Global Environment.com

Ozone Depletion

What is the Threat from Ozone Depletion?

Evolution of photosynthetic, oxygen-producing bacteria has produced a stratospheric global sunscreen: the ozone layer. Its presence for the past450 million years has allowed life to develop and expand on land and in the surface layers of aquatic systems.

The overwhelming consensus of researchers in this field is that ozone depletion by certain chlorine-and bromine-containing chemicals emitted into the atmosphere by human activities is a serious long-term threat to human health, animal life, and the sunlight-driven primary producers (mostly plants) that support the earth’s food chains and webs.

Ozone concentrations in the stratosphere have been measured since mid-1960s at more than 30 locations around the world and also by satellites since 1970. These measurements show that during the 1980s normal ozone levels dropped 5-155 in winter above the temperate and tropical zones of both hemispheres – three times the losses measured in the 1970s. Globally, the earth lost an average of about 4% of its stratospheric ozone between 1979 and 1994. According to a 1995 report by prominent atmospheric scientists, average global ozone levels are projected to drop 7-13% during the 2000-2005 period.

What causes Ozone Depletion?

This situation started when Thomas Midgley, Jr., s General Motors chemist, discovered the first chlorofluorocarbon (CFC) in 1930, and chemists then made similar compounds to create a family of highly useful CFC's.

These compounds seemed to be dream chemicals. Cheap to make, they became popular as coolants in air conditioners and refrigerators (replacing toxic sulphur dioxide and ammonia), propellants in aerosol spray cans, cleaners for electronic parts such as computer chips, sterilants for hospital instruments, fumigants for granaries and ship cargo holds, and bubbles in plastic foam used for insulation and packaging.

But CFC's were too good to be true. In 1974 calculations by chemists Sherwood Rowland and Mario Molina indicated that CFC's were creating a global chemical time bomb by lowering the average concentration of ozone in the stratosphere. They called for an immediate ban of CFC's in spray cans (for which substitutes were readily available).

They found that spray cans, discarded or leaky refrigeration and air conditioning equipment, and the production and burning of plastic foam products release CFC's into the atmosphere. Because these molecules are insoluble in water and are chemically unreactive, they are not removed from the troposphere. As a result – mostly through convection, random drift, and the turbulent mixing of the air in the troposphere – they rise slowly into the stratosphere, taking 10-20 years to make the journey.

In the stratosphere, under the influence of high-energy UV radiation, these molecules break down and release highly reactive chlorine atoms, which speed up the breakdown of highly reactive ozone (O3) into O2 and O in a cyclic chain of chemical reactions. This causes ozone in the stratosphere to be destroyed faster than it formed.

Each CFC molecule can last in the stratosphere for 65-110 years. During that time each chlorine atom released from these molecules can convert as many 100,000 molecules of O3 to O2 before it is removed from the stratosphere by forming HCl which diffuses downward to the troposphere and is removed by rain). If these calculations are correct, these molecules have turned into a nightmare of global ozone destroyers.

Although the warning of this problem was stated in 1974, it took 15 years of interaction between scientific and political communities before countries agreed to being phasing out CFC's. In 1995 Rowland and Molina received the Nobel prize in chemistry for their work.

What Other Chemicals Deplete Stratospheric Ozone?

CFC's are not the only ozone-eaters; other chemicals can release highly reactive and bromine atoms if they reach the stratosphere and are exposed to intense UV radiation.

One group consists of long-lived bromine-containing compounds such as halons and HBFC's, both used in fire extinguishers. Another is methyl bromide (Ch3Br), a widely used fumigant. Another group consists of chlorine-containing compounds such as carbon tetrachloride (CCl4), a cheap, highly toxic solvent, and toxic methyl chloroform, or 1,1,1-trichloroethane (C2H3Cl3), used as a cleaning solvent for clothes and metals and as a propellant in more than 160 consumer products, such as correction fluid, dry-cleaning sprays, spray adhesives, and other aerosols. Another source of ozone depletion is the emission of hydrogen chloride (HCl) into the stratosphere by space shuttles.

What are the Effects of Ozone Depletion?

Why should we care about ozone loss? From a human standpoint the answer is that with less ozone in the stratosphere, more biologically damaging UV-B radiation will reach the earth’s surface and give humans worse sunburns, more cataracts and more skin cancers.

According to UN Environment Programme estimates, the additional UV-B radiation reaching the earth’s surface resulting from an annual 10% loss of global ozone could lead to 300,000 additional cases of squamous cell cancer and basal cell cancer worldwide each year, 4,500-9,000 additional cases of potentially fatal malignant melanoma each year, and 1,5 million new cases of cataracts each year.

Cases of skin cancer and cataracts are increasing in Australia, New Zealand, South Africa, Argentina and Chile, where the ozone layer is very thin for several months each year.

Assuming that we phase out all ozone destroying chemicals over the next three decades, the EPA estimates that projected ozone thinning during the 1990s and 2000s will lead to12 million new cases of skin cancer and 200,000 additional skin cancer deaths in the United States alone.

Other effects from increased UV exposure are:

Suppression of the immune system. This makes the body more susceptible to infectious diseases and some forms of cancer.
An increase in eye burning, highly damaging acid deposition, and ozone in smog in the troposphere.
Lower yields of key crop such as corn, rice, cotton, soybeans, beans, peas, sorghum, and wheat.
A serious decline in forest productivity of the many tree species sensitive to UV-B radiation. This could reduce CO2 uptake and enhance global warming.
Increased breakdown and degradation of materials such as various types of paints, plastics, and outdoor materials.
Reduction in the productivity of surface dwelling phytoplankton, which could upset aquatic food webs, decrease yields of seafood eaten by humans, and possibly accelerate global warming by decreasing the oceanic uptake of CO2 by phytoplankton.
Damage to t5he ecological structure and function of lakes because of deeper penetration of UV light caused by synergistic interaction between ozone depletion, global warming, and acid deposition.

Humans can quickly make crucial adaptations to increased UV-B radiation by staying out of the sun, protecting their skin with clothing, and applying sunscreens. However, plants and other animals that help support us and other forms of life can’t make such changes except through the long process of biological evolution.

Solutions: Protecting the Ozone Layer

The scientific consensus of researchers in this field is that we should immediately stop producing ozone-depleting chemicals. Even with immediate action, models indicate that it will take 50-60 years for the ozone layer to return in 1975 levels and another 100-200 years for full recovery to pre-1950 levels.

Substitutes are already available for most uses of CFC's, and others are being developed. See table below:

Some Hopeful Progress

In 1987, 36 nations meeting in Montreal developed a treaty, commonly known as the Montreal Protocol, to cut emissions of CFC's into the atmosphere by 35% between 1989 and 2000.