The amount of carbon dioxide in the atmosphere has been 50% higher than in pre-industrial times.
That’s according to the latest analysis by the National Oceanic and Atmospheric Administration (NOAA) which suggests carbon dioxide measured at its Mauna Loa Atmospheric Baseline Observatory peaked at 421 parts per million (ppm) in May.
That level was not seen for millions of years, scientists have said.
The report suggests prior to the Industrial Revolution, carbon dioxide levels were around 280 ppm for almost 6,000 years of human civilisation.
NOAA Administrator Rick Spinrad said: “The science is irrefutable: humans are altering our climate in ways that our economy and our infrastructure must adapt to.
“We can see the impacts of climate change around us every day. The relentless increase of carbon dioxide measured at Mauna Loa is a stark reminder that we need to take urgent, serious steps to become a more Climate Ready Nation.”