Increasing levels of atmospheric carbon dioxide (CO₂) is one of the main causes of global warming. The steady rise of this potent greenhouse gas is clearly visible on the Keeling Curve, a leading indicator of human-caused climate change.

Charles David Keeling, a researcher at Scripps Institution of Oceanography, set up a CO₂ monitoring site high atop Mauna Loa on the Big Island of Hawaii in 1958. This remote spot in the middle of the Pacific Ocean is well removed from the localized influences of both carbon sources (factories) and sinks (forests) that could skew the data. According to NOAA, it is the world’s oldest continuous carbon dioxide monitoring station.

When the data recorded at the site is shown graphically, it resembles a “saw-toothed” curve. This is because CO₂ levels go up and down throughout the year with the life cycles of plants. Since most of the world’s landmass and vegetation are in the northern hemisphere, CO₂ levels start to go down in spring when plants draw in the gas during the process of photosynthesis. Then, after reaching a minimum in the autumn, CO₂ levels begin to go back up as plants die off and decay.

The curve’s long-term trend, however, has been definitively upward. Human activities, such as burning fossil fuels, have been releasing more CO₂ into the atmosphere than natural carbon sinks (plants and oceans) can take out.

When first established in 1958, the CO₂ level at Mauna Loa was 315ppm (parts per million). This autumn, it was more than 400ppm. To put this rapidly increasing number into perspective, consider that ice-core research shows that pre-industrial levels of carbon dioxide held steady around 280ppm from about 1000-1750 AD.

The Keeling Curve. Credit: Scripps and NOAA