Assessment of carbon sequestration potential in coastal wetlands

Abstract

This paper describes model (Marsh Equilibrium Model) simulations of the unit area carbon sequestration potential of contemporary coastal wetlands before and following a projected 1 m rise in sea level over the next century. Unit rates ranged typically from 0.2 to 0.3 Mg C ha-1 year-1 depending primarily on the rate of sea-level rise, tidal amplitude, and the concentration of suspended sediment (TSS). Rising sea level will have a significant effect on the carbon sequestration of existing wetlands, and there is an optimum tide range and TSS that maximize sequestration. In general, the results show that carbon sequestration and inventories are greatest in mesotidal estuaries. Marshes with tidal amplitudes <50 cm and TSS < 20 mg l-1 are unlikely to survive a 1 m rise in sea level during the next century. The majority of the United States coastline is dominated by tidal amplitude less than 1 m. The areal extent of coastal wetlands will decrease following a 1 m rise in sea level if existing wetland surfaces <1 m fail to maintain elevation relative to mean sea level, i.e. expansion by transgression will be limited by topography. On the other hand, if the existing vegetated surfaces survive, coastal wetland area could expand by 71%, provided there are no anthropogenic barriers to migration. The model-derived contemporary rate of carbon sequestration for the conterminous United States was estimated to be 0.44 Tg C year-1, which is at the low end of earlier accounts. Following a 1 m rise in sea level, with 100% survival of existing wetland surfaces, rates of carbon sequestration rise to 0.58 and 0.73 Tg C year-1 at TSS = 20 and 80 mg l-1, respectively, or 32–66% higher than the contemporary rate. Globally, carbon sequestration by coastal wetlands accounts for probably less than 0.2% of the annual fossil fuel emission. Thus, coastal wetlands sequester a small fraction of global carbon fluxes, though they take on more significance over long time scales. The deposits of carbon in wetland soils are large. There have been large losses of coastal wetlands due to their conversion to other land uses, which creates opportunities for restoration that are locally significant.