The agenda for the COP 26 summit

SOME 1,500 years before the birth of Christ, when the chariots of Ahmose I, the first pharaoh of the 18th dynasty, had brought all of Egypt back under the rule of Thebes, the level of carbon dioxide in Earth’s atmosphere was about 277 parts per million (ppm). When the Gautama Buddha attained enlightenment under the Bodhi Tree a millennium later, and when Socrates drained his cup of hemlock a century after that, the level of CO₂ had hardly changed at all. It was barely different when the Tang dynasty in China and the first Muslim caliphate arose in the 7th century AD, or when the Aztec empire fell to the conquistadors nine centuries later on the other side of the world. For most of history the composition of Earth’s atmosphere has been as unchanging a backdrop to the human drama as the arrangement of its continents, or the face of its Moon.

In the middle of the 19th century that changed. Very quickly by historical standards, and instantaneously by geological ones, the CO₂ level began to rise. Having stayed between 275ppm and 285ppm for millennia, by the 1910s it had reached 300ppm. By 2020 it was 412ppm (see chart). In a century or so a crucial aspect of Earth’s workings had undergone a change 100 times greater than had previously been seen in a millennium.