Anthropogenic CO₂ emissions absorbed by the oceans, dissolving as carbonic acid → shifts carbonate–bicarbonate equilibrium → reduces pH
Mean surface ocean pH (unitless, logarithmic scale).
Pre-industrial average pH ≈ 8.2; present global surface mean ≈ 8.05. This is a decline of ~0.1 pH units, equivalent to a ~30% increase in hydrogen ion concentration [H⁺].
A drop of 0.1 units (from 8.2 to 8.1) has already crossed the PB "zone of uncertainty"; safe operating space was defined at ≥ 8.1
Special mention should be made of ocean acidification which is driven by excess carbon dioxide emissions. The speed at which pH (a measure of acidity) is changing is a major risk. Whilst the absolute measures of measures change, it is the speed at which they change which poses the greatest risk.
