The greenhouse gas offset potential from seagrass restoration

Abstract

Awarding CO2 offset credits may incentivize seagrass restoration projects and help reverse greenhouse gas (GHG) emissions from global seagrass loss. However, no study has quantified net GHG removal from the atmosphere from a seagrass restoration project, which would require coupled Corg stock and GHG flux enhancement measurements, or determined whether the creditable offset benefit can finance the restoration. We measured all of the necessary GHG accounting parameters in the 7-km2Zostera marina (eelgrass) meadow in Virginia, U.S.A., part of the largest, most cost-effective meadow restoration to date, to provide the first seagrass offset finance test-of-concept. Restoring seagrass removed 9,600 tCO2 from the atmosphere over 15 years but also enhanced both CH4 and N2O production, releasing 950 tCO2e. Despite tripling the N2O flux to 0.06 g m−2 yr−1 and increasing CH4 8-fold to 0.8 g m−2 yr−1, the meadow now offsets 0.42 tCO2e ha−1 yr−1, which is roughly equivalent to the seagrass sequestration rate for GHG inventory accounting but lower than the rates for temperate and tropical forests. The financial benefit for this highly successful project, $87 K at $10 MtCO2e−1, defrays ~10% of the restoration cost. Managers should also consider seagrass co-benefits, which provide additional incentives for seagrass restoration.

Seagrass Recovery Following Marine Heat Wave Influences Sediment Carbon Stocks

“…Regardless of the exact mechanism, the net gain of sediment C at the outer sites shows that the meadow-wide MHW did not cause meadow-wide C losses. A related study mapping blue C in this meadow showed an increase in meadow-wide sediment C stocks from 2013 to 2016, despite the localized loss of sediment C in the inner meadow (Oreska et al, 2020). Our study confirms these findings and shows that disturbance from a MHW can have non-uniform effects on seagrass loss and recovery and on sediment C stock persistence at the meadow-scale (km 2 ).…”

“…Carbon offset credits may provide an avenue to finance seagrass restoration; voluntary credits are now available under a seagrass offset-credit accounting framework administered by Verra (formerly the Verified Carbon Standard Program). Assigning credits under this framework requires knowledge of the net carbon offset that a seagrass restoration project can generate over the project period, typically 30 years (Needelman et al, 2018;Oreska et al, 2020). As we show here, short-term disturbances can interrupt restoration trajectories, potentially altering the net carbon offset through time.…”

“…Climate change mitigation through blue C accumulation is one motivation for seagrass restoration efforts, and methodologies now exist to calculate and award carbon-offset credits from seagrass restoration projects (Emmer et al, 2015;Needelman et al, 2018). However, restoration of degraded meadows typically takes at least a decade (Greening and Janicki, 2006;McGlathery et al, 2012), and restored blue C stocks must persist for a period of 30 years to be eligible for carbon-offset credits (Oreska et al, 2020). Over the decadal time scales of seagrass restoration, short-term disturbance events threaten the persistence of slowly accumulating sediment C, potentially limiting the total blue C benefit from restoration.…”

Plant species determine tidal wetland methane response to sea level rise

“…Site salinity, a proxy for sulfate availability, is the best-established predictor of CH 4 emissions from tidal wetlands, but it weakly constrains emission rates 6,7 . Overall, CH 4 emissions from tidal wetlands are extremely variable, and many sites emit CH 4 at rates that exceed C sequestration in terms of CO 2 equivalents 2,8,9 . Drivers of variability in CH 4 emissions other than sulfate are poorly understood 7,10 .…”

Concerns and uncertainties relating to methane emissions synthesis for vegetated coastal ecosystems

“…There is strong interest in the role of vegetated coastal ecosystems in climate mitigation, arising from their high rates of carbon accumulation and significant ‘blue carbon’ stocks. However, quantifying those benefits also requires information on the counteracting emissions of greenhouse gases, primarily methane (CH 4 ) (Oreska et al., 2020; Rosentreter, Maher, Erler, Murray, & Eyre, 2018). In this context, we welcome the new global synthesis of methane emissions from mangrove forests, salt marshes and seagrasses by Al‐Haj and Fulweiler (2020), despite their worrying conclusion that the global marine methane production has previously been grossly underestimated with the possible loss of net climatic benefit of mangroves and salt marshes.…”