as natural climate solutions, freshwater wetlands offer several other benefits, such as providing habitat for a range of species, improving water quality, increasing water security, and protecting from floods (Chausson et al., 2020; Thorslund et al., 2017). Freshwater wetlands are, therefore, a comprehensive nature-based solution to mitigate the impacts of climate change.

There are several ways to restore degraded wetlands, including rewetting, active revegetation, and fencing. Rewetting is a common management intervention that is used to reinstate the natural hydrological connectivity of wetlands previously disconnected from their original waterway (Limpert et al., 2020; Meng et al., 2020). This approach often relies on redesigning the hydrology of degraded wetlands (e.g., digging out soil or sediment to create hummocks and tussocks) and the blocking of ditches and drains to deliver water to dried wetland areas to stimulate the development of wetland vegetation (Baldwin et al., 2018; Kayranli et al., 2010; Schwieger et al., 2021). Nevertheless, extended periods of dry conditions may cause the depletion of seed banks, thus preventing the return of native species of wetland plants following rewetting (Zedler, 2000). Restoration through rewetting is therefore more effective when combined with active revegetation, which involves the planting of seeds or plantlets of native wetland species in the restored wetlands (Spieles, 2022). In cases of severe wetland degradation, restoration efforts may also include the removal of non-native plant species such as farmed trees or the addition of topsoil to increase nutrient and substrate availability (Scott et al., 2020; Spieles, 2022). The fencing approach, conversely, is a passive approach to restoring wetlands through livestock exclusion. The presence of livestock can have several adverse effects on freshwater wetland ecosystems. For example, high-intensity grazing can reduce vegetation cover, thus substantially reducing a wetland’s potential for carbon sequestration and storage (Morris and Reich, 2013). Similarly, the presence of livestock can increase greenhouse gas emissions due to an increased nutrient input from manure (Bonetti et al., 2022). Limpert et al. (2021) found that excluding livestock grazing within wetlands in south-eastern Australia significantly increased soil carbon concentrations and lowered carbon emissions. Although these restoration techniques are well established, the time required to return a degraded wetland to its natural conditions and functions remains largely unclear.

Restoring degraded wetlands often produces an increase in methane (CH₄) and nitrous oxide (N₂O) emissions, complicating the assessment of wetland restoration as a natural climate solution (Gutknecht et al., 2006; Malerba et al., 2022; Serrano-Silva et al., 2014; Van Cleemput et al., 2007). Importantly, for freshwater wetlands to be an effective, long-term climate solution, the carbon they sequester and store must be greater than the CH₄ and N₂O they emit in terms of radiative forcing. Recent models predict that the increased CH₄ and N₂O emissions following restoration should be fully compensated by the concomitant increase in the net CO₂ uptake within 40–80 years after restoration (Günther et al., 2020; Zou et al., 2022). Empirical long-term studies investigating the greenhouse gas and carbon sequestration dynamics of restored wetlands, however, are rare. Nevertheless, wetland restoration will likely generate an initial warming effect on the environment as CH₄ and N₂O emissions overpower the cooling effects from carbon sequestration. As the warming potentials of CH₄ and N₂O in the atmosphere decrease over time, restored wetlands will switch from having a net radiative warming to having a net cooling effect on the climate (Nyberg et al., 2022). It is thus important to consider the true effects of CH₄ and N₂O to accurately assess the role of wetland restoration for climate change mitigation.

One way to estimate the net radiative effects of restored wetlands is to use the ‘switchover time’ framework (Neubauer, 2014; Neubauer and Megonigal, 2015). The switchover time accounts for the time since CH₄ and N₂O emissions and CO₂ sequestration have occurred and thus allows determining how long it takes for a restored wetland to have a net radiative cooling effect on the climate (Mitsch et al., 2013; Taillardat et al., 2020). Considering the time since emissions have occurred is particularly important since different greenhouse gases vary in their atmospheric perturbation lifetimes (Neubauer and Megonigal, 2015; Pierrehumbert, 2014). For example, it takes 12.4 years for a molecule of CH₄ to be oxidised to CO₂, whereas the breakdown of N₂O into nitrogen and oxygen through photolysis takes 121 years (Myhre et al., 2013). Other greenhouse gas metrics – such as the global warming potential (GWP), sustained GWP, or GWP* – are unable to estimate the time required for an ecosystem to have a net radiative cooling effect on the climate as such metrics fail to consider the time since greenhouse gas emissions have occurred and rather integrate the effect of greenhouse gas breakdown in the atmosphere for a fixed period of time (i.e., 100 years under the United Nations Framework Convention on Climate Change, UNFCCC). Estimating the switchover time of a restored wetland based on empirically measured CO₂ sequestration and CH₄ and N₂O emission rates is a powerful approach to evaluate the radiative role of restored freshwater wetlands in climate change mitigation.

In this work, we compiled published data and performed a global meta-analysis to determine the effects of freshwater wetland restoration on (1) the greenhouse gas exchange (CO₂ emissions or ecosystem respiration R_e, CH₄ and N₂O fluxes) and net CO₂ sequestration (net ecosystem exchange NEE, which takes into account CO₂ emissions and CO₂ uptake through photosynthesis), (2) the net ecosystem carbon budget (NECB), which allowed us to determine whether wetlands are net carbon sinks or sources, and (3) the net ecosystem radiative balance of restored wetlands. To do so, we compiled data from studies comparing greenhouse gas fluxes and carbon sequestration rates between restored wetlands and at least one degraded and/or natural wetland. We then calculated the switchover time of restored wetlands to determine the time required for these wetlands to have a net cooling effect on the climate after restoration.