rise globally at an average rate between 1.8–3.6 mm a−1, (ii) precipitation will increase in some regions of the wet tropics and decrease the dry tropics, and (iii) salinity will change in tandem with changes in precipitation.
Rising atmospheric CO2 and climate change are associated with shifts in temperature, circulation, stratification, nutrient input, oxygen content, and ocean acidification, with potentially wide‐ranging effects on the biology and ecology of marine organisms and their communities (Chapter 13).
2.7.1 Rising Atmospheric CO2
Mean atmospheric CO2 concentrations will increase to 441 ppm over the 2081–2100 period (Church et al. 2013; Collins et al. 2013). This projected increase is the net result of complex atmospheric, land, and ocean forces. While tropical atmospheric CO2 concentrations are rising, they respond to climatic events, such as El Niño. The tropical Pacific Ocean, for instance, plays an important role in modulating changes in atmospheric CO2 concentrations during El Niño events. ENSO is correlated with large interannual variability in global atmospheric CO2 concentrations. During the 2015–2016 ENSO event, there was a negative CO2 change during the development phase (spring–summer 2015) which was likely due to a reduction in local CO2 outgassing from the tropical Pacific Ocean; a positive CO2 change was measured during the mature phase (autumn 2015–2016), likely a reflection of an increase in atmospheric CO2 concentrations due to a combination of reduced vegetation uptake across pan‐tropical regions and enhanced biomass burning in Southeast Asia (Chatterjee et al. 2017). Thus, although atmospheric CO2 levels are increasing, there are still notable variations and oscillations over time and space and probably explain some of the observed variability in pCO2 concentrations in the world ocean.
2.7.2 Ocean Acidification
The global ocean has a large capacity to absorb atmospheric CO2 because CO2 dissolves and reacts with seawater to form bicarbonate (HCO3−) ions and protons (H+). Between one‐quarter and one‐third of the CO2 emitted into the atmosphere from the burning of fossil fuels, cement manufacturing and land‐use changes have been absorbed by the ocean (Kleypas and Langdon 2006; Turley and Findlay 2016). Over thousands of years, the changes in pH have been buffered by bases such as carbonate ions (CO32−). However, the rate at which CO2 is currently being absorbed is too rapid to be buffered sufficiently to prevent substantial changes in ocean pH. Consequently, the relative seawater concentrations of CO2, HCO3−, CO32− and pH have been altered. Since the Industrial Revolution, ocean pH has decreased globally by 0.1 unit (Figure 2.10 bottom), and it is predicted that ocean pH will decline by 0.4–0.5 unit by 2100 (Kleypas and Langdon 2006). It will take tens of thousands of years for these changes in ocean chemistry to be buffered through neutralization by carbonate sediments, and the level at which the ocean pH will eventually stabilize will be lower than it currently is. pH is sensitive to changes in salinity and total alkalinity and DIC concentrations, so organisms in coastal waters receiving river inputs are ordinarily exposed to greater variability in pH than organisms in the open ocean.
CO32− concentration directly influences the saturation, and consequently, the rate of dissolution of calcium carbonate (CaCO3) minerals in the ocean. Laboratory experiments and field observations indicate that ocean acidification is a threat to the survival of many marine organisms, especially organisms that use CaCO3 to produce shells, tests, and skeletons, such as corals (Andersson and Glenhill 2013). A shift in pH alters the saturation state of CaCO3 (called the ‘aragonite saturation state’) in seawater. The saturation state is expressed as:
where Ksp* is the solubility product for CaCO3 (as the mineral aragonite, arg) and [Ca2+] and [CO32−] are the calcium and carbonate concentrations, respectively. When Ωarg > 1, seawater is supersaturated with respect to mineral CaCO3 and the larger this value the more suitable the environment will be for organisms that produce CaCO3 shells and skeletons. When Ωarg < 1, seawater is undersaturated and is corrosive to CaCO3. When pH decreases, Ωarg decreases. The surface waters of the tropical oceans are currently supersaturated with respect to aragonite (mean Ωarg = 4.0 ± 0.2). However, Ωarg steadily declined from a calculated 4.6 ± 0.2 one hundred years ago and is expected to continue declining to 2.8 ± 0.2 by 2100 (Kleypas and Langdon 2006). The aragonite saturation state (Ωarg) is sensitive to the partial pressure of CO2 above the ocean as well as ocean temperature. In the latter case, the aragonite saturation of warm tropical oceans is higher than that of polar oceans because CO2 is more soluble in cold water. As atmospheric CO2 has increased, Ωarg of the world's oceans has decreased.
The acidification of the world's oceans is a complex process that is only now being understood. In the western tropical Pacific Warm Pool, acidification is proceeding, with declines in pH (−0.0013 ± 0.0001 a−1) and Ωarg (−0.0083 ± 0.0007 a−1) over the 1986–2016 period (Ishii et al. 2020). Oceanographic modelling indicates that the acidification of the Warm Pool occurs primarily through uptake of anthropogenic CO2 in the extra tropics which is then transported to the tropics through the Equatorial Undercurrent from below (Ishii et al. 2020). The rate of Warm Pool acidification can be expected to be modulated by the contribution of a long interior residence time (years to decades) acting in concert with accelerating CO2 increases in the atmosphere.
Acidification is an even more complex process in estuarine and coastal environments as carbonate chemistry is strongly regulated by changes in biological activity related to eutrophication and the delivery of nutrients by rivers and groundwater (Wallace et al. 2014). The increased loading of nutrients into estuaries and shallow inshore waters causes the accumulation of algal biomass and subsequent decomposition of this organic material, decreasing dissolved O2 levels and contributing towards hypoxia. Hypoxia increases pCO2 concentrations and upwelling processes can bring CO2‐enriched water in contact with coastal waters, amplifying the effects of acidification. Land‐use changes such as deforestation and fossil fuel combustion also produce increased dissociation products of strong acids (HNO3 and H2SO4) and bases (NH3) in coastal waters, causing decreases in surface water alkalinity, pH, and DIC. River discharge further reduces alkalinity as river waters, especially in the tropics, are typically more acidic than receiving coastal waters. Further, the decomposition of organic matter, especially in bottom sediments and wetland soils, can reduce pH and alter carbonate chemistry, producing an increase in pCO2 mostly due to microbial respiration. Nearly all estuarine and nearshore waters in the tropics naturally exhibit wide variations in salinity, pH, and carbonate chemical parameters, especially pCO2 and [CO32−] (Alongi 2020). Carbonate chemistry in the coastal zone thus responds more strongly to eutrophication than ocean acidification (Borges and Gypens, 2010).
2.7.3 Rising Temperatures, Increased Storms, Extreme Weather Events, and Changes in Precipitation
The rate of ocean heating has increased within the 0–700 m layer by 3.22 ± 1.61 ZJ (ZJ = 1021 joules) from 1969 to 1993 and 6.28 ± 0.48 ZJ from 1993 to 2017, representing a twofold increase in heat uptake (Bindoff et al. 2019). This has resulted in temperature rises as noted earlier. Warming has also strengthened vertical stratification, inhibiting
