Last week the concentration of carbon dioxide in the atmosphere was reported to have passed 400 parts per million for the first time in 4.5m years. It is also continuing to rise at a rate of about 2 parts per million every year. On the present course, it could be 800 parts per million by the end of the century. Thus, all the discussions of mitigating the risks of catastrophic climate change have turned out to be empty words.

Collectively, humanity has yawned and decided to let the dangers mount. Professor Sir Brian Hoskins, director of the Grantham Institute for Climate Change at Imperial College in London, notes that when the concentrations were last this high, “the world was warmer on average by three or four degrees Celsius than it is today. There was no permanent ice sheet on Greenland, sea levels were much higher, and the world was a very different place, although not all of these differences may be directly related to CO2 levels.”

His caveat is proper. Nonetheless, the greenhouse effect is basic physics: it is why the earth has a more pleasant climate than the moon. CO2 is a known greenhouse gas. There are positive feedback effects from rising temperatures, via, for example, the quantity of water vapour in the atmosphere. In brief, humanity is conducting a huge, uncontrolled and almost certainly irreversible climate experiment with the only home it is likely to have. Moreover, if one judges by the basic science and the opinions of the vast majority of qualified scientists, risk of calamitous change is large.