26 July 2023
Sea Level Rise: Non-Permanence and Blue Carbon
Dr Abigail Mabey
Blue Carbon Sector Coordinator
Sea level rise introduces non-permanence risk to coastal ecosystems, particularly within the Mangroves sub-sector. Understanding the potential impact of sea level rise in the context of the project area is essential to fully understand the associated non-permanence risk.
We project exposure to sea level rise and determine whether the local context is likely to exacerbate or mitigate a project’s non-permanence risk.
We then assess the ability of the project to mitigate this risk, through their activities and buffer pool contributions and in relation to the project’s commitment period.
Sea level rise is accelerating and is expected to continue beyond 2100 in all emission scenarios.¹ This is largely driven by two factors: increased water volumes from melting ice sheets and glaciers, and the thermal expansion of seawater. Rising sea levels will impact habitats and communities in coastal areas through processes such as coastal erosion, flooding, and saltwater intrusion into coastal aquifers.² For projects in the voluntary carbon market (VCM), there is a risk that future sea level rise may drive the reversal of carbon emissions previously avoided or removed by project activities.
Risk due to sea level rise is incorporated into our assessment of non-permanence and is applicable to projects in numerous sectors, as it is relevant to any coastal system where rising sea levels may hamper project activities. Typically, this risk is mostly observed within Nature-Based Solutions, with the most pertinent link being to Mangrove projects within our Blue Carbon sector.
Mangroves are trees or shrubs that grow in the coastal intertidal zone, typically in brackish or saline water. Mangrove protection and restoration can increase resilience in coastal areas and reduce coastal flooding.³ However, such benefits may not occur until the mangrove forest is well established as seedlings cannot provide coastal protection.⁴ Even established mangrove forests are vulnerable to sea level rise, however mangroves with limited exposure to negative environmental drivers (such as anthropogenic impacts) may be more resilient.⁵ Sea level rise can also have implications for the wider Forestry sector, as terrestrial tree species in coastal areas may be vulnerable to saltwater intrusion.⁶ Mangrove forests may also be included within the project areas of largely terrestrial projects - within our ratings coverage, three Avoided Deforestation project areas include mangroves alongside terrestrial tree species.
To assess the non-permanence risk posed by sea level change, we have a multi-factor approach:
1. Projected exposure to sea level rise. Using data from NASA and the IPCC 6th Assessment Report, we predict sea level rise within the project area for the project’s commitment period relative to a baseline from 1995 to 2014. Our analysis includes two scenarios, assuming 2°C and 3°C increases in global mean surface temperatures towards the end of the century (Figure 1). We consider the topography of the site, local hydrological regimes, the species planted, land use, sediment transport, and tectonic and isostatic uplift where relevant. These project-specific variables and geographic context are also important given that some regions experience faster rates of sea level rise than the global average. ⁷
2. Assessment of sea level rise in buffer pool contributions. We assess whether there has been an appropriate contribution to account for impacts from sea level rise within the project’s buffer pool contribution. Buffer pool contributions depend on the perceived impacts of sea level rise by the project. An appropriate assessment by the project may temper non-permanence risk in our assessment.
3. Relation to Commitment Periods. To assess non-permanence risk, we first identify the commitment period of issued credits, and then investigate the risk of reversal within that time. The stated commitment period may be the crediting period, or it may be a longer duration of time. With this approach, credit risks are assessed based on their contractual commitment to deliver greenhouse gas (GHG) benefits in a fungible way across all sectors, and also, specific to rated vintages. We model the projected sea level rise to the end of the commitment period, and assess the impact of that within the context of the project area. To see more detail on how we incorporate commitment periods into our assessment of non-permanence, see our risk factors series on non-permanence.
4. Capacity for ecosystem resilience. Mangrove forests may be able to adapt to sea level rise in two main ways: through inland migration⁸ and by increasing their vertical profile.⁹ Projects may include inland migration as a potential mitigation for the impacts of sea level rise. However, for this to occur, suitable land must be available for the mangrove forest to move inland. As part of our analysis, we interrogate the potential for inland mangrove migration. This involves investigating the current land use, land tenure, and whether the project conducts any activities to facilitate this migration.
Mangroves can also adapt to sea level rise through raising the vertical profile of the soil. The surface elevation of mangrove forests can be increased by factors such as groundwater influx which causes soil swelling, geological uplift or glacial isostatic rebound, and accumulation of sediments. These sediments are deposited due to lateral movement in mangrove ecosystems via hydrological processes and through biological processes facilitated by the mangrove ecosystem itself. This includes decomposition of deadwood, root accumulation, and the accumulation of sediment within vegetative structures. ¹⁰
If mangrove ecosystems are able to raise their vertical profile faster than sea levels rise, non-permanence risk is likely to be reduced.¹¹ To assess the likelihood of this, we examine the availability of sediment to the site (which may be impeded through upstream land management such as dam building)¹², historical sedimentation rates (where available), and the severity of other non-permanence risks which may limit the vertical growth of mangrove soils, or reverse it. For example, both flooding and increased nutrient availability have been demonstrated to limit root production in some circumstances, thereby reducing the contribution of roots to soil volume. Extreme weather events such as tropical storms can also reduce vertical elevation through induced mangrove mortality.¹³
Sea level rise varies regionally and can have substantial impacts on coastal ecosystems. Sea level change is of particular importance for Blue Carbon projects involved in mangrove restoration and conservation but affects all nature-based projects in coastal areas. Taking a project-specific view to assess the risk from sea level change to carbon permanence is essential, as the impacts and subsequent risk to issuance will vary depending on the ecosystem context and the project’s carbon accounting. At BeZero, we combine project-specific projections of sea level rise with detailed assessment of ecosystem resilience and information about risk mitigations by the project proponent to provide a thorough understanding of the non-permanence risk associated with sea level rise across sectors.