Climate Regulation as a Service from Estuarine and Coastal Ecosystems

Abstract

Coastal regions, at the interface between terrestrial and oceanic ecosystems, play an important role in global biogeochemical cycles. This chapter reviews the climate regulation services of estuarine and coastal ecosystems (ECEs) including tidal salt marshes, mangroves, seagrass beds, macroalgal forests, coral reefs, and coastal shelf ecosystems. ECEs regulate global and regional climates by sequestering or releasing carbon dioxide and other greenhouse gases (GHGs). ECEs are extremely productive biologically, with net primary production rates per unit area among the highest of any ecosystem. Consequently, ECEs play a globally significant role as carbon sinks, with carbon storage rates per unit area of many habitats far exceeding that of land habitats at the rate of about 10 times that of temperate terrestrial forests and 50 times that of tropical forests. Furthermore, sedimentation does not reach an equilibrium carbon balance as occurs in terrestrial systems, whose sequestration capacity is forecasted to decrease this century. Conversely, they are large potential sources of GHG’s if disturbed or mismanaged. Critically, carbon sequestration in many coastal habitats is superior to that of terrestrial habitats, as carbon is generally stored over long time frames (thousands of years) as a consequence of the large belowground biomass and the absence of fire threat. Furthermore, carbon is generally broken down anaerobically; hence, emissions of other potent GHGs such as methane and nitrous oxide are negligible. A review of literature provided sequestration rates for various coastal habitats. Using these in combination with global extent of selected habitats, this chapter finds that GHGs worldwide, mangroves, seagrass beds, and salt marshes combine to sequester a minimum of 136 000 tonnes C annually into long-term carbon storage. Assuming prices of CO 2 e from $10 to $90 per tonne, the value of the annual sequestration is $5 –45 billion. This is an underestimate due to data gaps and, the limited assessment of the area of these three coastal ecosystem habitats, and relates only to long-term storage. The figures do not include short-term carbon storage in biomass, and further unaccounted for carbon sequestration occurs in kelp forests, estuaries, and coastal shelf seas. Many ECEs are under threat globally from sea-level rise, coastal development, pollution, and other anthropogenic stressors, and protection and restoration of ECEs may be an important tool for mitigating climate change. Currently, these habitats are not included in the United Nations Framework Convention on Climate Change (UNFCCC) carbon accounting frameworks, and therefore are excluded from incentive schemes such as carbon markets and other incentive programs, but their inclusion deserves consideration due to their potential for mitigating global climate change. The global distribution of C sequestration in ECEs reveals that large areas of the tropics are home to the highest sequestration rates and occur in developing countries, which also have the highest rates of coastal habitat degradation. Schemes such as Reducing Emissions from Deforestation and Forest Degradation (REDD) may bring revenues and added benefits to developing countries for instigating projects and marine protected areas for conservation. Many small island nations and developing countries in the tropics are particularly vulnerable to climate change and contain large swathes of seagrasses and mangroves compared to overall land area, but do not contain large areas of forests or grasslands, which would apply to REDD in its current form. Coastal habitats thus not only present an untapped potential for inclusion in climate change mitigation schemes, but also present a little-recognized risk of loss of large carbon stocks if their degradation and destruction are not reduced or halted.