Measurement began in February 1958. Within two years his instruments showed that the world's store of airborne carbon dioxide had increased in that period from about 313 parts per million to about 315 parts per million. Keeling worked on Mauna Loa from 1958 until his death in 2005, during which time the proportion of carbon dioxide in the air rose to 380 parts per million. Combined with the work by Revelle and Suess, Keeling's meticulous, decades-spanning measurements convinced climate researchers that carbon dioxide was accumulating in the air.