Abstract
Data obtained since 1958 from the continuous plankton recorder show an increasing occurrence of jellyfish in the central North Sea that is positively related to the North Atlantic Oscillation (NAO) and Atlantic inflow to the northern North Sea. Since 1970, jellyfish frequency has been also significantly negatively correlated with mean annual pH, independent of NAO trends. Jellyfish frequency increased in the mid‐1980s, coincident with the reported regime shift in the North Sea and tracking trends in phytoplankton color. As models produced under all climate‐change scenarios indicate a move toward a positive NAO, and pH of the oceans is predicted to decrease with rising CO 2 , we suggest that jellyfish frequency will increase over the next 100 yr.