Already I find this course is doing its job. There was I with my memories of GCSE chemistry 25 years ago (gulp!) thinking that at least I understood the way greenhouse gases work. Oh no…. Turns out that the term ‘greenhouse gases’ is not at all the best way to conceptualise our planet’s temperature systems.
The short-wave heat radiation entering the greenhouse, and the long-wave heat radiation partially leaving the greenhouse (which I’d learnt about at school) are only a very small part of the story. In a real greenhouse, the main heating mechanism is in fact the trapping of air inside the structure, preventing the escape of heat. Whereas for our planet, the key heating mechanism is the presence of certain gases in the atmosphere. These gases act as a blanket, trapping heat and re-radiating some of it back to the surface of the planet.
Without these blanket gases our planet would be, on average, a not very cosy -18C. However, as these gases are present, we have instead an average of 15C – much better for life. So, until the anthropocene, these blanket gases were doing a very important job, moderating the temperature of the planet and thus facilitating life.
So what are the key gases? Again, my chemistry GCSE let me down. CO2 is important, but not as important as H2O (as water vapour). Water vapour, it turns out, is the main blanket gas. CO2, methane, ozone, nitrous oxide and human made CFCs are also key contributors. Molecule for molecule, methane is a more active blanket gas than CO2, but it is also far less abundant in the atmosphere.