The data collection started by Keeling and continued at Mauna Loa is the longest continuous record of atmospheric carbon dioxide in the world and is considered a reliable indicator of the global trend in the mid-level troposphere. Keeling's research showed that the atmospheric concentration of carbon dioxide grew from 315 parts per million (ppm) in 1958 to 380 (ppm) in 2005, with increases correlated to fossil fuel emissions. There has also been an increase in seasonal variation in samples from the late 20th century and early 21st century.