Phytoplankton play a key role in biogeochemical cycling and climate processes. Precise quantitative measurements of chlorophyll-a (Chl-a), a measure of phytoplankton biomass, have only been available globally since 1997 from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). In the North Atlantic, semi-quantitative measurements of chlorophyll (Phytoplankton Color Index, PCI) for >50 years have been collected by the Continuous Plankton Recorder. Here we demonstrate a significant correlation between PCI and SeaWiFS Chl-a from 1997--2002. Combining both time series allows quantification of the stepwise increase in biomass in the mid-1980s; this regime shift corresponded to a 60% increase in Chl-a. This was a result of an 80% increase in Chl-a during winter, alongside a smaller summer increase. This new high-resolution data set on the monthly variation of Chl-a in the North Atlantic since 1948 is now available for the development and validation of climate models, and for interpretation of ecological changes related to climate.