Ocean surface pH is a measure of the acidity or alkalinity of seawater at the ocean surface. The ocean absorbs carbon dioxide (CO₂) from the atmosphere. Around 29% of the CO₂ emitted by human activities during the decade 2015–2024 was absorbed by the ocean. The absorbed CO₂ changes the carbonate chemistry of seawater and reduces its pH. Although surface seawater remains alkaline, these changes are acidifying the ocean. Surface ocean pH is monitored using sustained carbon observations that are integrated into global reconstructions. Ocean surface pH is not changing uniformly across regions.