As CO2 from fossil fuel emissions enters the ocean, it is becoming acidified. Global surface pH has already decreased by more than 0.1 units since preindustrial times and might decrease by 0.100.35 additional units by 2100. The level of supersaturation of calcium carbonate has decreased and the partial pressure of carbon dioxide has increased over much of the upper ocean. We present predicted changes in ocean surface pH from the Canadian Earth System Model (CanESM-1). Results presented at a 2010 workshop in Sendai, Japan, of experiments on marine animals under elevated CO2 conditions indicate that few general conclusions across species can as yet be made. Recommendations include protocols that mimic natural diel and seasonal fluctuations in and that extend over multiple generations. Projections from CanESM-1 suggest that other changes associated with climate change have little influence on acidification relative to the addition of CO2 and that freshwater dilution at high northern latitudes exacerbates acidification and results in undersaturation with respect to carbonate minerals at higher pH than in undiluted seawater. Finally, we recommend a framework that considers the effects of multiple stressors associated with a changing climate on the "thermal window" of activity of marine animals. © 2011 International Council for the Exploration of the Sea.