Declining ocean greenness and phytoplankton blooms in low to mid-latitudes under a warming climate

Marine phytoplankton are crucial to oceanic ecosystems, yet trends in their activity, monitored through chlorophyll a, remain uncertain due to observational limitations. We generated an ocean chlorophyll a dataset (2001 to 2023) across low to mid-latitudes (45°N to 45°S) using multisource data and a deep learning approach. Our analysis suggests widespread decline in ocean greenness, with chlorophyll a concentrations decreasing at a rate of (-0.35 ± 0.10) × 10^-3 milligrams per cubic meter per year (mg m^-3 year^-1). The decline is steeper in coastal regions [(-0.73 ± 0.22) × 10^-3 mg m^-3 year^-1]. The frequency of high chlorophyll a concentration events in coastal waters has decreased at a relative rate of -1.78% per year. These trends are predominantly driven by rising sea surface temperatures, which enhance ocean stratification, suppress nutrient upwelling, and limit phytoplankton growth. These findings suggest a long-term decline in marine primary production and a reduced occurrence of phytoplankton blooms, potentially disrupting trophic interactions and oceanic carbon cycling.