Most ozone lies in the lower stratosphere between 15 and 30 km in altitude. Here it absorbs harmful (UVB) radiation from the sun.
If it were brought to sea level, the ozone layer would be 3 mm thick. Within the Antarctic ozone hole this can shrink to less than 1 mm.
Stratospheric ozone is being depleted by some of the human-made gases that have been released into the atmosphere, including those known as chlorofluorocarbons (CFCs). They can eventually migrate upward into the stratosphere.
The ozone hole
The ozone hole was first detected over Antarctica when scientists compared the amount of ozone found in the early 1980s with measurements dating back to 1956. The ozone hole forms each year when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica for a period of about two months during September and October.
Each winter a polar vortex forms in the stratosphere over the Antarctic. There, without sunlight, temperatures drop to as low as −85° Celsius in the lower stratosphere. At these very low temperatures, ice clouds form that act as sites where chlorine- and bromine-containing compounds (the halons) are converted to compounds that will catalytically destroy ozone. In spring when the sunlight returns to Antarctica, the destruction of ozone within the polar vortex starts and rapidly accelerates. It reaches a maximum in early October and slowly declines so that things have returned to normal by the end of December.
Ozone depletion is much less in the Arctic
The ozone hole over Antarctica has no direct northern hemisphere counterpart because the meteorologies of the two polar regions are very different. The South Pole is part of a very large land mass (Antarctica) that is completely surrounded by ocean. These conditions produce very low stratospheric temperatures that in turn lead to formation of clouds (polar stratospheric clouds). The clouds that form at low temperatures lead to chemical changes that promote rapid ozone loss during September and October of each year, resulting in the ozone hole. In contrast, the Earth's surface in the northern polar region lacks the land/ocean symmetry characteristic of the southern polar area. As a consequence, Arctic stratospheric air is generally much warmer than in the Antarctic, and fewer clouds form there. Therefore, the ozone depletion in the Arctic is much less than in the Antarctic.
Increases in UVB radiation damage living things
About 2% of the light the sun emits is in the form of high-energy, ultraviolet (UV) radiation. Some of this UV radiation (UV-B) causes damage to living things, including sunburn, skin cancer and eye damage.
The amount of solar UV radiation received at any particular location on the Earth's surface depends upon:
- the position of the Sun above the horizon
- the amount of ozone in the atmosphere and
- local cloudiness and pollution
Scientists agree that in the absence of changes in clouds or pollution, decreases in atmospheric ozone will increase ground-level UV radiation. Large increases in UVB have been observed in Antarctica during Spring.
Stratospheric ozone losses have caused a cooling of the global lower stratosphere, which results in less infrared radiation reaching the surface, and offsetting the greenhouse effect.
Ozone depletion caused by human-made chemicals is continuing and is expected to persist until chlorine and bromine levels are reduced. The total combined abundance of ozone-depleting compounds in the lower atmosphere peaked in about 1994 and is now slowly declining. Total chlorine is declining, but total bromine is still increasing. Worldwide monitoring has shown that stratospheric ozone has been decreasing for the past two decades or more. Globally averaged losses have been about 5% since the mid-1960s, with cumulative losses of about 10% in the winter and spring and 5% in the summer and autumn over locations such as Europe, North America, and Australia.
Record low ozone levels have been observed in recent years, and much greater future global depletions in ozone would have been highly likely without reductions in human emissions of ozone-depleting gases. However, worldwide compliance with current international agreements is rapidly reducing the yearly emissions of these compounds. As these emissions cease, the ozone layer will gradually improve over the next several decades. The recovery of the ozone layer will be gradual because of the long times required for CFCs to be removed from the atmosphere.
Watch the 2012 ozone hole form and then disappear (below).