Poleward greening of the global oceans

Phytoplankton, which sit at the base of marine food web, are responsible for a large fraction of oceanic carbon sequestration. These microorganisms convert carbon dioxide into oxygen through photosynthesis and fundamentally affect marine ecosystems. Phytoplankton contain chlorophyll—a pigment that absorbs blue wavelengths and reflects green wavelengths—and thereby influence ocean color. When populations of phytoplankton increase, water appears greener. Although this enables satellite observation of their spatial distribution in the ocean, it is challenging to detect the underlying large positive or negative trends because of small signal strength compared with the background noise level. On page 1337 of this issue, Zhao et al. (1) report an analytical approach that averages ocean chlorophyll data over latitudinal bands to determine a global trend in phytoplankton population over the past two decades. This has notable implications for marine ecology and fisheries, particularly those in tropical and subtropical regions.