Climate Benefits of Saltmarsh Restoration Greatly Overstated by Mason et al. (2023)

Abstract

The meta-analysis by Mason et al. (2023) provides many important insights into carbon storage and dynamics in saltmarsh ecosystems. However, we consider that their estimate of the net global mean climate benefit of saltmarsh restoration of 64.70 t CO₂e ha⁻¹ year⁻¹ is too high, by at least an order of magnitude.

Mason et al. determined the above value by adding atmospherically measured net CO₂ uptake (also known as net ecosystem exchange, NEE) to sediment-based organic carbon accumulation (C-acc) rates, then subtracting CO₂e values for methane (CH₄) and nitrous oxide (N₂O) emissions, using global arithmetic means for all parameters. However, the same atoms are involved in NEE and C-acc; such carbon was therefore double-counted (disregarding C imports/exports, a simplification also made by Mason et al.). Furthermore, most NEE data in Mason et al.'s database were daytime, growing-season measurements at low or mid-tide, using chambers. It is invalid to extrapolate these short-term CO₂ fluxes to annual rates since photosynthesis by saltmarsh vegetation either does not occur or is much reduced during nighttime, winter, and tidal immersion.

Eddy correlation NEE measurements in the database better integrate gas exchanges over large spatial areas and temporal periods. Their global mean value provides a carbon removal estimate of 7.8 t CO₂ ha⁻¹ year⁻¹, combining data for restored and natural salt marshes. Correcting for CH₄ and N₂O fluxes (using arithmetic mean values from Mason et al.'s table 1), changes this estimate to −1.3 t CO₂e ha⁻¹ year⁻¹; that is, indicating potential for net climate warming. Mason et al.'s CH₄ and N₂O data are also mostly from chamber measurements; however, they are less likely to be affected by diel bias.

Although of interest, atmospherically based determinations of carbon sequestration are indirect, and the relatively few eddy correlation studies (n = 34) are geographically biased. We, therefore, consider that C-acc rates provide a better (upper) estimate of climate benefit, being more directly derived from depth profiles of sediment organic carbon content and sediment accretion rates. Using data from Mason et al.'s table 1, global arithmetic mean C-acc (expressed in terms of CO₂ uptake) is 16.2 and 7.8 t CO₂ ha⁻¹ year⁻¹ for restored (n = 82) and natural (n = 312) salt marshes, respectively. With corrections for CH₄ and N₂O fluxes made separately for restored and natural sites, global mean values become −0.8 and 2.6 t CO₂e ha⁻¹ year⁻¹ respectively, for the two conditions, showing the nonnegligible (yet highly uncertain, Rosentreter et al. 2021) importance of these emissions, contrary to the conclusion drawn by Mason et al.

We therefore disagree with Mason et al. that saltmarsh restoration has clear potential to offset carbon emissions: the maximum benefit would seem < 0.05% of current CO₂ emissions rather than the 0.51% that they claim. Nevertheless, we recognize the value of protecting salt marshes (and their restoration, wherever feasible) for the many other environmental services that these ecosystems provide (Vegh et al. 2019).

Phillip Williamson: conceptualization, investigation, methodology, writing – original draft, writing – review and editing. Robert W. Schlegel: formal analysis, validation, writing – review and editing. Jean-Pierre Gattuso: conceptualization, validation, writing – review and editing. Julian E. Andrews: validation, writing – review and editing. Tim D. Jickells: validation, writing – review and editing.

The authors declare no conflicts of interest.

This article is a Letter to the Editor on Mason et al., https://doi.org/10.1111/gcb.16943. See also response to this letter by Mason et al., https://doi.org/10.1111/gcb.17526.