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.
The image for 2026 shows the actual scale based on the numbers above.
