THE LIVING WORLD
Unit One. The Study of Life
1. The Science of Biology
1.6. Science in Action: A Case Study
In 1985 Joseph Farman, a British earth scientist working in Antarctica, made an unexpected discovery. Analyzing the Antarctic sky, he found far less ozone (O3, a form of oxygen gas) than should be there—a 30% drop from a reading recorded five years earlier in the Antarctic!
At first it was argued that this thinning of the ozone (soon dubbed the “ozone hole”) was an as-yet-unexplained weather phenomenon. Evidence soon mounted, however, implicating synthetic chemicals as the culprit. Detailed analysis of chemicals in the Antarctic atmosphere revealed a surprisingly high concentration of chlorine, a chemical known to destroy ozone. The source of the chlorine was a class of chemicals called chlorofluorocarbons (CFCs). CFCs (the purple balls 1 in figure 1.6) have been manufactured in large amounts since they were invented in the 1920s, largely for use as coolants in air conditioners, propellants in aerosols, and foaming agents in making Styrofoam. CFCs were widely regarded as harmless because they are chemically unreactive under normal conditions. But in the atmosphere over Antarctica, CFCs condense onto tiny ice crystals 2; in the spring, the CFCs break down and produce chlorine, which acts as a catalyst, attacking and destroying ozone, turning it into oxygen gas without the chlorine being used up 3.
The thinning of the ozone layer in the upper atmosphere 25 to 40 kilometers above the surface of the earth is a serious matter. The ozone layer protects life from the harmful ultraviolet (UV) rays from the sun that bombard the earth continuously. Like invisible sunglasses, the ozone layer filters out these dangerous rays. So when ozone is converted to oxygen gas, the UV rays are able to pass through to the earth 4. When UV rays damage the DNA in skin cells, it can lead to skin cancer. It is estimated that every 1% drop in the atmospheric ozone concentration leads to a 6% increase in skin cancers.
The world currently produces less than 200,000 tons of CFCs annually, down from 1986 levels of 1.1 million tons. As scientific observations have become widely known, governments have rushed to correct the situation. By 1990, worldwide agreements to phase out production of CFCs by the end of the century had been signed. Production of CFCs declined by 86% in the following 10 years.
Nonetheless, most of the CFCs manufactured since they were invented are still in use in air conditioners and aerosols and have not yet reached the atmosphere. As these CFCs move slowly upward through the atmosphere, the problem can be expected to continue. Ozone depletion is still producing major ozone holes over the Antarctic.
But the worldwide reduction in CFC production is having a major impact. The period of maximum ozone depletion will peak in the next few years, and researchers’ models predict that after that the situation should gradually improve, and that the ozone layer will recover by the middle of the 21st century. Clearly, global environmental problems can be solved by concerted action.
Figure 1.6. How CFCs attack and destroy ozone.
CFCs are stable chemicals that accumulate in the atmosphere as a by-product of industrial society 1. In the intense cold of the Antarctic, these CFCs adhere to tiny ice crystals in the upper atmosphere 2. UV light causes the breakdown of CFCs, producing chlorine (Cl). Cl acts as a catalyst, converting O3 into O2 3. As a result, more harmful UV radiation reaches the earth's surface 4.
Key Learning Outcome 1.6. Industrially produced CFCs catalytically destroy ozone in the upper atmosphere.