Remote sensing measurements of ocean colour (i.e. the detection of phytoplankton pigments) provide the only global-scale focus on the biology and productivity of the ocean’s surface layer. Phytoplankton are microscopic plants that live in the ocean, and like terrestrial plants, they contain the pigment chlorophyll, which gives them their greenish colour. Different shades of ocean colour reveal the presence of differing concentrations of sediments, organic materials and phytoplankton. The ocean over regions with high concentrations of phytoplankton is shaded from blue-green to green, depending on the type and density of the phytoplankton population. From space, satellite sensors can distinguish even slight variations in colour which cannot be detected by the human eye.