(11) Air

Ozone Depletion



How Have We Depleted Ozone in the Stratosphere and What Can We Do about It?

Concept 15-6A Widespread use of certain chemicals has reduced ozone levels in the stratosphere, which allows more harmful ultraviolet radiation to reach the earth’s surface.

Concept 15-6B To reverse ozone depletion, we must stop producing ozone-depleting chemicals, and adhere to the international treaties that ban such chemicals

Human Activities Threaten the Ozone Layer

A layer of ozone in the lower stratosphere keeps about 95% of the sun’s harmful UV radiation from reaching the earth’s surface. Measurements made using balloons, aircraft, and satellites show considerable seasonal depletion (thinning) of ozone concentrations in the stratosphere above Antarctica and the Arctic. Similar measurements reveal a lower overall thinning everywhere except over the tropics.

Based on these measurements and mathematical and chemical models, the overwhelming consensus of researchers in this field is that ozone depletion in the stratosphere poses a serious threat to humans, other animals, and some primary producers (mostly plants) that use sunlight to support the earth’s food webs (Concept 15-6A).

This situation began when Thomas Midgley, Jr., a General Motors chemist, discovered the first chlorofluorocarbon (CFC) in 1930. Chemists soon developed similar compounds to create a family of highly useful CFCs, known by their trade name as Freons.

These chemically unreactive, odorless, nonflammable, nontoxic, and noncorrosive compounds seemed to be dream chemicals. Inexpensive to manufacture, they became popular as coolants in air conditioners and refrigerators, propellants in aerosol spray cans, cleaners for electronic parts such as computer chips, fumigants for granaries and ship cargo holds, and bubbles in plastic foam used for insulation and packaging.

It turned out that CFCs were too good to be true. Starting in 1974 with the work of chemists Sherwood Rowland and Mario Molina (Individuals Matter, below), scientists demonstrated that CFCs are persistent chemicals that destroy protective ozone in the stratosphere. Measurements and models indicate that 75–85% of the observed ozone losses in the stratosphere since 1976 were caused by CFCs and other ozone-depleting chemicals (ODCs) released into the atmosphere by human activities beginning in the 1950s.

Ozone Levels over the Earth’s Poles Drop for a Few Months Each Year

In 1984, researchers analyzing satellite data discovered that 40–50% of the ozone in the upper stratosphere over Antarctica disappeared each year during October and November. This observed loss of ozone has been called an ozone hole. A more accurate term is ozone thinning because the ozone depletion varies with altitude and location.

When the southern hemisphere’s winter ends and partial sunlight returns to Antarctica in October, huge masses of ozone-depleted air above Antarctica flow northward and linger for a few weeks over parts of Australia, New Zealand, South America, and South Africa. This raises biologically damaging UV-B levels in these areas by 3–10% and in some years by as much as 20%. In 2006, there was a record seasonal loss of ozone over an area of Antarctica about the size of North America.

In 1988, scientists discovered that similar but usually less severe ozone thinning occurs over the Arctic from February to June, resulting in a typical ozone loss of 11–38% (compared to a typical 40–50% loss above Antarctica). When the mass of air above the Arctic breaks up each spring, large masses of ozone-depleted air flow south to linger over parts of Europe, North America, and Asia.

Models indicated that the Arctic is unlikely to develop the large-scale ozone thinning found over the Antarctic. They also project that ozone depletion over the Antarctic and Arctic will be at its worst between 2010 and 2019.

Why Should We Worry about Ozone Depletion?

Why should we care about ozone loss? One effect is that more biologically damaging UV-A and UV-B radiation will reach the earth’s surface (Concept 15-6A). This will give people worse sunburns, more eye cataracts, and more skin cancers.

The most dangerous type of skin cancer is malignant melanoma. It kills about one-fourth of its victims (younger than age 40) within 5 years, despite surgery, chemotherapy, and radiation treatments. Each year it kills at least 48,000 people (including 7,700 Americans), mostly Caucasians, and the number of cases and deaths is rising in many countries. People who experience three or more blistering sunburns before age 20 are five times more likely to develop malignant melanoma than are those who have never had severe sunburns.

The most serious threat from ozone depletion is that the resulting increase in UV radiation can impair or destroy phytoplankton, especially in Antarctic waters. These tiny marine plants play a key role in removing CO2 from the atmosphere and when they die and sink to the ocean floor, they take their carbon out of circulation for millions of years as a part of the carbon cycle.

Furthermore, ozone depletion and global warming can interact to further decrease the vital populations of phytoplankton in Antarctic waters. Scientists project that global warming can slow down the upwelling of nutrients that support these populations of phytoplankton. In other words, populations of Antarctic phytoplankton could decrease sharply because of a combination of fewer nutrients and increased UV radiation.

We Can Reverse Stratospheric Ozone Depletion

According to researchers in this field, we should immediately stop producing all ODCs (Concept 15-6B). However, even with immediate and sustained action, models indicate it will take about 60 years for the ozone layer to return to 1980 levels and about 100 years for recovery to pre-1950 levels. Good news. Substitutes are available for most uses of CFCs, and others are being developed. In 1987, representatives of 36 nations met in Montreal, Canada, and developed the Montreal Protocol. This treaty’s goal was to cut emissions of CFCs (but not other ODCs) by about 35% between 1989 and 2000. After hearing more bad news about seasonal ozone thinning above Antarctica in 1989, representatives of 93 countries met in London in 1990 and then in Copenhagen, Denmark, in 1992. They adopted the Copenhagen Protocol, an amendment that accelerated phasing out key ODCs. These landmark international agreements, now signed by 189 countries, are important examples of global cooperation in response to a serious global environmental problem. If nations continue to follow these agreements, ozone levels should return to 1980 levels by 2068 (18 years longer than originally projected) and to 1950 levels by 2100 (Concept 15-6B). The longer healing time results from a connection between global warming of the troposphere and repair of the ozone layer.Warming of the troposphere makes the stratosphere cooler, which slows down the rate of its ozone repair.

The ozone protocols set an important precedent by using prevention to solve a serious environmental problem. Nations and companies agreed to work together to solve this global problem for three reasons. First, there was convincing and dramatic scientific evidence of a serious problem. Second, CFCs were produced by a small number of international companies. Third, the certainty that CFC sales would decline over a period of years unleashed the economic and creative resources of the private sector to find even more profitable substitute chemicals. However, the most widely used substitutes cause some ozone depletion and must also be phased out.

Sherwood Rowland and Mario Molina-A Scientific Story of Courage and Persistence

In 1974, calculations by chemists Sherwood Rowland and Mario Molina at the University of California–Irvine indicated that CFCs were lowering the average concentration of ozone in the stratosphere. They shocked both the scientific community and the $28-billion-per-year CFC industry by calling for an immediate ban of CFCs in spray cans, for which substitutes were available.

The research of these two scientists led them to four major conclusions. First, these persistent CFCs remain in the atmosphere. Second, over 11–20 years these compounds rise into the stratosphere through convection, random drift, and the turbulent mixing of air in the lower atmosphere.

Third, once they reach the stratosphere, the CFC molecules break down under the influence of high-energy UV radiation. This releases highly reactive chlorine atoms (Cl), as well as atoms of fluorine (F) and bromine (Br), all of which accelerate the breakdown of ozone (O3) into O2 and O in a cyclic chain of chemical reactions. As a consequence, ozone is destroyed faster than it forms in some parts of the stratosphere.

Fourth, each CFC molecule can last in the stratosphere for 65–385 years, depending on

its type. During that time, each chlorine atom released during the breakdown of CFC can

convert hundreds of O3 molecules to O2. The CFC industry, a powerful, well-funded adversary with a lot of profits and jobs at stake (led by DuPont), attacked Rowland’s and Molina’s calculations and conclusions. The two researchers held their ground, expanded their research, and explained their results to other scientists, elected officials, and the media. After 14 years of delaying tactics, DuPont officials acknowledged in 1988 that CFCs were depleting the ozone layer and they agreed to stop producing them.

In 1995, Rowland and Molina received the Nobel Prize in chemistry for their work. In awarding the prize, the Royal Swedish Academy of Sciences said that they contributed to “our salvation from a global environmental problem that could have catastrophic consequences.”


Effects of Ozone Depletion Human Health

Worse sunburns

More eye cataracts

More skin cancers

Immune system suppression

Food and Forests

Reduced yields for some crops

Reduced seafood supplies from reduced phytoplankton

Decreased forest productivity for UV-sensitive tree species


Increased eye cataracts in some species

Decreased populations of aquatic species sensitive to

UV radiation

Reduced populations of surface phytoplankton

Disrupted aquatic food webs from reduced phytoplankton

Air Pollution and Materials

Increased acid deposition

Increased photochemical smog

Degradation of outdoor paints and plastics

Global Warming

While in troposphere, CFCs act as greenhouse gases


Reducing Exposure to UV Radiation

Stay out of the sun, especially between 10 A.M. and 3 P.M.

Do not use tanning parlors or sunlamps.

When in the sun, wear protective clothing and sunglasses that protect against UV-A and UV-B radiation.

Be aware that overcast skies do not protect you.

Do not expose yourself to the sun if you are taking antibiotics or birth control pills.

When in the sun, use a sunscreen with a protection factor of at least 15.

Examine your skin and scalp at least once a month for moles or warts that change in size, shape, or color and sores that keep oozing, bleeding, and crusting over. If you observe any of these signs, consult a doctor immediately.

Volcanic Eruptions, Climate Change, and Sustainability

We have seen that human activities play a major role in warming the troposphere and depleting ozone in the stratosphere. Occasional large volcanic eruptions also emit CO2 and other pollutants into the lower atmosphere. But about three-fourths of current emissions of CO2 come from human activities, especially the burning of fossil fuels. Thus, energy policy and climate policy are closely connected. The four scientific principles of sustainability can be used to help reduce the problems of air pollution, global warming, and stratospheric ozone depletion. We can reduce reduce inputs of air pollutants, greenhouse gases, and ODCs into the atmosphere by relying more on direct and indirect forms of solar energy than on fossils fuels; reducing the waste of matter and energy resources and recycling and reusing matter resources; mimicking biodiversity by using a diversity of carbon-free renewable energy resources based on local or regional availability; and reducing human population growth and wasteful resource consumption.

We can also find substitutes for ODCs and emphasize pollution prevention. Each of us has an important role to play in protecting the atmosphere that sustains life and supports our economies.

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