In 1958, Charles David Keeling began making daily measurements of the concentration of atmospheric carbon dioxide (CO2) at the Mauna Loa Observatory on the Big Island of Hawaii. These measurements were begun as part of a one-year initiative, the International Geophysical Year, but because of Keeling's persistence, the daily record has continued through the present, almost without interruption. The Mauna Loa record, now known as the Keeling Curve, continues to be collected under the direction of Keeling's son, Ralph.
The current climb in carbon dioxide started before Keeling started watching; more recent research has shown that preindustrial levels of carbon dioxide (from 1000-1750 AD) ranged between 275 and 285 ppm.1 All of this growth appears to stem from human activity, due to the use of fossil fuels, so that today, about one in four CO2 molecules in the atmosphere comes from us.
The small annual zigzag visible on the curve is timed with the seasons. Carbon dioxide levels drop during the northern hemisphere spring and summer, when plants are taking CO2 out of the atmosphere to grow. In the fall and winter, plants and leaves die off and decay, releasing CO2 back into the atmosphere and causing a small spike. Since most of the world's seasonal vegetation is in the northern hemisphere, as is the measuring station at Mauna Loa, the seasonal trend in the Keeling Curve record from Mauna Loa is based on northern hemisphere seasons. The detailed and logical “breathing” of the planet that the Keeling Curve shows is just one of many indicators of its sensitivity and accuracy.
The history of this finding is described by Earthguide.
Isolated in the middle of the Pacific Ocean and at over 11,000 feet above sea level, the upper north face of Mauna Loa volcano is an ideal location to make measurements of atmospheric carbon dioxide that reflect global trends, not local influences such as factories or forests that might boost or drop carbon dioxide within their vicinity. The CO2 sensors at Mauna Loa are positioned such that they sample an incoming breeze direct from the ocean, unaffected by human activities, vegetation or other factors on the island. (The Mauna Loa Observatory is high enough that the incoming breeze rides above the thermal inversion layer.)
Volcanoes are considerable sources of carbon dioxide themselves. However, the sampling location was chosen to be normally upwind of Mauna Loa's vent, and Keeling perfected methods for detecting and correcting intervals when the wind blew the wrong way.
Measurements at about 100 other sites have confirmed the long-term trend shown by the Keeling Curve, although no sites have a record as long as Mauna Loa. Monitoring networks include the global Fluxnet network, with sites concentrated in North America and Western Europe; the AmeriFlux network, with 35 stations in Canada, Costa Rica and the United States; and the EuroFlux network, with 18 sites in northwestern Europe. This international ground network is continuously being upgraded, and there are extensive inter-calibration efforts.