# Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests
## 3 Climate Change Impacts
### Rising Temperatures, Increased Storms, Extreme Weather Events, and Precipitation Changes  
During 2011-2020, the increase in global surface temperature (GST) since 1850-1900 was 1.09 (range: 0.95-1.20) \({}^{\circ}\)C with a further likely 1.5 \({}^{\circ}\)C increase during 2021-2400 and a very likely crossing of the 2 \({}^{\circ}\)C threshold during the 2040-2060 period. Over the 2081-2100 period, average GST is very likely to be higher by 1.0-1.8 \({}^{\circ}\)C in the low CO\({}_{2}\) emissions scenario and by 3.3-5.7 \({}^{\circ}\)C in the high emissions scenario [49]. By 2300, GST will be in the range of 0.9-9.6 \({}^{\circ}\)C higher depending on the rate of CO\({}_{2}\) emissions. Similarly, global mean sea surface temperatures (SST) have increased 0.88 \({}^{\circ}\)C (range: 0.68-1.01) since 1900 and are projected to increase by 0.86 \({}^{\circ}\)C (0.43-1.47 \({}^{\circ}\)C) and by 2.89 \({}^{\circ}\)C (2.01-4.07 \({}^{\circ}\)C) by 2100 under low and high rates of CO\({}_{2}\) emissions, respectively. Considering the projected rises in air and sea temperatures, mangroves will respond in most cases positively up to a critical threshold. Warmer temperatures affect mangroves by (1) altering species composition, (2) changing physiological patterns, (3) increasing primary production and ecosystem respiration at least up to critical thresholds, and (4) expanding species ranges to higher latitudes where range is temperature-limited but not limited by other factors [52]. Rises in temperature may also result in decreased survival in arid and increasingly arid regions, (2) increase water vapor deficit, (3) increased secondary production, and (4) shifts in species dominance and biodiversity. Rates of leaf photosynthesis peak for most species at or below a daily mean of 30 \({}^{\circ}\)C and leaf CO\({}_{2}\) assimilation rates of many species decline as temperatures increase from 33 to 35 \({}^{\circ}\)C [6]. It is therefore likely that with small temperature increases (daily mean of <35 \({}^{\circ}\)C), mangroves will produce more carbon via higher primary production, although metabolic balance (P versus R) is likely to shift in favor of net heterotrophy (P:R \(\leq\) 1) with rising temperatures. Data from Chinese mangrove forests over long stretches of time indicate that warming during the past 600 years has resulted in a decline in forest development, suggesting that the rise in temperature during the Anthropocene has been too rapid to accommodate a positive mangrove response [59]. Losses of blue carbon due to temperatures above the critical threshold may be compensated by the continuing expansion of mangroves into higher latitudes in the Gulf of Mexico, Florida, New Zealand, Australia, southern China, and southern Africa, with air temperature and rainfall best explaining this range expansion [60]. Obviously, new mangroves will result in an increase in biomass and soil carbon although the latter depends on freshwater availability, soil moisture, hydrologic regime, salt marsh, and mangrove species and productivity as well-developed soils already exist prior to replacement by larger mangrove forests [61]. In many locations where mangroves are replacing less productive or marginal salt marshes, especially in arid areas, there are large increases in soil carbon stocks with mangrove invasion although it may take time for significant development of mangrove soil carbon stocks [62]. Differences in carbon fluxes between salt marshes and invading mangroves is poorly understood with studies indicating variable results across the ecotone [63; 64]. While mangrove encroachment into salt marshes appears to be driven primarily by increasing temperatures and reduced frequency of extreme cold events, changes in precipitation patterns and increasing frequency of storms cannot be ruled out as co-factors. Increased temperatures have affected the frequency and intensity of extreme weather events, such as droughts, storms, and cyclones [65].