The U.S. National Oceanic and Atmospheric Administration revised down readings that showed the amount of carbon dioxide in the atmosphere surpassed a threshold not seen for 3 million years.
Preliminary data published May 10 showed the gas reached a concentration of 400.03 parts per million, or ppm, at NOAA’s Mauna Loa monitoring station in Hawaii the day before. The figure for May 9 has now been revised down to 399.89 ppm, and is still deemed preliminary.
The level, not passed for at least 3 million years, is considered a landmark by scientists and environmentalists, who say carbon emissions caused by burning fossil fuels are warming the planet and must be reined in before they cause irreversible changes to weather, sea levels and Arctic ice cover.
While hourly measurements at the station have frequently surpassed 400 ppm in the past month, the daily level has not. The most recent data, for May 11, shows a reading of 399.46 ppm, the same level as shown by separate readings taken at the same observatory by the University of California, San Diego’s Scripps Institution of Oceanography, which uses different equipment to analyze its data.
The Mauna Loa data is important because it’s the longest set of continuous measurements of atmospheric concentrations of carbon dioxide. Charles David Keeling, a geochemist at Scripps, began taking readings there in 1958. Scientists use measurements of gases in ice bubbles trapped in ice, as well as analysis of ancient sediments, to determine carbon dioxide levels in the atmosphere hundreds of thousands of years ago.
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