A national approach to greenhouse gas abatement through blue carbon management

Highlights

• We assess the potential inclusion of blue carbon within Australia's Emissions Reduction Fund, emphasizing issues and approaches that have global relevance.

• We identify twelve potential management actions then quantify and discuss the five most promising activities, encompassing the protection, restoration and creation of mangroves, tidal marshes and seagrasses.

• On a per area basis, mean abatement intensity of organic carbon was highest for the management activity ‘(re)introduction of tidal flow’ which may result in mean annual abatement of 13 – 15 Mg C_org ha⁻¹ yr⁻¹ for mangrove and 6 – 8 Mg C_org ha⁻¹ yr⁻¹ for tidal marsh.

• Our approach offers a template that uses best available information to identify options for carbon abatement through management of coastal landscapes.

Abstract

There is increasing interest in protecting, restoring and creating ‘blue carbon’ ecosystems (BCE; mangroves, tidal marshes and seagrasses) to sequester atmospheric CO₂-C and thereby contribute to climate change mitigation. While a growing number of countries aspire to report greenhouse gas emission and carbon sequestration changes from these ecosystems under voluntary international reporting requirements, few countries have domestic policy frameworks that specifically support the quantification and financing of carbon emission abatement through BCE management.

Australia, as home to approximately 5–11% of global blue carbon stocks, has a substantial interest in the development of blue carbon policy. Here we assess the potential inclusion of blue carbon within Australia's Emissions Reduction Fund, emphasizing issues and approaches that have global relevance. We used a participatory workshop of scientific experts and carbon industry stakeholders to identify blue carbon management actions that would meet the requirements of the Fund. In total, twelve actions were assessed for their greenhouse gas emission abatement potential and the ability to measure abatement reliably, using a combination of available data and qualitative and quantitative methods, including expert knowledge.

We identify and discuss the five most relevant and promising activities, encompassing the protection, restoration and creation of mangroves, tidal marshes and seagrasses. On a per area basis, mean abatement intensity of organic carbon (C_org) was highest for the (re)introduction of tidal flow resulting in establishment of mangrove (13–15 Mg C_org ha⁻¹ yr⁻¹) and tidal marsh (6–8 Mg C_org ha⁻¹ yr⁻¹), followed by land use planning for sea-level rise for the creation of new mangrove habitat (8 Mg C_org ha⁻¹ yr⁻¹). The avoided disturbance of existing mangroves, tidal marshes and seagrasses has the twofold benefit of avoiding remineralisation of existing stocks, plus the future annual abatement associated with the net sequestration of atmospheric CO₂-C as C_org with the continued functioning of these BCE. Our approach offers a template that uses best available information to identify options for carbon abatement through management of coastal landscapes, and details current knowledge gaps and important technical aspects that need to be considered for implementation in carbon crediting schemes.

Introduction

Among the multiple ecosystem services provided by mangroves, tidal marshes and seagrasses (Fig. 1), their capacity to sequester atmospheric carbon dioxide carbon (CO₂-C) in the form of organic carbon (C_org) and mitigate climate change has generated interest among scientists and policy makers (Bouillon et al., 2008; Duarte et al., 2013; Fourqurean et al., 2012; Macreadie et al., 2017a; McLeod et al., 2011). This is because it is estimated that coastal ecosystems contribute nearly half of the yearly C_org accumulation in marine sediments, despite occupying just 0.2% of the ocean surface (Chmura et al., 2003; Duarte et al., 2013; McLeod et al., 2011). Additionally, high sulfate concentrations in coastal waters promote biogeochemical conditions which minimise the production of methane (CH₄), a powerful greenhouse gas (GHG) emitted from most freshwater ecosystems (Poffenbarger et al., 2011). Consequently, mangroves, tidal marshes and seagrasses are considered as established ‘blue carbon’ ecosystems (BCEs; Lovelock and Duarte, 2019; Nellemann et al., 2009) due to their exceptional capacity to sequester atmospheric CO₂-C and store it as C_org over millennial time-scales (Lo Iacono et al., 2008; McKee et al., 2007; Rogers et al., 2019a).

Globally, destruction and degradation of all natural ecosystems is responsible for approximately 12–20% of the CO₂ released to the atmosphere (Houghton et al., 2012; Le Quéré, 2009). Cumulative coastal development is causing a net decline in the area of BCE, estimated at 1–3% yr⁻¹ globally (Hamilton and Casey, 2016; Short et al., 2011; Valiela et al., 2001; Waycott et al., 2009). Loss of BCE can result in the remineralisation of the C_org stored in living plant biomass as well as the loss of future CO₂-C sequestration potential. Erosion of disturbed blue carbon soils can result in the remineralisation of the soil C_org accumulated over millennia, leading to GHG emissions and thereby hampering efforts to mitigate climate change (Lovelock et al., 2017a).

Early efforts to mitigate CO₂ emissions through biosequestration focused on terrestrial forest ecosystems, including the development of global climate change mitigation solutions, such as the Reducing Emissions from Deforestation and Forest Degradation program (REDD+; IPCC (2003), payments for ecosystem services (Thomas, 2014) and Nationally Appropriate Mitigation Actions. In 2009, the United Nations Framework Convention on Climate Change (UNFCCC) identified opportunities to benefit from the high C_org sink capacity of BCE (Nellemann et al., 2009). Additionally, the Intergovernmental Panel on Climate Change (IPCC) completed the 2013 Wetland Supplement to the 2006 assessment. The supplement provides guidance for GHG accounting (Hiraishi et al., 2014) acknowledging the role of BCE in sequestering carbon dioxide carbon (CO₂-C) and providing GHG emission factors to estimate emissions associated with degradation as well as sequestration resulting from the restoration of these ecosystems. Activities detailed under the Wetland Supplement relevant to BCE include rewetting (conversion from drained to saturated soils by restoring inundation with saline water and reestablishment of vegetation); drainage for agriculture, forestry, mosquito control; extraction for construction of aquaculture, salt production and/or coastal infrastructure; as well as forest and soil management practices (Fig. 2).

Tools for the accounting and crediting of carbon payments also exist under the voluntary carbon market for BCE protection, restoration and creation (Mack et al., 2012; Plan Vivo, 2013; Restore America's Estuaries and Silvestrum, 2015; Sapkota and White, 2019).

Under the Paris Agreement, nations can independently decide how to lower their emissions through their Nationally Determined Contributions (NDC). Each nation can develop their NDC actions to include the conservation and/or restoration of ecosystems – including BCE – within their climate change mitigation or adaptation solutions. Following IPCC guidance, nations also have flexibility in how they estimate their carbon inventories, including the use of published global default values and activity data (Tier 1; high uncertainty); country or site specific data (Tier 2); or highly specific data and/or repeated measurements (Tier 3; lower uncertainty) (Eggleston et al., 2006).

At present, approximately 50 countries have recognised the importance of BCE as part of their NDC or climate action plans (International Partnership for Blue Carbon, 2017). With nations required to revise their NDC every five years, under a principle of increasing ambition, there is growing opportunity for more coastal nations to include BCE in their accounts (Herr and Landis, 2016). Despite this increasing recognition of blue carbon, few nations have domestic policy frameworks which specifically support the quantification and financing of carbon abatement through BCE management.

While BCE voluntary market mechanisms, including the Verified Carbon Standard (VCS) VM0033, have been designed for broad implementation (Restore America's Estuaries and Silvestrum, 2015), they may not align with the carbon accounting, reporting and policy requirements of national (or sub-national) governments (Fig. 2). This may present barriers to implementation of BCE voluntary market mechanisms, as is currently the case in Australia (Bell-James and Lovelock, 2019a).

Australia has more than 15,000 km² of tidal marshes, 10,000 km² of mangroves and 125,000 km² of seagrass meadows (Table 1). This is a considerable portion of the global BCE extent: approximately 3–37% of tidal marshes, 2–8% of mangroves and 15–43% of seagrasses - representing substantial C_org stocks and CO₂-C sequestration (Macreadie et al., 2017b; Serrano et al., 2019). Consequently, Australia is placed among the nations with the largest potential to benefit from developing blue carbon-focussed climate change mitigation schemes.

Australia's NDC states that Australia's economy-wide emissions reduction target will be developed into an emissions budget covering the period 2021–2030. Australia is one of 14 nations which include coastal wetlands in its NDC under the Land Use, Land Use Change and Forestry sector, with intention to embrace the IPCC 2006 Guidelines and IPCC 2013 Wetlands Supplement to account for its GHG targets (Herr and Landis, 2016). In 2016, Australia began reporting coastal wetland land-use change and/or management activities (incorporating mangroves, tidal marsh, seagrass and aquaculture) in its National Inventory Report to the UNFCCC. The Australian government has also begun to develop domestic blue carbon strategies to contribute to mitigating GHG emissions and climate change. Together, these steps could lead to BCE being specifically included in Australia's NDC, and provide a framework for other nations with similar objectives.

The Australian government has established the Emissions Reduction Fund (ERF; $AUD ~2.55 Billion) to encourage the adoption of management strategies that result in either the reduction of GHG emissions or the sequestration of atmospheric CO₂. The ERF is enacted through the Carbon Credits (Carbon Farming Initiative) Act 2011. Under the ERF, businesses, farmers and community groups can earn Australian Carbon Credit Units (ACCUs) by undertaking projects to reduce emissions or sequester atmospheric CO₂. ACCUs can also be traded outside of the ERF (i.e. by industry through voluntary market mechanisms) if an abatement project is accredited as an ‘eligible offsets project’ under the ERF. These projects must be in accordance with approved Methods - legislative instruments that define which activities are eligible to earn carbon credits, and how abatement is to be measured, verified and reported. All Methods must also comply with offsets integrity standards set out in legislation.

One of the key integrity standards is that carbon abatement generated through a Method must be able to be used to meet Australia's climate change targets under the Kyoto Protocol or other international agreements. That is, it must fall within the scope of land uses and/or activities which Australia has elected to include within its National Inventory Report. As noted above, this includes management activities in mangrove, tidal marsh and seagrass, although for seagrass only dredging projects are currently considered. A further requirement for incorporating Australia's BCE into the broader framework of the nation's carbon economy, is an assessment of additionality. For carbon abatement to be considered additional, management activities must remove additional CO₂-C from the atmosphere (sequestering it as C_org in vegetation or soil) and/or avoided GHG emissions(CO₂, N₂O and CH₄) that would otherwise be unlikely to occur in the absence of the incentive provided by the ERF.

In 2016, the Australian Department of the Environment and Energy commissioned a technical review of opportunities for including blue carbon activities within the ERF (Kelleway et al., 2017). Here we detail the approach, findings and recommendations of that technical review and offer a template for other nations seeking to identify options for carbon abatement through management of coastal landscapes and BCE. There are two broad objectives in this study: (1) to undertake a detailed assessment of anthropogenic management activities that have the potential to enhance C_org storage and/or reduce/avoid emissions of GHG in Australian mangroves, tidal marshes and seagrasses within the context of the nation's policy and legislative frameworks; and (2) provide recommendations on anthropogenic blue carbon management activities that could be applied and prioritised for potential Method development under the ERF. This includes estimation of likely carbon abatement potential (where possible), identification of barriers or constraints to implementation and outlining the steps required to address the identified barriers or constraints.