The value of carbon sequestration and storage in coastal habitats
Introduction
Ecosystem services are commonly defined as “the outputs of ecosystems from which people derive benefits” (NEA, 2011). The ecosystem service of carbon sequestration and storage, linked to the provision of an equable climate, is a rapidly growing research field (Chung et al., 2011). Whilst there is extensive literature regarding the role of terrestrial habitats as a source and sink of greenhouse gases, the role of marine and coastal habitats is comparatively unknown. Recent research has shown however that ‘blue carbon’, that is carbon sequestrated and stored by marine and coastal habitats (Nellemen et al., 2009), could play a significant role in the global carbon budget (McLeod et al., 2011, Chung et al., 2011). At present, an estimated one third (∼2 Gt C yr−1) of anthropogenic CO2 emissions are sequestered by the oceans (Orr, 2001, Takahashi et al., 2002). In addition, coastal habitats such as mangroves, sand dunes and saltmarsh have the capacity to sequester carbon at a rapid rate (Chmura et al., 2003, Jones et al., 2008, Alongi, 2012) and, on accreting coasts, this may occur to considerable depth or lateral extent (Chmura et al., 2003). The relative carbon storage potential of coastal habitats is now considered to play a significant role in the regulation of both local and global climate (Nellemen et al., 2009, Irving et al., 2011, Pendleton et al., 2012).
Coastal habitats are at risk and in decline across the world (French, 1997, Martinez et al., 2004). Drivers of this decline include urban and industrial development, aquaculture, agriculture, tourism, forestry, coastal erosion and sea level rise (Jones et al., 2011). For example, ‘reclamation’ of coastal land for agricultural or industrial use alone, here termed ‘land claim’, has accounted for an estimated 25% loss of intertidal land in estuaries worldwide (French, 1997). In the UK, coastal margin habitats have been subject to considerable land use change over the last 100 years (French, 1997, Delbaere, 1998), with land claim through draining occurring on an industrial scale since the 1700s (Hansom et al., 2001). With conversion, degradation or loss comes a decline in their potential to sequester and store carbon. Pendleton et al. (2012) estimate that 0.15–1.02 Pg (billion tons) of CO2 are released annually through the conversion of vegetated coastal ecosystems resulting in economic damage, estimated to be in the order of $US 6–42 billion annually (£ 4–27 billion).
The social and economic significance of this ecosystem service in coastal systems is however poorly represented in policy and management decisions, and rarely features in global climate change mitigation discussions or documentation. In the context of coastal management, it is critical to recognise that any change in type, functioning and area of these ecosystems has the potential to influence carbon sequestration and storage (Everard et al., 2010). In addition to policy at a global scale, this capacity is also of importance for local scale ecosystem service accounting, for example when making decisions on coastal flood defence options such as managed realignment (Andrews et al., 2006).
Global level studies have raised awareness of coastal carbon (Pendleton et al., 2012), yet there is a continuing need for methodological development regarding the calculation of carbon sequestration rates, carbon stocks, and the valuation of this ecosystem service. In addition there is an on-going requirement for site specific data to support meaningful national and local scale policy. It is the aim of this study to address these issues using the UK as a case study.
The UK is selected as a case study partly because few studies have been published in this area. For example, published carbon sequestration rates in saltmarsh rely predominantly on US studies of saltmarsh (Kirwan and Mudd, 2012) which are geomorphologically different from European saltmarshes. Sequestration rates in saltmarsh have been estimated from extrapolation of sedimentation rates and carbon content of established saltmarsh sediments (Cannell et al., 1999, Adams et al., 2012) but do not quantify carbon stocks. In sand dune grasslands and dune wetlands, chronosequence approaches have been used to estimate carbon sequestration rates (Jones et al., 2008). However, no study has yet attempted an inventory of carbon stocks in these habitats and the implications of coastal change for carbon stocks are largely unquantified.
This paper is organised in three sections. Firstly, carbon sequestration rates and stocks are calculated. This study provides the first comprehensive inventory of carbon stocks and sequestration for the principal UK coastal margin habitats of saltmarsh, sand dune and machair dune grassland, including change over time from 1900 to 2060. Information is collated from published sources, the grey literature and unpublished data to calculate C stocks, and estimate the impact of future change. Given the rate of conversion of coastal habitats to other land uses globally and within Europe, the implications of this decline, both for the carbon stocks held and for future carbon sequestration, are explored. It is essential to understand the extent and stability of those carbon stocks and therefore to understand the permanence of storage.
Secondly, a valuation of the ecosystem service of carbon sequestration is undertaken. ‘Carbon stock’ is used to define the carbon stored in the given ecosystem, often shown in static units of g/m2 or g/m3. This is different to an ‘ecosystem service stock’, which in the case of carbon storage and sequestration is the ecosystem structure and processes (Luisetti et al., 2013), sometimes known as the natural capital. Neither stock is valued here. The carbon stock currently stored in coastal ecosystems is not valued as although stand-alone environmental values are valuable in raising awareness (Costanza et al., 1997, Beaumont et al., 2008), they do not aid the decision making process with regard to balancing trade-offs and selecting between different development options.
From the ecosystem service stock flows a variety of ecosystem services, one of which is carbon sequestration, or the rate of carbon uptake, which is generally measured in dynamic units such as g m−2 yr−1. The carbon stock has the potential to increase, via the ecosystem service of carbon sequestration (a positive flow of this ecosystem service), or decrease via habitat destruction and an accompanying release of CO2 (which could be interpreted as a negative flow of this ecosystem service, or dis-service). It is the net carbon sequestered which is valued here, and both aspects, the potential service and dis-service, are explored. The third and final section discusses the significance of these figures in terms of global coastal management and future recommendations are made. This approach will provide information to policy makers and coastal managers, and an improved methodology which will be transferable to coastal habitats elsewhere.
Section snippets
Carbon sequestration and storage by UK coastal margin habitats
Sand dune habitats and sandy beaches, saltmarsh and machair dune grassland comprise 93% of the UK coastal margin habitat, the remainder consisting of vegetated shingle and shingle beaches, saline lagoons and maritime cliffs and slopes and small islands. In this study the focus is on the first three habitats, henceforth termed sand dunes, saltmarsh and machair since very little is known about the carbon stock or sequestration rates in vegetated shingle, maritime cliff grasslands or saline
Valuing the ecosystem service of carbon sequestration and storage by UK coastal margin habitats
Understanding the underlying processes which support C sequestration and storage enables the quantification of this ecosystem service, and in turn more effective sustainable environmental management. However, valuation, both monetary and non-monetary, can facilitate transparency in the discussion of trade-offs between different ecosystem services (and associated beneficiaries) when different development options are considered.
To avoid double counting, valuation of ecosystem services should
Discussion
Coastal habitats are declining in both area and habitat quality because of increasing pressures related to climate change (such as the effect of coastal erosion and sea level rise) as well as increasing pressure resulting from human-driven change (conversion of habitat due to development). This study has used the UK as a case study to quantify the extent of habitat decline that has already occurred, and provided plausible future projections of continued habitat loss. The potential implications
Conclusions
This study has investigated how the value of the carbon sequestration service in coastal margin habitats changes over time providing, where possible, a hindcast and a forecast. It has demonstrated that coastal habitats can have significant value in terms of CO2 stored and sequestered. If the current extent of UK coastal habitat is maintained, their sequestration capacity over the period 2000–2060 is valued to be in the region of £1 billion (3.5% discount rate). However, if current habitat loss
Acknowledgements
This paper builds on research undertaken in the National Ecosystem Assessment and the authors gratefully acknowledge the support received. Funding from the Wales Environment Hub and Scottish Natural Heritage is also acknowledged. The authors would like to thank the anonymous reviewers for their valuable critiques of previous versions of this paper.
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