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The measurement, called the Keeling Curve after Charles David Keeling, the scientist who began tracking carbon dioxide there in 1958, is a global benchmark for atmospheric carbon levels.
Instruments perched on NOAA's mountaintop observatory recorded carbon dioxide at about 419 parts per million last month, more than the 417 parts per million in May 2020.
The amount of carbon in the air now is as much as it was about 4 million years ago, a time when sea level was 78 feet (24 meters) higher than it is today and the average temperature was 7 degrees Fahrenheit higher than it was before the Industrial Revolution, said a report on the emissions.
"We are adding roughly 40 billion metric tons of CO2 pollution to the atmosphere per year," Tans wrote in the report. "That is a mountain of carbon that we dig up out of the Earth, burn, and release into the atmosphere as CO2 -- year after year."
The carbon dioxide levels measured were not affected by the eruption of Hawaiian volcanoes, Tans said, adding the station is situated far enough from active volcanoes that measurements are not distorted, and occasional plumes of carbon dioxide are removed from the data.
The scientists urged the global community to transition to solar and wind energy instead of fossil fuels, warning that the world has been unable to slow, let alone reverse, annual carbon dioxide levels thus far.
"The solution is right before our eyes," said Tans. "If we take real action soon, we might still be able to avoid catastrophic climate change."