evidence: Ocean acidification (OA) is caused by excess CO₂ in seawater. Atmospheric concentrations of CO₂ have now reached 414ppm (from about 280ppm in 1750) and the oceans are a major sink, having absorbed nearly half of all anthropogenic emissions since the beginning of the Industrial Revolution some two centuries ago. But rising levels of CO₂ in the oceans change the acidity of seawater, measured as pH. Ocean acidity has remained remarkably stable for more than 800,000 years, but has increased by about 30% in the last 200 years.