The North Atlantic Ocean is the most intense marine sink for anthropogenic carbon dioxide (CO2) in the world's oceans, showing high variability and substantial changes over recent decades. However, the contribution of biology to the variability and trend of this sink is poorly understood. Here we use in situ plankton measurements, alongside observation-based sea surface CO2 data from 1982 to 2020, to investigate the biological influence on the CO2 sink. Our results demonstrate that long term variability in the CO2 sink in the North Atlantic is associated with changes in phytoplankton abundance and community structure. These data show that within the subpolar regions of the North Atlantic, phytoplankton biomass is increasing, while a decrease is observed in the subtropics, which supports model predictions of climate-driven changes in productivity. These biomass trends are synchronous with increasing temperature, changes in mixing and an increasing uptake of atmospheric CO2 in the subpolar North Atlantic. Our results highlight that phytoplankton play a significant role in the variability as well as the trends of the CO2 uptake from the atmosphere over recent decades.