The Northern Hemisphere breathing — spring leaf-out drawing carbon down, autumn decay releasing it. Six or seven parts per million, once a year: the sawtooth is the year turning. Underneath it, each breath lands a little higher than the last.

Roughly 315 when Keeling switched the analyser on in March 1958. In the 1960s the line climbed by under 1 ppm a year; over the past decade, nearer 2½.

Through eight hundred thousand years of ice cores — eight ice ages and their thaws — the air stayed between about 172 and 300. It left that range early in the twentieth century. Today's level is likely the highest in more than two million years.

Warm days each year in Berkhamsted — daily maximum of 20° or above, 1940 to 2025. The steady line is the ten-year mean: mid-fifties through the 1940s, around seventy now.