Measurements from Mauna Loa Observatory on Hawaii's Big Island averaged 424 parts per million (ppm) in May 2023, up 3 ppm from last May, according to the National Oceanic and Atmospheric Administration (NOAA). Atmospheric CO2 peaks in the Northern Hemisphere in May, so this month is used as a reference value. The new record builds on last year's all-time high and pushed the total concentration of CO2 in the atmosphere by 50 percent above pre-industrial levels. (Nick Kilvert)

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