The climate benefit of seagrass blue carbon is reduced by methane fluxes and enhanced by nitrous oxide fluxes

Abstract

Blue carbon is carbon stored long-term in vegetated coastal ecosystems, which constitutes an important sink for atmospheric carbon dioxide (CO2). However, because methane (CH4) and nitrous oxide (N2O) have higher global warming potentials (GWP) than CO2, their production and release during organic matter diagenesis can affect the climate benefit of blue carbon. Here, we present a meta-analysis synthesizing seagrass CH4 and N2O fluxes and long-term organic carbon burial rates, and use these data to estimate the reduced climate benefit (offsets) of seagrass blue carbon using three upscaling approaches. Mean offsets for individual seagrass species (34.7% GWP20;1.0% GWP100) and globally (33.4% GWP20;7.0% GWP100) were similar, but GWP20 offsets were higher, and GWP100 offsets were lower than globally, for the Australian region (41.3% GWP20;1.1% GWP100). This study highlights the importance of using long-term organic carbon burial rates and accounting for both CH4 and N2O fluxes in future seagrass blue carbon assessments.

Unveiling Profound Seasonal and Spatial Dynamics of Greenhouse Gases (CO2, CH4, N2O) in the Northern Benguela Upwelling System Unnamed Article

“…To standardize the comparison of GHG emissions in a common metric, we also computed CH 4 and N 2 O emissions in terms of CO 2 equivalent (CO 2 -e) emissions, following the methodology outlined by Eyre et al (2023);…”

“…We firstly determined the average fluxes for each sub-region during winter and summer and subsequently, CO 2 -e values were determined using the 20-year and 100-year global warming potentials (GWP) for each gas (Eyre et al 2023). The GWP 20 values for CH 4 and N 2 O stand at 79.7 and 273, while the GWP 100 values for CH 4 and N 2 O are 27 and 273 respectively, according to IPCC (2021).…”

It's time to broaden what we consider a ‘blue carbon ecosystem’

“…C Ecosystem level climate gas exchange perspectives: If there is a need to consider ecosystems in a complete climate gas exchange perspective, then significantly improved understanding of the complete climate gas balance is required. Such a balance has begun to be understood in marine ecosystems more widely (e.g., Bonaglia et al, 2022;Eyre et al, 2023;Roth et al, 2023) an additional level of complexity to priorities A-C. We already know that climate change affects the capacity for ecosystems to store carbon (e.g. .…”

“…In this context, while many incubation experiments for either net ecosystem metabolism or calcification do exist (as do theoretical carbon budgets) quantification of carbon recycling, and importantly burial efficiency is still lacking at suitable timescales. Ecosystem level climate gas exchange perspectives : If there is a need to consider ecosystems in a complete climate gas exchange perspective, then significantly improved understanding of the complete climate gas balance is required. Such a balance has begun to be understood in marine ecosystems more widely (e.g., Bonaglia et al., 2022; Eyre et al., 2023; Roth et al., 2023) but in particular remains very opaque for non‐classical blue carbon systems. Methane and nitrous oxide budgets (in addition to carbon dioxide) are also needed for non‐classical ecosystems to resolve the complete climate relevant budget—in systems that bury significant organic carbon, that burial may be balanced (as carbon equivalents) by substantial releases of climate gasses. Impacts of climate change : Projected climate change provides an additional level of complexity to priorities A–C.…”

“…Macreadie et al., 2019) and carbon losses due to methane and nitrous oxide efflux (e.g. Bonaglia et al., 2022; Eyre et al., 2023; Roth et al., 2023). Such recent science underpins investment in blue carbon conservation and using habitat restoration as a pathway for mitigating climate change.…”