The concentration of CO2 in the atmosphere reached in May a level 50% higher than during the pre-industrial era, and never seen on Earth for around 4 million years, an American agency warned on Friday.

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Global warming caused by humans, in particular through the production of electricity using fossil fuels, transport, the production of cement or even deforestation, is clearly responsible for this new high, underlined the US Oceanic and Atmospheric Observation Agency (NOAA).

May is usually the month with the highest carbon dioxide levels each year.

In May 2022, the bar of 420 parts per million (ppm), the unit of measurement used to quantify pollution in the air, was crossed. In May 2021, this rate was 419 ppm, and in 2020, 417 ppm.

These measurements are taken at the Mauna Loa observatory in Hawaii, ideally located high on a volcano, allowing it not to be influenced by local pollution.

Prior to the Industrial Revolution, CO2 levels hovered consistently around 280ppm for nearly 6,000 years of human civilization before it, according to NOAA.

The level reached today "is comparable" to what it was "between 4.1 and 4.5 million years ago, when CO2 levels were near or above 400 ppm", a the agency said in a statement.

Back then, sea levels were 5 to 25 meters higher, enough for many of today's major cities to lie underwater. And large forests occupied parts of the Arctic, according to studies.

CO2 is a greenhouse gas that has the effect of trapping heat, gradually causing global warming. It persists in the atmosphere and oceans for thousands of years.

This warming is already having dramatic consequences, recalled the NOAA, including the multiplication of heat waves, droughts, fires or floods.

"Carbon dioxide is at levels our species has never seen before -- and that's nothing new," NOAA scientist Pieter Tans said in the statement. “We have known this for half a century and failed to do anything meaningful. What will it take to wake us up?”