Climate Benefits of Saltmarsh Restoration Greatly Overstated by Mason et al. (2023),Global Change Biology

当前位置： X-MOL 学术 › Glob. Change Biol. › 论文详情

Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)

Climate Benefits of Saltmarsh Restoration Greatly Overstated by Mason et al. (2023)
Global Change Biology ( IF 10.8 ) Pub Date : 2024-10-18 , DOI: 10.1111/gcb.17525
Phillip Williamson, Robert W. Schlegel, Jean‐Pierre Gattuso, Julian E. Andrews, Tim D. Jickells

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.

Several other carbon accounting issues complicate the quantification of the climate mitigation benefit of saltmarsh restoration. They include

Nonnormal distributions. The frequency distributions of C-acc rates in Mason et al.'s database are highly skewed, as found for other blue carbon ecosystems (e.g., Breithaupt and Steinmuller 2022). Under such circumstances, medians or geometric means are generally considered preferable to arithmetic means in representing the central tendency: such values for restored salt marshes are given in Figure 1a. If medians are used both for C-acc rates and their CH₄ and N₂O corrections, net global climate benefits for restored and natural salt marshes become 9.6 and 3.9 t CO₂e ha⁻¹ year⁻¹ respectively.
Effect of time since restoration. The enhanced C-acc rate in restored salt marshes compared to natural ones is relatively short-lived (Figure 1b): 12 years after restoration, there is no statistical difference (Mann–Whitney U test).
Lateral carbon transport. A high proportion (> 25%) of carbon accumulating in salt marshes can be allochthonous (externally derived), either from terrestrial or marine sources (Williamson and Gattuso 2022; Smeaton et al. 2024). Such carbon is conventionally excluded from climate benefit accounting since additionality (Gustavsson et al. 2000) cannot be verified. High C-acc in recently restored sites can arise from a pulse of allochthonous carbon on re-flooding, prior to the re-establishment of halophilic vegetation (Mossman et al. 2022).
Regional and subregional variability. Mason et al.'s data set shows a 25-fold and 3-fold intercontinental variability in mean C-acc rates for restored and natural salt marshes, respectively, with 3-fold variability for the latter on a national basis within Europe (Smeaton et al. 2024). Data on African and South American salt marshes are currently lacking. A single global value is therefore potentially misleading, being dominated (76%) by North American studies.
Climate change impacts. Saltmarsh productivity is expected to significantly decline under future warming and sea-level rise (Bindoff et al. 2019; Li et al. 2020). Currently determined C-acc rates are therefore likely to overestimate future benefits.

Details are in the caption following the image — **FIGURE 1**
Open in figure viewerPowerPoint

Additional analyses of the global data set of carbon accumulation rates for restored salt marshes expressed as t CO₂ ha⁻¹ year⁻¹ from the Mason et al. (2023) database. (a) Frequency distribution showing strong skewness (kurtosis), with range of values for their central tendency. Mason et al.'s estimate of net climate benefit also shown, after correcting for CH₄ and N₂O fluxes. (b) Carbon accumulation rates as a function of time since restoration, showing that high initial rates are not sustained.

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).

中文翻译：

Mason 等人（2023 年）大大夸大了盐沼恢复的气候效益

Mason 等人（2023 年）的荟萃分析为盐沼生态系统中的碳储存和动态提供了许多重要见解。然而，我们认为他们对盐沼恢复 64.70 吨 CO₂e ha⁻¹ 年⁻¹ 的全球净平均气候效益的估计太高了，至少高了一个数量级。

Mason 等人通过将大气测量的净 CO₂ 吸收（也称为净生态系统交换，NEE）添加到基于沉积物的有机碳积累率（C-acc）中，然后减去甲烷（CH₄）和一氧化二氮（N₂O）排放的 CO₂e 值，使用所有参数的全局算术平均值来确定上述值。然而，相同的原子参与 NEE 和 C-acc;因此，这种碳被重复计算（忽略 C 进口/出口，Mason 等人也进行了简化）。此外，Mason 等人数据库中的大多数 NEE 数据是使用暗室在低潮或涨潮中期的白天生长季节测量。将这些短期 CO₂ 通量外推到年速率是无效的，因为盐沼植被的光合作用要么在夜间、冬季和潮汐浸泡期间不发生，要么减少很多。

数据库中的涡流相关 NEE 测量更好地整合了大空间区域和时间周期的气体交换。它们的全球平均值提供了 7.8 吨 CO₂ ^ha-1 年^-1 的碳去除估计值，结合了恢复盐沼和天然盐沼的数据。校正 CH₄ 和 N₂O 通量（使用 Mason 等人表 1 中的算术平均值），将此估计值更改为 -1.3 t CO₂e ^ha-1 年^-1;也就是说，表明气候净变暖的可能性。Mason 等人的 CH₄ 和 N₂O 数据也主要来自腔室测量;但是，他们不太可能受到 Diel 偏倚的影响。

尽管令人感兴趣，但基于大气的碳封存测定是间接的，并且相对较少的涡流相关性研究（n = 34）在地理上存在偏差。因此，我们认为 C-acc 速率提供了更好的（较高）气候效益估计，更直接地来自沉积物有机碳含量和沉积物吸积速率的深度剖面。使用 Mason 等人表 1 中的数据，恢复（n = 82）和自然（n = 312）盐沼的全球算术平均值 C-acc（以 CO₂ 吸收量表示）分别为 16.2 吨和 7.8 吨 CO₂ ^ha-1 年^-1。通过分别对恢复站点和自然站点的 CH₄ 和 N₂O 通量进行校正，在这两种条件下，全球平均值分别变为 -0.8 和 2.6 t CO₂e ^ha-1 ^year-1，显示出这些排放的重要性不可忽视（但高度不确定，Rosentreter 等人，2021 年），这与 Mason 等人得出的结论相反。

其他几个碳核算问题使盐沼恢复的气候缓解效益的量化变得复杂。它们包括

非正态分布。Mason 等人的数据库中 C-acc 率的频率分布高度偏斜，这在其他蓝碳生态系统中发现（例如，Breithaupt 和 Steinmuller 2022）。在这种情况下，通常认为中位数或几何平均值在表示集中趋势方面优于算术平均值：图 1a 中给出了恢复盐沼的此类值。如果中位数同时用于 C-acc 速率及其 CH₄ 和 N₂O 校正，则恢复盐沼和天然盐沼的全球气候净效益分别为 9.6 吨和 3.9 吨 CO₂e ha⁻¹ 年⁻¹。
自修复以来时间的影响。与自然盐沼相比，恢复盐沼中增强的 C-acc 率相对较短（图 1b）：恢复后 12 年，没有统计学差异（Mann-Whitney U 检验）。
横向碳传输。盐沼中积累的碳中很大一部分（> 25%）可以来自陆地或海洋来源（Williamson 和 Gattuso 2022;Smeaton 等人，2024 年）。由于无法验证额外性（Gustavsson 等人，2000 年），这种碳通常被排除在气候效益核算之外。在亲盐植被重新建立之前，最近恢复的地点的高 C-acc 可能是由再洪水时的异质碳脉冲产生的（Mossman 等人，2022 年）。
区域和次区域可变性。Mason 等人的数据集显示，恢复盐沼和天然盐沼的平均 C-acc 速率的洲际差异分别为 25 倍和 3 倍，后者在欧洲全国范围内的变化是 3 倍（Smeaton 等人，2024 年）。目前缺乏关于非洲和南美洲盐沼的数据。因此，单一的全球价值可能具有误导性，由北美研究主导（76%）。
气候变化影响。在未来变暖和海平面上升的情况下，预计盐沼生产力将显着下降（Bindoff 等人，2019 年;Li et al. 2020）。因此，目前确定的 C-acc 率可能会高估未来的收益。