Mauna Loa’s daily carbon-dioxide reading first exceeded 400 ppm on May 9, 2013. The milestone came from the long-running atmospheric record known as the Keeling Curve.
Charles David Keeling began continuous CO₂ measurements at Hawaii’s Mauna Loa Observatory in 1958. The remote site was chosen partly because its high elevation and location in the Pacific helped provide air samples representative of the broader atmosphere, away from many local pollution sources.
The record rises overall because fossil-fuel combustion and other human activities add carbon dioxide to the air. It also wiggles seasonally: concentrations usually peak in Northern Hemisphere spring, then decline as vegetation absorbs CO₂ during the growing season. The annual cycle rides on top of the long-term upward trend.
The 400-ppm event concerned a daily mean, not every instant of every day and not a global average. Monthly and annual values can differ slightly from a particular daily reading. The milestone was notable because atmospheric CO₂ had remained below that level throughout the modern instrumental record, although evidence from ancient air and geological proxies shows that Earth’s distant past had higher concentrations.