The US National Oceanic and Atmospheric Administration (NOAA) has published data collected at its Mauna Loa atmospheric baseline observatory in Hawai’i showing that atmospheric CO2 concentration at Mauna Loa has reached 387 part per million by volume (ppmv), the highest concentration of carbon dioxide in Earth’s atmosphere in the last 650,000 years.
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On average, the annual mean increase per year is accelerating, averaging 2.1 ppmv per year since 2000. Pre-industrial revolution carbon dioxide levels were about 280 ppm, or about 60% of today’s levels.
Historical atmospheric CO2 concentrations that predate direct measurement are calculated using a range of proxy data collected from air bubbles trapped in ice cores that have been extricated from areas such as the West Antarctica Ice Sheet (WAIS) and the Greenland Ice Sheet (GIS). Ice cores have been drilled as deep as 3,100 meters, or almost two miles.
The Mauna Loa observatory, which is sited on the Mauna Loa volcano, was the site of early CO2 monitoring by Charles David Keeling of the Scripps Institution of Oceanography, beginning in 1957.