Changes in ocean chemistry due to anthropogenic CO₂ emissions affect marine life, nutrient cycles and biocalcification. Ocean acidification has been identified as a major consequence of rising atmospheric CO₂ levels. This makes understanding the response of calcareous plankton to acidification and other effects of global change an urgent challenge. With generous support from the ESF EuroCLIMATE Programme and PAGES, and with participation from SOLAS and IMBER, we are hosting a Workshop/Networking Activity on Atmospheric CO₂, ocean acidification and ecological changes in planktonic calcifying organisms. The objective of the workshop is to bring together a diverse mix of experts, from the cellular and genetic to the ecological and global carbon cycle level. Questions to be addressed include  how the predicted acidification is likely to affect the calcifying plankton, what the possible secondary consequences may be, and what research is needed to allow robust predictions.