The average for the day was 399.73 and researchers at the observatory expect this figure, too, to exceed 400 in the next few days.

The Arctic, by contrast, gets a lot of polluted air from Europe and North America.

The last time such values prevailed on Earth was in the Pliocene epoch,4m years ago, when jungles covered northern Canada.

Mauna Loa is the benchmark for CO2 measurement (and has been since 1958, see chart) because Hawaii is so far from large concentrations of humanity.

This year the average reading for the whole month will probably also reach 400ppm, according to Pieter Tans, who is in charge of monitoring at Mauna Loa, and the seasonally adjusted annual figure will reach 400ppm in the spring of 2014 or 2015.

AT NOON on May 4th the carbon dioxide concentration in the atmosphere around the Mauna Loa Observatory in Hawaii hit 400 parts per million (ppm).

Mauna Loa's readings are one of the world's longest-running measurement series.