The ocean acidification (OA) literature is replete with laboratory studies that report species sensitivity to seawater carbonate chemistry in experimental treatments as an “effect of OA”. I argue that this is unintentionally misleading, since these studies do not actually demonstrate an effect of OA but rather show sensitivity to CO2. Documenting an effect of OA involves showing a change in a species (e.g. population abundance or distribution) as a consequence of anthropogenic changes in marine carbonate chemistry. To date, there have been no unambiguous demonstrations of a population level effect of anthropogenic OA, as that term is defined by the IPCC.