The uptake of anthropogenic CO₂ by the global ocean is leading to significant changes in seawater chemistry, primarily through ocean acidification. By the end of the century, atmospheric CO₂ levels are projected to reach 800 ppm, causing a decrease in surface water pH from pre-industrial levels of about 8.2 to around 7.8. This increase in acidity will reduce carbonate ion concentrations, leading to aragonite undersaturation in most open-ocean surface waters by 2020 in the Arctic and 2050 in the Southern Ocean. By 2095, all of the Arctic and parts of the Southern Ocean and North Pacific will be undersaturated with respect to aragonite. For calcite, undersaturation will occur by 2095 in most of the Arctic and some parts of the Bering and Chukchi seas. These changes will significantly impact calcifying organisms, such as planktonic coccolithophores, pteropods, mollusks, and corals, by reducing their ability to produce shells or skeletons. The study uses data from the Global Ocean Data Analysis Project (GLODAP) and the National Center for Atmospheric Research Community Climate System Model 3.1 (CCSM3) to project these changes and highlights the potential for profound impacts on marine ecosystems if CO₂ emissions are not significantly reduced.The uptake of anthropogenic CO₂ by the global ocean is leading to significant changes in seawater chemistry, primarily through ocean acidification. By the end of the century, atmospheric CO₂ levels are projected to reach 800 ppm, causing a decrease in surface water pH from pre-industrial levels of about 8.2 to around 7.8. This increase in acidity will reduce carbonate ion concentrations, leading to aragonite undersaturation in most open-ocean surface waters by 2020 in the Arctic and 2050 in the Southern Ocean. By 2095, all of the Arctic and parts of the Southern Ocean and North Pacific will be undersaturated with respect to aragonite. For calcite, undersaturation will occur by 2095 in most of the Arctic and some parts of the Bering and Chukchi seas. These changes will significantly impact calcifying organisms, such as planktonic coccolithophores, pteropods, mollusks, and corals, by reducing their ability to produce shells or skeletons. The study uses data from the Global Ocean Data Analysis Project (GLODAP) and the National Center for Atmospheric Research Community Climate System Model 3.1 (CCSM3) to project these changes and highlights the potential for profound impacts on marine ecosystems if CO₂ emissions are not significantly reduced.