This raster dataset depicts the measurements of ocean acidification at global coverage. Changes in CO2 concentration alter the aragonite saturation state (ASS) of the ocean, among other chemical properties of seawater, and as ASS levels drop the ability of calcifying species such as corals, diatoms, and shelled invertebrates to create calcium carbonate structures declines. The global distribution of ASS values has been modeled at 1-degree resolution for pre-industrial (circa 1870) and modern times (2000-2009). We use the difference between these values as an estimate of the human-derived driver of changes in ocean acidification. This dataset is part of the Global Map of Human Impacts to Marine Ecosystems, a collection of data and analyses related to anthropogenic drivers of change in marine ecosystems. The goal of this research is to estimate and visualize the global impact that humans are having on the oceans' ecosystems. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.