M ost carbon dioxide released into the atmosphere as a result of the burn-ing of fossil fuels will eventually be absorbed by the ocean 1 , with potentially adverse consequences for marine biota 2–4 . Here we quantify the changes in ocean pH that may result from this continued release of CO 2 and compare these with pH changes estimated from geological and historical records. We find that oceanic absorption of CO 2 from fossil fuels may result in larger pH changes over the next several centuries than any inferred from the geological record of the past 300 million years, with the possi-ble exception of those resulting from rare, extreme events such as bolide impacts or catastrophic methane hydrate degassing. When carbon dioxide dissolves in the ocean it lowers the pH, making the ocean more acidic. Owing to a paucity of relevant observations, we have a limited understand-ing of the effects of pH reduction on marine biota. Coral reefs 2 , calcareous plankton 3 and other organisms whose skeletons or shells contain calcium carbonate may be particu-larly affected. Most biota reside near the surface,where the greatest pH change would be expected to occur, but deep-ocean biota may be more sensitive to pH changes 4 . To investigate the effects of CO 2 emis-sions on ocean pH, we forced the Lawrence Livermore National Laboratory ocean gen-eral-circulation model 5 (Fig. 1a) with the pressure of atmospheric CO 2 ( p CO 2 ) observed from 1975 to 2000, and with CO 2 emissions from the Intergovernmental Panel on Climate Change’s IS92a scenario 1 for 2000–2100.Beyond 2100,emissions follow a logistic function for the burning of the remaining fossil-fuel resources (assuming 5,270 gigatonnes of carbon (GtC) in 1750; refs 6, 7). Simulated atmospheric CO
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