My NASA Earth Observatory newsletter this week contained a beautiful representation of the Ozone ‘hole‘. You can read everything you ever wanted to know about this phenomenon at the NASA Ozone Hole watch site, including some beautiful videos showing the formation of the ozone hole as Antarctica enters its dark winter. Seeing these movies – with their day-to-day fluctuations – shows just how complex this phenomenon is. But its measurement is generally summarised in two statistics: the extent of the hole, and its minimum depth. I have made graphs of these quantities below and it seems that the extent and the severity of the hole have stabilised in recent years. This is pretty much as predicted by the modeling of the phenomena following the Montreal Protocol controlling emissions of CFC chemicals.
Over most of the Earth the ozone layer is formed daily as the sunlight reaches the thin gases in the upper atmosphere. Visible light passes straight through these molecules, but the ultra-violet light is absorbed, exciting the molecules and stimulating the formation of ozone – a molecule with three atoms of oxygen linked together (O3) instead of the normal two (O2). The ozone is simultaneously formed by the ultra-violet light, and is especially efficient at absorbing ultra-violet light. The ozone formation layer extends through tens of kilometres of the extremely thin atmosphere. The Dobson unit measures the thickness of the ozone if it were all compressed to atmospheric pressure at ground level. Each unit corresponds to 0.001 mm of ozone at atmospheric pressure. So the normal ozone thickness of 330 Dobson units corresponds to only 3.3 millimetres of ozone at atmospheric pressure. I find this fact profoundly disturbing: life on Earth is protected from the withering ultra-violet light from the Sun by a layer of gas this thin. It seems astonishing that we are protected by something so fragile, and yet so vast in its extent.
Update 24th September 2010: A kind reader in the comments pointed out a couple of pages on the NASA Earth Observatory site which describe the world which would have happened if we had not banned CFCs. It’s a scary scenario, and it makes me feel good that people around the world can agree on such things.