A visualisation of polar stratospheric clouds and the Antarctic ozone hole by The Australian Community Climate Earth-System Simulator Chemistry Climate Model.
by Kane Stone
Ozone plays a very important role in our atmosphere. Although it is toxic to breathe directly, without it, life as we know it could not exist.
The ozone layer, situated between 10 and 30 km above us in the stratosphere, acts as a shield for the surface and its inhabitants by absorbing DNA-damaging ultraviolet radiation from the sun. Around 97-99% of UVB and UVC rays are absorbed by ozone and oxygen.
The successful international effort to protect the ozone layer definitely gave us a reason to celebrate World Ozone Day on 16 September.
The Montreal Protocol is an international agreement signed on 16 September 1987 to protect the ozone layer by limiting the production and use of ozone-depleting substances, mostly chlorofluorocarbons and halons. This protocol is widely hailed as the most successful environmental policy of our time.
The latest Scientific Assessment of Ozone Depletion, a requirement of the Montreal Protocol, has stated that ozone is on the road to recovery. Ozone in the tropics and mid-latitude, which has been depleted by 2.5% since 1980, is expected to recover back to 1980 levels by the middle of this century. Antarctic ozone is also scheduled to recover, but not until the later part of this century.
Not only has The Montreal Protocol succeeded in protecting the ozone layer, thus preventing up to two million annual cases of skin cancer by 2030, as ozone depleting substances are also very potent greenhouse gases it has also had a large impact on slowing human induced climate change. For example, emissions of ozone depleting substances in 1987 were equivalent to emitting 10 gigatonnes of CO2 per year. This has since been reduced by 90 percent.
Total column ozone over Antarctica for 14 September 2014. Blue and purple colours show lower amounts of ozone. Image credit: NASA/Ozone Watch.
The formation of the ozone hole each spring over Antarctica is the result of combination of a number of different dynamical and chemical processes.
Ozone depleting substances make their way into the stratosphere through upwelling in the tropics. Here they mostly reside in reservoir species, such as chlorine nitrate, preventing the large-scale destruction of ozone. However, the reservoir species find their way to the poles through the global overturning circulation.
When winter comes, strong upper atmospheric westerlies, called the polar vortex, at around 50˚S trap the reservoir species within. The combination of the polar night and the very stable air within lowers the temperatures dramatically. Once below 195˚K (-78˚C), polar stratospheric clouds form. Not only do polar stratospheric clouds provide a surface to rapidly convert the chlorine reservoir species into chlorine radical precursors, they also denitrify and dehumidify the stratosphere, forcing chlorine to remain in these more active radical forms by removing their reservoir partners.
Now, all that is required is enough energy to drive the reactions responsible for the catalytic destruction of ozone. Unfortunately, this energy is in abundant supply as soon as the sun returns in spring. When this occurs, chlorine radicals are unleashed, catalytically destroying ozone and continuing to do so until the break up of the polar vortex during December. This allows nitrogen rich air from lower latitudes to rush in and tie up the chlorine radicals back into their reservoir species, ending the destruction.
As you can probably guess, simulating such a complex process is not easy; it requires a very powerful chemistry climate model to fit all the moving parts together in a coherent manner. Luckily for us, such models exist, and continue to advance as the science progresses and computers get faster.
At the University of Melbourne, we are running one of these models, dubbed The Australian Community Climate Earth-System Simulator Chemistry Climate Model (ACCESS-CCM), and we are using it to contribute to the international Chemistry Climate Model Initiative (CCMI). This project brings together the current generation of chemistry climate models to perform simulations to address current science questions and accompany future ozone assessment reports and climate reports from the Intergovernmental Panel on Climate Change.
We have completed two main simulations with this model so far. A hind-cast simulation (reference-C1) from 1960-2010 and a future projection run (reference-C2) from 1960-2100.
The output of October average total column ozone is shown in the figure below compared to the ensemble of the 2nd Chemistry Climate Model Validation project (CCMVal-2, the previous iteration of CCMI) and observations. Our model is simulating a slightly higher amount of ozone than the CCMVal-2 ensemble and observations, however a similar amount of depletion is seen. In our future projection run, October ozone over Antarctica returns to 1980 levels by around 2060, but never quite returns to 1965 levels by the end of the time series.
ACCESS-CCM hind-cast and future projection simulated October average total column ozone between 60˚S and 90˚S compared to observations and CCMVal-2 multi-model ensemble. All solid lines have undergone a 10-year running mean, which removes the year-to year variability shown in the observations and highlights the longer-term variations.
Having access to such a powerful model, means we also have access to a lot of data for visualisation purposes. The video above, using model output at six hourly time steps, shows the evolution of ozone and polar stratospheric clouds and the formation of the Antarctic ozone hole over the course of a typical year in the 2000s. I hope you enjoyed World Ozone Day!