As oxygen increased in the Earth's atmosphere, so did the levels of ozone, O3. The formation of the ozone layer in the upper atmosphere is generally believed to have played an important role in the development of life on Earth, particular in the development of life on land. The accumulation of oxygen molecules in the atmosphere allowed for the production of ozone. Gradually, the increasing levels of ozone led to the formation of the stratosphere, a region of the upper atmosphere where temperature increases with altitude largely as a result of the absorption of solar radiation by ozone. The resulting screening of lethal levels of solar UV radiation by the ozone layer is thought to have been important to allowing life to migrate from the oceans onto land.
Ozone, O3, is composed of three oxygen atoms and is a gas at atmospheric pressures and temperatures. Approximately 90% of the atmospheric ozone is in the stratosphere. Most of the remaining ozone is in the troposphere, the lower region of the atmosphere extending from the Earth's surface up to roughly 10 km at mid-latitudes and 16 km in the tropics. At mid-latitudes, the peak concentrations of ozone occur at altitudes between 20 and 30 km. At high latitudes, the peak occurs at lower altitudes, largely as the result of atmospheric transport processes and the lower height of the tropopause (the transition region between the troposphere and stratosphere).
Ozone in the stratosphere is often called 'good' ozone because it protects life on Earth from harmful levels of UV radiation from the Sun. Therefore, a decrease in the amount of ozone would allow an increase in the amount of the UV radiation from the Sun to reach the Earth's surface. Corresponding to an increase in UV are projected significant impacts on ecosystems and human health, including increases in incidences of skin cancer, eye cataracts, damage to genetic DNA, and suppression of the efficiency of the immune system. It is the concerns about increased biologically harmful levels of UV from the decreasing levels of ozone that has largely been the driver for policy actions to protect the ozone layer.
On the other hand, ozone near the Earth's surface is called 'bad' ozone because of its direct effects on plants, ecosystems, and humans. Ozone pollution is a concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe. Many urban and suburban areas throughout the world have high levels of 'bad' ozone during summer months, and the effects of winds can carry these high ozone levels to rural areas.
Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. 'Bad' ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue. Ground-level ozone also damages vegetation and ecosystems. It leads to reduced agricultural crop and commercial forest yields, reduced growth and survi-vability of tree seedlings, and increased susceptibility to diseases, pests, and other stresses such as severe weather.
Ozone can also radiatively affect the Earth's climate. Ozone absorbs solar radiation but it also is a so-called greenhouse gas that can absorb infrared radiation from the Earth that otherwise would be emitted to space. It is the balance between the solar and infrared radiative processes that determines the net effect of ozone on climate. Decreases in ozone in the stratosphere above about 30 km (roughly 18 miles above the Earth's surface) tend to increase the surface temperature as a result of the increased absorption of solar radiation, effectively increasing the solar energy that warms the Earth's surface. Below about 30 km, decreases in ozone tend to cool the surface temperature, as the infrared greenhouse effect dominates in this region. Scientific analyses have shown that the decrease in stratospheric ozone over recent decades have had a cooling effect, counteracting a fraction of the warming effect over this time from increasing concentrations of carbon dioxide and other greenhouse gases.
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