Ocean Greening Amid Warming: Evidence from Stringently-screened Ocean Color Data

The subtlety of oceanic color shifts, compounded by noise in satellite records, has made it uncertain whether the open ocean is undergoing a change akin to terrestrial greening. Using optical indices derived from a new, stringently screened MODIS-Aqua remote sensing reflectance dataset, we show that the ocean (60° S–60° N) has become markedly greener over the past two decades, with 73% of latitudinal bands exhibiting a shift toward greener hues and 24% showing statistically significant trends. By separating optical signals from different constituents, we attribute this greening primarily to increases in phytoplankton pigments, accompanied by smaller increases in other optically active components. We suggest the observed greening results from a complex interplay of warming-driven competitive advantages of picophytoplankton, mixed layer dynamics and atmospheric dust deposition. Importantly, we find that previously perceived chlorophyll-a concentration (Chl) decline in mid-low latitudes (40° S–40° N) are largely driven by radiance data of lower-certainty; removing these data reverses the trend. Moreover, the water corresponding to these data is not warming, further challenging the conventional causal mechanism that links warming to reduced Chl. Collectively, these results reveal that Earth’s largest biome is undergoing a subtle yet detectable greening in response to climate change.