# Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests
## 3 Climate Change Impacts
### Sea-Level Rise (SLR)  
Rising sea-level due to warming seas and melting ice is arguably the main factor determining the survival of mangrove forests, considering their intertidal existence [48]. Global sea-level (GMSL) increased 0.2 m over the 1901-2018 period at a rate of 1.7 mm a\({}^{-1}\) with an accelerated rate of 3.7 mm a\({}^{-1}\) over 2006-2018 with increased GMSL beyond 2300 [49]. The paleoenvironmental record indicates that over millennial timescales, mangroves have been exposed to different sea-level trajectories, showing a broad capacity to adjust to sea-level variations, as sediment supply, organic matter sequestration, and belowground root growth show considerable natural resilience in response to SLR [50]. Four main factors facilitate mangrove resistance and resilience to SLR: (1) the rate of sea-level change relative to the mangrove soil surface, (2) species composition, (3) the physiographic setting, including the slope of the forest relative to that of the land the mangroves currently occupy, and (4) the presence/absence of obstacles to landward migration [51]. An analysis of mangrove accretion versus local mean SLR [52] found that the threshold accretion rate between survival and drowning was 6 mm a\({}^{-1}\). Between 9800 and 7500 years ago, mangroves expanded at a rate driven mainly by the rate of relative SLR, but it was highly likely (90% probability) that they were unable to sustain accretion when relative SLR exceeded 6.1 mm a\({}^{-1}\)[53]. Accepting that rates of SLR greater than 6 mm a\({}^{-1}\) represent a critical threshold for mangrove survival, a comparison with a recent IPCC analysis (Table 3) of predicted rates of SLR under five CO\({}_{2}\) emissions scenarios and five different temperature rise scenarios during the 2040-2060 and 2080-2100 periods [49], infers that mangroves will likely survive in future with declining (\(-10\) to \(-15\) Gt CO\({}_{2}\) a\({}^{-1}\)) CO\({}_{2}\) emissions and/or global surface air temperature rises of 1.5-2.0 \({}^{\circ}\)C (Table 3). The impact of submergence due to SLR on mangrove carbon stocks and fluxes will likely be profound. Once available nutrients and oxygen are exhausted and trees are dead, there will be an increase in peat deposits which will result in a net increase in carbon burial of large amounts of dissolved and particulate carbon. Export of forest litter to the adjacent rising sea will be initially large as the dying aboveground biomass decomposes; initial necromass will decompose and release CO2 and CH4 to the atmosphere and tidal water. Labile soil carbon will still be mineralized until nutrients are exhausted. As trees are killed by submergence, live belowground roots will also begin to decompose slowly, leading to soil/peat collapse just below the root zone. Mangroves that survive SLR will expand and accumulate carbon as SLR will not necessarily lead to blue carbon loss. Encroachment up the intertidal zone (assuming available space) will allow mangroves to build soil surface elevations vertically via soil accumulation, root growth, litter and woody debris accumulation, and microbial and fauna development [4] resulting in long-term soil carbon accumulation. (Sufficient available space for mangrove encroachment landward due to low human population densities is called 'low coastal squeeze' whereas insufficient colonization space with SLR due to dense populations is called 'high coastal squeeze'). Paleorecords indicate continuous peat development throughout the Holocene in some locales where SLR rates were <5 mm a\({}^{-1}\) and peat accumulation of up to 10 m on some carbonate islands [54]. Mangrove peat formation is a complex process, including being built up by root growth stimulated by the emergence of vertical space created by rising coastal waters [4]. Over time, the peat expands vertically as root growth occurs in low oxygen zones that reduce decomposition below the new root zone layer, resulting in carbon accumulation.