Previous satellite-based studies have suggested an expansion of subtropical gyres concomitant with decreasing chlorophyll concentrations over time due to ocean warming, raising major concerns about a potential increase in the ocean desertification and its effect on the global climate. However, these studies have relied on the analysis of limited period and/or single gyre and do not provide a comprehensive overview of the temporal evolution of phytoplankton biomass in these systems. Here, 25-year (1998-2022) time series of satellite ocean colour and sea surface temperature data and mixed layer depth are examined to investigate their long-term trends in the five major subtropical gyres on Earth. Main results show that, despite the chlorophyll decrease observed in the most oligotrophic zone of all gyres, phytoplankton biomass is rather constant over time, suggesting that chlorophyll changes in these systems are mainly driven by phytoplankton physiological adaptation to the ongoing warming.