Oceans going acid

Ocean acidification occurs as a result of increased CO2 levels in seawater. CO2 and water molecules may lead to the formation of carbonic acid to maintain chemical equilibrium. The added carbonic acid molecules reacting with water molecules to give a bicarbonate ion and a hydronium ion.

Figure 1 Chemical process leading to ocean acidification. Source: NOAA

Figure 2 Historical pH level (1850) Broadgate et al., 2012

Since the outset of the Industrial Revolution, pH has decreased from 8.2 to 8.1, which represents a 30% increase in acidity (NOAA). And at the current environmental change rate, levels are going to keep rising. In 2100, modelled results for high CO2 emissions scenario predicted a decrease up to 7.1 pH units.

Figure 3 Projected pH levels (in 2100). Source: Broadgate et al., 2012

Ocean acidification is intensified by nitrogen and sulfur inputs from fossil-fuel combustion.

Human-induced alkalinity change is altering surface pH and atmosphere/ocean CO2 exchanges (Doney et al. 2007).

We can argue that the predictability of chemical reactions make future pH projections easier (Logan 2010). In comparison to climate change, ocean acidification would thus be easier to model. It is however not so straightforward. First, different CO2 release scenarios lead to different consequences. Even so, under one scenario (business as usual) we find a large range of prediction: 100% to 150% increase in hydrogen ion (IPCC). We can nonetheless agree that the increase is alarming.

Evidence proves that ocean acidification is likely to alter oceans ability to act as a net CO2 sink (Logan 2010). Lower surface carbonate ion levels will impede oceans' carbon uptake (Orr et al. 2005). However a decrease in calcium carbonate export from the high latitudes would increase carbonate ion levels at the surface. It would increase carbon uptake by 6 to 13 Pg (Petagrams) (Heinz 2004). This is built on a modelled laboratory extrapolation. As we have seen, simulated carbon uptakes by the oceans is easily inflated.