Deadly Threat No. 2: Ocean acidification

Following on from my introductory video a few days ago, today I will be discussing ocean acidification. As the second ‘deadly’ threat, it stems from the ocean’s ability to absorb atmospheric carbon dioxide (CO₂). Thus far, it is estimated that the ocean has absorbed about 30% of the emitted anthropogenic CO₂ (Bijma et al., 2013). Whilst this has benefitted humanity by diminishing the rise in atmospheric CO₂ and, therefore, reduced the rate of global warming, it has been at the expense of the ocean’s chemistry.

The CO₂ exchange between atmosphere and ocean is governed by the differences in CO₂ concentration. With today’s atmospheric CO₂ concentration higher than that of the ocean, the sea is absorbing CO₂ in attempts to reach equilibrium. As the equation below shows, when this atmospheric CO₂ dissolves into seawater, it undergoes hydration to form carbonic acid (H₂CO₃). This then dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃^-), before the latter once again dissociates into another H⁺ and carbonate ion (CO₃^2-).

CO₂(aq) + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO­₃^- ⇄ 2H⁺ + CO₃^2- 

Figure 1 makes this look a little more friendly!

Figure 1. Ocean acidification. Source.

For typical ocean surface conditions, about ninety per cent of the total dissolved inorganic carbon (DIC) occurs as HCO₃^-, nine per cent as CO₃^2- and one per cent as H₂CO₃ or CO₂(aq) (Feely et al., 2009). However, with rising atmospheric CO₂ levels, the ocean is absorbing larger amounts of CO₂ and consequently, the concentration of CO₂(aq), HCO₃^- and H⁺ is increasing. Due to pH = –log₁₀[H⁺], we are seeing a reduction in the ocean’s pH - notice the progression from less acidic conditions on the left-hand side of Figure 1 to more acidic conditions (where the H⁺ are) on the right-hand side. Since the pre-industrial period, the ocean surface layers have acidified by 0.1 pH units (IPCC, 2013). Indeed, over the last 30 years, there has been a steady decrease of 0.02 pH units per decade (Bijma et al., 2013). Figure 2 displays the trends in atmospheric CO2 and ocean pH that have been observed. This level of acidification is 'unprecedented', causing the ocean to be more acidic than it has ever been in the last 300 million years (Laffoley et al., 2013). In the future, ocean acidification is predicted to continue, following the trend in atmospheric carbon dioxide (IPCC, 2013). This will undoubtedly have serious consequences for marine life. 

Figure 2. Observations of CO2 (parts per million) in the atmosphere and pH of surface seawater from Mauna Loa and Hawaii Ocean Time-series (HOT) Station Aloha, Hawaii, North Pacific (IGBP, IOC, SCOR, 2013) .

H⁺ also reacts with CO₃^2- to produce additional HCO₃^-. Thus, alongside an increase in H⁺, the dissolution of CO₂ in seawater decreases the concentration of CO₃^2-. This affects marine calcifiers that build their shells and skeletons out of CaCO₃ (more on specific calcifiers next time) (Guinotte and Fabry, 2008).

The saturation state of seawater (Ω) determines whether a mineral will precipitate (form) or dissolve and, therefore, affects calcification (Barker and Ridgwell, 2012). It is expressed as:

The subscript 'seawater' refers to the in-situ concentration of the mineral and 'saturation' refers to the concentration when the mineral is saturated. The denominator in the equation is also known as the apparent solubility product, K’_sp (Feely et al., 2009). It is a function of temperature, salinity and pressure and differs between calcium carbonate minerals. When conditions are supersaturated (Ω > 1), calcification is favoured and when conditions are undersaturated (Ω < 1), dissolution is preferred. Today, most global surface waters are supersaturated with respect to CaCO₃, meaning there are more than enough CO₃^2- ions for shell-building, although a few locations are close to undersaturation. However, with progressing ocean acidification, [CO₃^2-]_seawater will reduce, causing Ω to decrease (Barker and Ridgwell, 2012). This has the potential to decrease calcification rates for a number of species (Doney et al., 2009). The continued decrease in [CO₃^2-]_seawater  also means that saturation horizons (Ω = 1) are shoaling (getting shallower), increasing the vulnerability of marine calcifiers. High latitude surface waters are the first locations to experience this shoaling due to the increased solubility of CO₂ in colder waters (Barker and Ridgwell, 2012).

To sum up, ocean acidification is reducing seawater pH, carbonate ion (CO₃^2-) concentrations and saturation states for important minerals required by marine calcifiers. These reductions all have direct impacts on a wide range of marine life and I will be focusing on these next time (I promise there will be less chemistry!).