Week 1: Tim Lenton

Practice what you preach, as they say. So I have decided to do some reflective learning and blog about my experience with our Climate Change MOOC. It’s been great this week, seeing so much engagement with the course material on the discussion threads. But it left me looking for one place where I could respond to some of the issues that were catching fire, without having to repeat myself. Hopefully this will be it. So here goes with this week’s favourite topic – ozone…

Yes, we threw a curveball in the very first question on the course. A lot of people were surprised to hear that ozone (chemical formula O3) is a greenhouse gas, especially knowing that we have been trying to protect the ozone layer for the last twenty years. So, let’s tackle that one head on.

The first point to grasp is that ozone is present in two layers of our atmosphere – the well-mixed bottom layer that we breathe, known as the ‘troposphere’ – and the next layer up, known as the ‘stratosphere’ (because it is vertically stratified, i.e. layered).

It is in the stratosphere that the ozone layer forms, and it is the absorption of high energy (UV) sunlight by the ozone layer that heats up the stratosphere and gives it its stratification (with temperature increasing as a function of height).

Down in the troposphere ozone is a short-lived gas, concentrated near the surface, and produced as a by-product of chemical reactions acting on a range of mostly human pollutant gases, including oxides of nitrogen, carbon monoxide, methane, and other ‘volatile organic carbon’ species.

In both atmospheric layers, ozone functions as a ‘blanket gas’ absorbing heat radiation coming off the Earth and thus helping warm the surface. However, the warming associated with the stratospheric ozone layer is natural, and the ozone layer is doing a wonderful service shielding us from ultraviolet radiation, which we couldn’t live without. The ozone in the troposphere on the other hand has been increased in concentration by human activities, thus contributing to climate change – and it has some other nasty effects, like inhibiting plant productivity.

The depletion of the stratospheric ozone layer that was caused by human-produced chlorofluorocarbons (CFCs) did, as would be expected, tend to cool the planet, but only by a small amount when globally averaged. That cooling was more than outweighed by warming due to the CFCs themselves, which are potent ‘blanket gases’. And both effects are small compared to the contribution of carbon dioxide (CO2) to recent warming.

Interestingly, the creation of the ozone hole, as well as letting more UV radiation down to the Earth’s surface, has affected the climate regionally in Antarctica and the Southern Ocean, tending to keep things cool there, and leading to a strengthening of the winds encircling the planet above the Southern Ocean. Those strengthening winds have in turn tended to blow more sea-ice away from the areas where it is made around Antarctica, causing the surprising increase in area of Antarctic sea-ice that is so beloved of climate sceptics.

Hopefully that gives some glimpse of the beautiful, interconnected complexity of the climate system. Happily the stratospheric ozone layer is on the mend, but unfortunately the compounds we replaced CFCs with (the HCFCs) are still potent ‘blanket gases’. One day we’ll learn…

Professor Tim