Assessing the climate benefit of rewetting peatlands: Analysis of suitable metrics and implications of non-permanence for carbon farming schemes

Peatland rewetting is a nature based solution and considered a low hanging fruit that may substantially contribute to reaching the net zero goal. Its overall climate impact is very much determined by the interplay of CO₂ and CH₄ emissions that both strongly and non-linearly depend on water table depth. Rewetting peatlands acts instantaneously, and its overall climate effect depends on the considered time horizon.

In this study, which extends an approach developed for carbon sequestration in mineral soils (Leifeld & Keel 2022), the radiative forcing [W m^-2] of peatland rewetting is calculated along a gradient of water table depths of between -60 and +2 cm, based on recently published parameterizations for the temperate zone (Tiemeyer et al. 2020; Evans et al. 2021), for a time horizon of up to 1000 years. Two metrics, the cumulated radiative forcing and the switchover time, are considered.

Switchover times, i.e. the length of time after which the positive radiative forcing due to increases in CH₄ emissions at a restored peatland is overtaken by the cumulative negative radiative forcing due to CO₂ uptake, range from 398 to almost 798 years, depending on water table. Switchover can only be reached when the system is a net CO₂ sink, i.e. at water table depths of -7 cm or less. Both metrics, the cumulative radiative forcing and the switchover time, reveal optimum water table depths of between -6 and -2 cm (i.e., below surface). However, relative to a business-as-usual scenario with a water table of -60 cm, any water table raise improves the overall radiative balance of the rewetted system from the very beginning.

In case of non-permanence of rewetting (e.g., accidental drainage), radiative forcing calculations can be used to derive suitable, biophysically based risk buffer accounts for carbon markets. For example, keeping a peatland rewetted for only 30 instead of 100 years as contractually settled still yields a 51 % climate benefit relative to a business-as-usual. The implications of these findings are discussed in the light of the carbon farming schemes proposed by the EU.

 

References

Evans, C.D. et al., 2021, Nature 593, 548–552. Leifeld, J. & Keel, S.G., 2022, Geoderma 423, 115971. Tiemeyer, B. et al., 2020, Ecological Indicators 109, 105838.