More than 60 years ago, scientists were uncertain how much carbon dioxide was present in the atmosphere: measurements were taken sporadically, and published values varied widely. In 1957, the U.S. Weather Bureau (now called the National Weather Service) built Mauna Loa Observatory. The facility's remote location near the summit of Hawaii's 4,000-meter-high volcano offered access to air undisturbed by industry and other polluting factors.
There, a young researcher from the Scripps Institution of Oceanography named Charles (Dave) Keeling, who in years prior had developed accurate means to measure the gas, began the CO2 measuring program that persists to this day. Keeling's air samples revealed not only carbon dioxide's natural short-term cycles but also a long-term rise forced by human activity.
Mauna Loa Observatory is now a part of NOAA's Earth System Research Laboratory (ESRL). It continues to monitor greenhouse and ozone-depleting gases to aid research and understanding of global climate and atmospheric change.
This dataset shows the annual cycle of CO2 in the atmosphere that Keeling discovered: the distinct rise and fall of carbon dioxide levels over a year's time. The data are from January 1 to December 31, 2012.
On the graph are daily average atmospheric carbon dioxide measurements recorded at Mauna Loa Observatory. The unit is parts per million, or ppm - the number of CO2 molecules present in every million molecules of air. The annual pattern is caused by the uptake and release of CO2 from seasonal plant growth on the vast landmasses of the Northern Hemisphere. The annual maximum CO2 concentration in the Northern Hemisphere occurs around May. The spring buildup happens because decaying plants have been releasing carbon throughout the winter. The annual minimum concentration occurs around October, after new growth has withdrawn CO2 from the air during photosynthesis.
Mapped onto the globe is CarbonTracker, a global model of how CO2 moves across the atmosphere. CarbonTracker was developed by NOAA ESRL. It simulates CO2 concentrations around the world based on observed quantities collected at Mauna Loa and other sampling sites (white dots). CarbonTracker also takes into account how molecules move through the atmosphere. On this map of CarbonTracker data, CO2 's spring buildup in the Northern Hemisphere is distinctly visible as a purple band on the top half of the globe in May. The Southern Hemisphere shows low levels (yellow) at that time. For more on CarbonTracker, see these datasets:
During Keeling's first measurements in 1958, the maximum concentration of CO2 reached 318 ppm in late May. In time, he realized that CO2 's annual cycles were superimposed on a long-term trend: average CO2 levels were rising 2.2 percent a year. He attributed this rise to the addition of CO2 from fossil fuel combustion and recognized its implications for enhancing the greenhouse effect. In 2013, CO2 levels at Mauna Loa Observatory reached 400 ppm for several days, a long-expected symbolic milestone of the human impact on Earth's atmosphere. In 2014, levels exceeded 400 ppm daily for three months straight.