Stratospheric Ozone (Global)

Contact Lucien Froidevaux

The stratospheric ozone (O3) layer (near 20 km altitude) is an absorber of ultraviolet light from the sun; this absorption protects humans from the potentially deadly effects of skin cancer, and can also shield animals and the marine food chain, as well as plants, from undesirable UV-related consequences.

Health effects tied to excessive UV exposure include skin cancer, cataracts, and a decline of the immune response system; while there are significant changes in average UV exposure from low to high latitudes, additional exposure (at any latitude) represents some increase in the risks. Decreases in the ozone layer have therefore been a cornerstone of atmospheric research for the past several decades, motivated by the realisation that industrial release of chlorofluorocarbon (CFC) gases at the Earth's surface were linked to a gradual depletion of the ozone layer, as well as the seasonal "ozone hole" phenomenon over Antarctica caused by enhancements in the ozone-destroying forms of chlorine and bromine in the northern hemisphere during the cold winter and spring periods.

The different variations in ozone between the northern and southern hemispheres are related to the interplay of dynamical and chemical effects. A circling whirlpool of winds isolates the so-called polar vortex region at high latitudes in winter. More vigorous wave activity in the North leads to a shorter-lived winter polar vortex than in the South, and this reduces the net ozone loss in the northern hemisphere.

On a global scale, ozone depletion is typically measured with respect to pre-1980 abundances; values of overhead (column) ozone abundances in the past few years have been lower than the pre-1980 levels by 3 to 6% (for mid- to high latitudes in the North and South, respectively).

Thanks to internationally-agreed reductions in CFC emissions after the 1987 Montreal Protocol on Subtances that Deplete the Ozone Layer (with its many subsequent amendments), global ozone is expected to recover to pre-1980 levels in the 2nd half of the 21st century. The slow recovery process arises because of the very long lifetime of the main CFC gases in the upper atmosphere (sunlight destroys these compounds very slowly).

There is mounting evidence that a slow path towards such a recovery is being achieved, although continued attention to unexpected chemistry and the variations in ozone is still a very useful endeavor.