Although the environmental drivers of long-term phytoplankton changes have been extensively investigated, the relative importance of ocean acidification within the broader suite of environmental drivers remains poorly understood. Long-term chlorophyll trends in the global ocean exhibit pronounced spatial heterogeneity, with the dominant environmental drivers differing markedly among oceanic regions. Environmental factors, including sea surface temperature, seawater pH, photosynthetically active radiation, mixed layer depth, wind stress, sea surface salinity, and nutrients, contribute differently to long-term Chl trends across oceanic regions. Based on monthly global observations of chlorophyll concentration and multiple environmental variables from 2003 to 2022, this study quantitatively evaluates the relative contributions of individual environmental drivers to long-term Chl trends and the spatial heterogeneity of these contributions, thereby providing new observational evidence for understanding how marine ecosystems respond to global climate change through the combined effects of multiple environmental drivers.