The release of CFCs or chlorofluorocarbons into the atmosphere through human activities has caused a massive hole in the ozone layer right above Antarctica and if unchecked, melting icecaps may inundate several regions of the earth in the future.
Ozone is a gas with a pungent odor whose molecule contains three oxygen atoms. At about 6–10 miles above the Earth’s surface and extending up to 30 miles, in a region of space called the stratosphere, you will find 90% ozone. The stratospheric region with the highest ozone concentration is commonly known as the “ozone layer”. The remaining ozone, about 10%, is found in the troposphere, which is the lowest region of the atmosphere, between Earth’s surface and the stratosphere.
Ozone at ground level in the troposphere is bad because it causes photochemical smog. The smog results when ultra-violet light falls on and reacts with nitrogen oxide from vehicle exhausts. Because of this, Ozone affects lung function, aggravates asthma and other chronic respiratory diseases.
On the other hand, ozone in the stratosphere performs a very useful function by acting as a blanket that blocks most of the sun’s high-frequency ultraviolet rays. These UV rays can cause skin cancer and cataracts in humans, as well as reproductive problems in several forms of life including even the single-celled phytoplankton at the bottom of the ocean food chain.
How Does Ozone Form in the Atmosphere?
When ultraviolet light strikes oxygen molecules containing two oxygen atoms (O2), it splits them into individual oxygen atoms (atomic oxygen), which then combines with unbroken O2 to create ozone, O3. Being unstable, this ozone once again splits into a molecule of O2 and an atom of atomic oxygen under the action of ultraviolet light. This continuing process called the ozone-oxygen cycle.
The ozone layer is very effective at screening out UV-B; Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface. Ozone cannot stop UV-A, the longer wavelength ultraviolet radiation which reaches the earth’s surface. However, this type of UV radiation is significantly less harmful to DNA.
The thickness of the ozone layer varies widely throughout the world, being smaller near the equator and larger towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere.
In May 1985 scientists with the British Antarctic Survey announced the discovery of a huge hole in the ozone layer over Antarctica. They announced that Ozone levels over the northern hemisphere had been dropping by 4% per decade. They described the larger seasonal drops in the ozone levels around the south pole as a ozone hole.
The ozone hole is not technically a “hole” with no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic during the Southern Hemisphere spring (August–October).
Stratospheric temperatures in the Northern Hemisphere during winter/spring are generally slightly warmer than those in the Southern Hemisphere. Therefore ozone losses over the Arctic have been much smaller than over the Antarctic during the 1980s and early 1990s. However, the Arctic stratosphere has gradually cooled over the past few decades, and Ozone holes have been observed at the Arctic regions too recently. This is a dangerous trend, because unlike the Southern Polar hemisphere, the Northern Polar hemisphere is well populated.
Ozone hole is caused by chemicals called CFCs, or chlorofluorocarbons. CFCs escape into the atmosphere from refrigeration and propellant devices and processes. In the lower atmosphere, they are so stable that they persist for decades. Eventually, some of the CFCs reach the stratosphere where chemical reactions take place primarily on the surface of polar stratospheric clouds, ice particles, or liquid droplets, which form at high altitudes in the extreme cold of the polar regions. Ultraviolet light breaks the bond holding chlorine atoms (Cl) to the CFC molecule. Chlorine then destroys ozone molecules by “stealing” their oxygen atoms. The breakdown of ozone in the stratosphere makes it unable to absorb ultraviolet radiation. Consequently, the unabsorbed ultraviolet-B radiation is able to reach the Earth’s surface. The extent of ozone destruction is extremely sensitive to small changes in stratospheric temperature.
Another culprit responsible for the ozone depletion is nitrous oxide (N2O). The major sources of nitrous oxide are industrial processes and combustion engines of various vehicles. They are also emitted from livestock manure and sewage. Like CFCs, Nitrous oxide is stable when emitted at ground level, but breaks down when it reaches the stratosphere to form nitrogen oxides that trigger ozone-destroying reactions.
In 1987 several UN countries gathered at Montreal, Canada, and signed a treaty to protect the stratospheric ozone layer. The Montreal Protocol stipulated that the production and consumption of compounds that deplete ozone in the stratosphere—chlorofluorocarbons, halons, carbon tetrachloride, and methyl chloroform—are to be phased out by 2005.
Chemical manufacturers soon created substitutes for CFCs with little added costs; thus, our life styles remained greatly unaffected by the switch-over from CFC’s. This has had the effect of putting a slow stopper on the Ozone hole.
Now, the issue of a possible connection between ozone hole and global warming is a controversial subject even among scientists. In fact, there is no unanimity in either of the assertions that Antarctica is warming or cooling. The British Antarctic Survey says categorically Antarctica to be both warming around the edges and cooling at the center at the same time. Thus it is not possible to say whether it is warming or cooling overall. Because there are too many parameters governing the global temperatures, it is difficult to correlate the theoretical temperature rise at the Antarctic caused by a thinner ozone layer with global climatic changes. It is useful to remember here that Ozone itself is a greenhouse gas and its thinning over the region reduces heat trapped over it and helps create sea spray that forms reflective, cooling clouds.