Anthropogenic carbon and ocean pH

The burning of fossil fuels and forests increases atmospheric CO₂ content, which drives a CO₂ flux into the ocean, and thereby makes the ocean more acidic. The effects of increased CO₂ and decreased ocean pH may individually and in combination have significant consequences for marine biota We present ocean pH change results from ocean geochemistry and ocean general circulation models for both atmospheric CO₂ change scenarios and ocean carbon sequestration scenarios. Unsurprisingly, the pH decrease in the ocean reflects the spatial and temporal distribution of anthropogenic carbon. However, the pH response of the ocean depends sensitively on the rate at which carbon is added to the ocean. When CO₂ changes occur over hundreds of thousands of years and longer, ocean carbonate-ion concentration is buffered by interaction with carbonate sediments, buffering ocean pH. However, when CO₂ changes occur over decades and centuries, ocean alkalinity remains roughly constant, tending to make ocean pH relatively sensitive to changes in ocean carbon content. A doubling of atmospheric CO₂ over a few centuries would decrease ocean pH by ~0.3 units. This is roughly the magnitude of pH variation over the past ~50 million years inferred from boron isotopes. There is no evidence of such rapid pH variation of this magnitude in the geologic record (with possible exceptions for rare catastrophic events). If CO₂ is emitted as per the IPCC IS92a ``Business as Usual'' scenario, or even as per most of the proposed CO₂ stabilization scenarios, ocean pH may decrease by an amount and at a rate not experienced by the Earth for the past few tens of millions of years. The biological consequences of such pH changes are uncertain at present.