Global chlorophyll distribution. Computer illustration showing the amount of chlorophyll present in the oceans, and the amount of vegetation on land. The Normalised Difference Vegetation Index (NDVI) measures the amount and health of plants on land, while chlorophyll A measurements indicate the amount of phytoplankton (photosynthetic plankton) in the ocean. Land vegetation and phytoplankton both consume atmospheric carbon dioxide. Purple and blue represent low levels of chlorophyll, while green, yellow and red indicate progressively higher concentrations. On land, areas of little vegetation are brown, while blue-green represents dense vegetation.