Direct measurements of atmospheric carbon dioxide (CO2) concentrations have been recorded at Mauna Loa since 1958. Its mid-Pacific location makes it an ideal place to collect atmospheric data. The units of atmospheric carbon dioxide measurements are ppm—parts per million. For every million molecules of air in our atmosphere, some number of them are carbon dioxide molecules.
Since 1958, the concentration of atmospheric CO2 has risen from 315 ppm to over 412 ppm (as of December, 2019). This is depicted in the graph shown below, known as the Keeling Curve (click to enlarge). The Keeling Curve is named for Charles David Keeling of the Scripps Institution of Oceanography, who was the first person to make frequent regular measurements of atmospheric CO2 concentrations at Mauna Loa.
The periodic annual fluctuations in the graph reflect seasonal changes. The Northern Hemisphere has far more land area than the Southern Hemisphere, and most of the land area in the Southern Hemisphere is desert. As plants in the Northern Hemisphere grow leaves each spring and summer, they remove some CO2 from the air via photosynthesis, causing CO2 levels to drop. There is a lag time for this effect to show in the data, so the lowest annual CO2 concentrations occur in the fall. During Northern Hemisphere fall and winter, plants lose their leaves, and the decrease in photosynthesis causes the CO2 level to rise. There is a lag time for this change to show in the data as well. Thus, highest CO2 concentrations occur in May each year. This seasonal fluctuation is the natural cycling of carbon from an atmospheric gas to solid plant material and back.
When you view multiple years of data together, the obvious upward trend is not part of the natural cycling of carbon between the atmosphere and the biosphere. Human activities are altering the carbon cycle—both by adding more CO2 to the atmosphere and, through land use changes such as deforestation, influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the Industrial Revolution (1760 - 1840). Until that time, atmospheric CO2 had not been higher than 280 ppm. Direct measurements of gas bubbles in ice cores from Antarctica show that atmospheric CO2 levels did not exceed 280 ppm during the previous 800,000 years.
The main human activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation. Certain industrial processes also emit CO2. Additionally, land use changes such as deforestation remove part of the land sink and thereby cause atmospheric CO2 levels to be higher.