Ozone Recovery and its Future Impact

The Antarctic ozone hole was discovered by three British Antarctic survey scientists in 1985 to the disbelief of the scientific community, as no one had anticipated the extent of the decline in the polar ozone. Shortly after, chlorofluorocarbon (CFC) (responsible for the depletion of the ozone) production was curbed thanks to the negotiations of the international treaty, the ‘Montreal Protocol’ in 1987, and phased out completely in developed countries in 1995. Scientists of the American Geophysical Union announced in 2003 that the ozone depletion was slowing due to the regulations on CFC usage.

Today it is now believed that the ozone hole is expected to be closed completely sometime after 2050. But what effect has this ‘ozone hole’ had on our climate and will its closure have an impact on our atmosphere? Two recent studies address just such concerns, one by Polvani et al. published early 2011 and the other by McLandress et al. published in 2010.

The ozone layer in the stratosphere (roughly 13-40km above the surface of the earth) absorbs the harmful shortwave ultraviolet (UV) radiation from the sun, protecting us and life on this planet. The depletion that occurred over the South Pole didn’t simply just increase UV radiation but also affected the Southern Hemisphere atmospheric circulation. The depletion of the ozone causes a cooling to occur in the stratosphere due to a decrease in UV absorption. This cooling causes a polewards shift in the subtropical jet disturbing the tracks of storms occurring at mid to high latitudes. The ozone depletion is shown to also cause the southern boundary of the Hadley atmospheric circulation cell to extend southward towards the pole. This shifts the stable dry subtropical high pressure belt (+30^oS), which is responsible for the subtropical dry zones, further south. Coincidentally increases in anthropogenic greenhouse-gas concentrations have the same effect as depletion in the ozone. So for roughly the past 40 years these two anthropogenic forces (but mainly ozone depletion) have been working together and have shifted the summer Southern Hemisphere mid-latitude jet stream between 230-300 km polewards.

For the recovery of the ozone hole, the opposite effect will now occur as the polar stratosphere will heat up to the levels it was in the past but the expectations of greenhouse-gas concentrations are anticipated to continue rising, the two factors are expected to oppose each other as shown in Figure 1.

The locations of the westerly jet stream and the southern boundary of the Hadley cell, along with the storm tracks and subtropical dry zones associated with them, are affected by increasing greenhouse-gas concentrations and the recovery of the Antarctic ozone hole. Ozone-hole recovery causes them to shift towards the Equator (blue arrow) and increasing greenhouse-gas concentrations drive them towards the South Pole (red arrow).

Figure 1: Impact of greenhouse-gas increase and ozone-hole recovery on climate.

(Source: http://www.nature.com/nclimate/journal/v1/n1/fig_tab/nclimate1065_F1.html)

Polvani et al. and McLandress et al. use climate models (using different methodologies) to determine the effects of each climate forcing on atmospheric circulations as well as the combined effect of the two forcing, producing similar results for the 21^st century. Both studies found no significant changes in the location of circulation and precipitation patterns during summer in the Southern Hemisphere since the two forcings cancel each other out. It is stressed that this cancellation are a summertime occurrence, while other seasons will be dominated by the greenhouse-gas forcing as ozone forcing weakens and a shift of the jet stream and Hadley cell boundary towards the South Pole will be seen.

This is a very interesting phenomenon and has the potential of having a profound impact on South Africa’s climate. We could see an expansion in dry areas towards the South Pole and the winter rainfall region may experience a lot less rainfall as the subtropical high pressure belts moves further south effectively blocking the rain bearing cold fronts of the mid-latitude cyclones from reaching the continent. A rather worrying concern for our country.

For those of you that are interested here are some references…

McLandress, C.; Shepherd, T. G.; Scinocca, J. F.; Plummer, D. A.; Sigmond, M.; Jonsson, A. I.  and Reader, M. C. 2010. Separating the dynamical effects of climate change and ozone depletion: Part 2. Southern Hemisphere Troposphere. Journal of Climate Online ISSN: 1520-0442

Perlwitz, J. 2011. Atmospheric science: Tug of war on the jet stream. Journal name: Nature Climate Change Volume: 1,Pages: 29–31 Year published: (2011) DOI: doi:10.1038/nclimate1065 Published online 29 March 2011

Polvani, L. M.; Previdi, M. and Deser, C. 2011. Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends. Geophys. Res. Lett., 38, L04707, doi:10.1029/2011GL046712.

Polvani, L. M.; Waugh, D. W.; Correa, G. J. P. and Son, S-W. 2011. Stratospheric Ozone Depletion: The Main Driver of Twentieth-Century Atmospheric Circulation Changes in the Southern Hemisphere. J. Climate, 24, 795–812. doi: 10.1175/2010JCLI3772.1