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(上新世)4m years ago, when jungles covered northern Canada. There have already been a few readings above 400 ppm elsewhere—those taken over the Arctic Ocean in May 2012, for example—but they were exceptional. Mauna Loa is the benchmark(标准)for CO ₂ measurement 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 400 ppm, according to Pieter Tans, who is in charge of monitoring at Mauna Loa, and the seasonally adjusted annual figure will reach 400 ppm 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 315 ppm. That means they have risen by a quarter in 55 years. In the early 1960s they were going up by 0. 7 ppm a year. The rate of increase is now 2. 1 ppm—three times as fast—reflecting the relentless rise in green-house-gas emissions. As a rule of thumb, CO ₂ concentrations will have to be restricted to about 450 ppm if global warming is to be kept below 2 degrees. 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 450 ppm. There seems no chance of that. Emissions are still going up. At current rates, the Mauna Loa reading will rise above 450 ppm in 2037. Which is true according to the last paragraph