In May, NOAA's measurements at the Mauna Loa observatory averaged 419.13 parts per million. Scientists at Scripps calculated a monthly average of 418.92 ppm. It's the highest level since accurate measurements began 63 years ago.
Atmospheric carbon dioxide measured at Mauna Loa Observatory reached a seasonal peak of 417.1 parts per million for 2020 in May, the highest monthly reading ever recorded. Monthly CO2 values at Mauna Loa first breached the 400 ppm threshold in 2014, and are now at levels not experienced by the atmosphere in several million years.