An atmospheric research station in Hawaii recorded its highest concentrations of carbon dioxide since accurate measurements began 63 years ago.
According to US government agency the National Oceanic and Atmospheric Administration, the coronavirus pandemic has barely impacted rising levels of the greenhouse gas.
NOAA said the Mauna Loa Atmospheric Baseline Observatory's measurements for May 2021 averaged 419 parts per million (ppm), up from 417 ppm in May 2020.
The Scripps Institution of Oceanography at the University of California San Diego was also involved in the research.
A senior scientist with NOAA's Global Monitoring Laboratory Pieter Tans said we are adding roughly 40 billion metric tons of CO2 pollution to the atmosphere per year.
He said if we want to avoid catastrophic climate change, the highest priority must be to reduce CO2 pollution to zero at the earliest possible date.