Keeling Curve - Mauna Loa Measurements

Mauna Loa Measurements

Due to funding cuts in the mid-1960s, Keeling was forced to abandon continuous monitoring efforts at the South Pole, but he scraped together enough money to maintain operations at Mauna Loa, which have continued to the present day.

The measurements collected at Mauna Loa show a steady increase in mean atmospheric CO₂ concentration from about 315 parts per million by volume (ppmv) in 1958 to 392 ppmv as of July 2011. This increase in atmospheric CO₂ is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years. Since carbon dioxide is a greenhouse gas, this has significant implications for global warming. Measurements of carbon dioxide concentration in ancient air bubbles trapped in polar ice cores show that mean atmospheric CO₂ concentration has historically been between 275 and 285 ppmv during the Holocene epoch (9,000 BCE onwards), but started rising sharply at the beginning of the nineteenth century. However, analyses of stomatal frequency in tree leaves indicate that mean atmospheric CO₂ concentration may have reached 320 ppmv during the Medieval Warm Period (800–1300 CE) and 350 ppmv during the early Holocene.

Though Mauna Loa is not an active volcano, Keeling and collaborators made measurements on the incoming ocean breeze and above the thermal inversion layer to minimize local contamination from volcanic vents. In addition, the data are normalized to negate any influence from local contamination. Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve, though no sites have a record as long as Mauna Loa.

The Keeling Curve also shows a cyclic variation of about 5 ppmv in each year corresponding to the seasonal change in uptake of CO₂ by the world's land vegetation. Most of this vegetation is in the Northern hemisphere, since this is where most of the land is located. The level decreases from northern spring onwards as new plant growth takes carbon dioxide out of the atmosphere through photosynthesis and rises again in the northern fall as plants and leaves die off and decay to release the gas back into the atmosphere.

Due in part to the significance of Keeling's findings, the NOAA began monitoring CO₂ levels worldwide in the 1970s. Today, CO₂ levels are monitored at about 100 sites around the globe.

Carbon dioxide measurements at the Mauna Loa observatory in Hawaii are made with a type of infrared spectrophotometer called a capnograph by, John Tyndall, its inventor in 1864, but now known as a nondispersive infrared sensor.

Keeling died in 2005. Supervision of the measuring project was taken over by his son, Ralph Keeling, a climate science professor at the Scripps Institution.

When C. D. Keeling published his 1960 paper, there did not appear to be any evidence of oceanic absorption. Keeling conjectured that this might be attributable to fluctuations in the causation of the seasonal variation, within the small number of years since his measurements had begun. However it is now understood that allowance should be made for about 50% absorption by the oceanic sinks. Keeling father and son have contributed to the work which has since yielded a much fuller understanding of the relation between atmospheric CO2 and the entirety of the carbon cycle, including the effects of land and ocean sinks.