Anthropogenic climate change is affecting marine ecosystems by altering the strength of phytoplankton blooms and driving shifts in the seasonality (phenology) of productivity. Here, we analyze a new 30-member Large Ensemble of climate change projections to quantify the sensitivity of phytoplankton bloom phenology (initiation, peak timing, and net growth period length) to anthropogenic forcing. Forced changes in the duration of net growth vary widely across the global ocean, with high latitudes experiencing a reduction of up to one month, and the tropics and subtropics experiencing an extension of up to one month. Changes in duration reflect shifts in both bloom initiation and peak bloom timing, which result from subtle decoupling between phytoplankton growth and zooplankton predation driven by temperature, nutrients and light variations. Changes in bloom strength and timing will alter the flow of energy in the marine ecosystem, with implications for higher trophic levels and fisheries.