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). 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 last time such values prevailed on Earth was in the Pliocene epoch, 4m years ago, when jungles covered northern Canada. There have already been a few readings above 400ppm elsewhere—those taken over the Arctic Ocean in May 2012, for example—but they were exceptional. Mauna Loa is the benchmark for CO 2 measurement (and has been since 1958, see chart) because Hawaii is so far from large concentrations of humanity. The Arctic, by contrast, gets a lot of polluted air from Europe and North America.

The concentration of CO 2 peaks in May, falls until October as plant growth in the northern hemisphere’s summer absorbs the gas, and then goes up again during winter and spring. 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.

Mauna Loa’s readings are one of the world’s longest-running measurement series. The first, made in March 1958, was 315ppm. That means they have risen by a quarter in 55 years. In the early 1960s they were going up by 0.7ppm a year. The rate of increase is now 2.1ppm—three times as fast—reflecting the relentless rise in greenhouse-gas emissions.

As a rule of thumb, CO 2 concentrations will have to be restricted to about 450ppm if global warming is to be kept below 2°C (a level that might possibly be safe). Because CO 2 stays in the atmosphere for decades, artificial emissions of the gas would have to be cut immediately, and then fall to zero by 2075, in order to achieve 450ppm. There seems no chance of that. Emissions are still going up. At current rates, the Mauna Loa reading will rise above 450ppm in 2037.

Editor's update (May 10th, 5pm GMT): The daily measure of carbon concentration at Mauna Loa rose over 400 parts per million on May 9th, just after our article went to press. On May 9th, the daily average was 400.03.