How green is Earth? That's a question that NASA's Sea-viewing Wide Field-of-view-Sensor Project, known as SeaWiFS, has been asking for the past 13 years. This image shows SeaWiFS' average measurements from 1998 to 2010.

SeaWiFS, on board the OrbView 2 (aka SeaStar) satellite measures the wavelengths of light reflected by phytoplankton (microscopic marine plants) and algae that use chlorophyll for photosynthesis. Over the oceans, SeaWiFS data indicate the concentration of phytoplankton floating on the surface. Scanning land, the sensor calculates the density of vegetation as a normalized difference vegetation index (NDVI). An NDVI of 0 means no green leaves, whereas an NDVI near 1.0 indicates a thick forest.

SeaWiFS data show that photosynthesizing organisms have declined in certain ocean gyres (large-scale surface current patterns), said Jim Yoder, a scientist at the Woods Hole Oceanographic Institution, in a NASA article commemorating the end of SeaWiFS's mission. (The instrument stopped communicating with Earth in December 2010 and its operators officially ended the mission in February.)

Scientists are still debating whether the decline is due to natural ocean cycles or climate change.

—Francie Diep