The highest value that the concentration of CO2 in the atmosphere reaches over the course of the year is usually recorded in May. The news that it had already exceeded 400 parts per million (ppm) by the end of March is therefore exceptional and alarming — for two reasons.

The first is that, according to the levels published daily on the Keeling curve website and the values recorded by the National Oceanic and Atmospheric Administration (Noaa), the concentration of CO2 has already shattered last year's record, surpassing 402 ppm. The second is that it means that until mid-May this level could continue to rise, breaking record after record and remaining above the symbolic figure of 400 ppm for weeks.

402 ppm is the highest figure ever reached in the last 800,000 years; it has never happened in the entire existence of humankind. This means we have entered a completely new, unpredictable phase, the consequences of which can only be hypothesised: melting ice, rising sea levels, an increase in extreme weather events — from floods to heatwaves.

Real-time monitoring of atmospheric CO2 began in 1958 on the summit of the Mauna Loa volcano in Hawaii. At the time, the average concentration was around 313 ppm. The measurements were compiled into a graph known as the Keeling curve (above, from 1700 to the present day), named after the American scientist Charles David Keeling, who produced it for the first time on behalf of the Scripps Institution of Oceanography.