How are ozone depletion and climate change related?

Stratospheric ozone depletion and climate change are both effects of human activities on the global atmosphere. They are distinct environ-mental problems, but are linked in a number of ways. Some of the main potential interactions are the following:

Ozone-depleting chemicals contribute to global warming

one-depleting chemicals can have an impact on the Earth's heat balance as well as on the ozone layer because many of them are greenhouse gases. For example, CFCs 11 and 12 (the two main chlorofluorocarbon compounds that destroy ozone) are, respectively, 4000 and 8500 times more powerful greenhouse gases than carbon dioxide (over a period of 100 years). Fluorocarbon chemicals developed as substitutes for CFCs are also powerful greenhouse gases.
Ozone depletion can affect climate

one is itself a greenhouse gas and the ozone layer play a role in maintaining the planet's overall temperature balance. Depletion of the ozone layer is currently thought to reduce the greenhouse effect.

On the other hand, increased exposure of the Earth's surface to UV-B due to ozone depletion could alter the cycling of greenhouse gases, such as carbon dioxide, in ways that could increase global warming. In particular, increased UV-B is likely to suppress primary production in terrestrial plants and marine phyto-plankton, so reducing the amount of carbon dioxide they absorb from the atmosphere.

Global warming could aggravate ozone depletion

Global warming is expected to increase average temperatures in the lower atmosphere-but it could cool the stratosphere. This could increase ozone depletion even with the same concentrations of man-made chemicals reaching the stratosphere because very cold temperatures favour special sorts of reactions that deplete ozone more rapidly.

How is UV radiation level changing at the Earth's surface?

Direct measurement of UV-B radiation levels is technically complicated. However, there is overwhelming scientific evidence that ozone depletion leads to more UV-B reaching the Earth's surface, and that the amount of increase can be predicted from trends in ozone levels. On this basis, UV-B at mid-latitudes is calculated to have increased by 8-10 per cent over the last 15 years (the calculation is for UV-B radiation at a wavelength of 310 nanometres at latitudes 45° north and south over the period 1979-1994). Calculated increases in UV-B to date are larger at higher latitudes and for shorter wavelengths.

The first persistent increase in UV-B over densely populated areas due to ozone depletion was measured in 1992/93. Several studies found large increases at northern mid and high-latitudes. Measurements at Toronto, Canada, suggested that UV-B at 300 nanometres was 35 per cent higher than four years previously.

Large increases in UV-B have occurred in Antarctica due to the annual ozone hole. In 1992, when ozone depletion was especially severe, UV-B (in the range 298-303 nanometres) at the South Pole was four times higher than in 1991. Surrounding regions have also been affected, because when the polar vortex breaks down in the spring, large quantities of ozone-depleted air drift toward lower latitudes.

At a measurement station in southern Argentina, biologically weighted levels of UV (a measure taking into account the greater damage caused by shorter wavelengths) were 45 per cent higher in December 1991 than is usual at this latitude. The increase was equivalent to moving the site 20 per cent closer to the equator.

Based on simulation models, peak levels of biologically weighted UV-B reaching the Earth due to ozone depletion are expected to be significantly higher than measured to date. Relative to 1960, estimated maximum increases for erythema induction and DNA damage at mid-latitudes are shown in the table below. As with the estimates of maximum ozone depletion given above, the figures are subject to uncertainty; and they assume full compliance from all parties in the global effort to phase out ozone-depleting substances.

How does UV radiation affect human skin?

One of the most obvious effects of UV-B radiation is sunburn, known technically as erythema. Dark-skinned people are protected from most of this effect by pigment in their skin cells. UV-B can also dam-age the genetic material in skin cells, which can cause cancers. For fair-skinned people, life-long exposure to high levels of UV-B increases the risk of non-melanoma skin cancers. Researchers have suggested that these kinds of skin cancers are likely to increase by 2 per cent for each 1 per cent decrease of stratospheric ozone. There is also some evidence that increased exposure to UV-B, especially in childhood, can increase the risk of developing more dangerous melanoma skin cancers.

How does UV radiation affect the eye?

In humans, exposure to UV-B from unusual directions can cause snow blindness-actinic keratitis-a painful acute inflammation of the cornea. Chronic exposure can also damage the eye. Enhanced levels of UV-B could lead to more people suffering from cataracts-a clouding of the lens that impairs vision. Cataracts are a leading cause of blindness, even though they can be effectively treated through surgery in regions well provided with medical care.

How does UV radiation affect the body's defences against disease?

Exposure to UV-B can suppress immune responses in humans and animals. Increased UV-B could therefore reduce human resistance to a number of diseases, including cancers, allergies and some infectious diseases. In areas of the world where infectious diseases are already a major problem, the added stress from increased UV-B could be significant. This is especially true for diseases, such as leishmaniasis, malaria and herpes, against which the body's major defence is in the skin. Exposure to UV-B can also affect the body's ability to respond to vaccinations against diseases.

The effects of UV-B on the immune system are not dependent on skin color. Dark-skinned and fair-skinned people are equally at risk.

What effect does UV radiation have on plants?

Many species and varieties of plants are sensitive to UV-B, even at present-day levels. Increased exposure could have complex direct and indirect effects, both on crops and natural ecosystems. Experiments have shown that increased exposure to UV-B of crops such as rice and soy beans results in smaller plants and lower yields. Increased UV-B could alter crop plants chemically, potentially reducing nutritional value or increasing toxicity. If further ozone depletion is not prevented, we will have to search for UV-B tolerant crop varieties or breed new ones.

The implications for natural ecosystems are difficult to predict, but could be significant. UV-B has a number of indirect effects on plants, such as altering plant form, biomass allocation to parts of the plant and production of chemicals that prevent insect attack. Increased UV-B could therefore lead to ecosystem-level effects, such as changes in the competitive balance between plants, animals that eat them and plant pathogens and pests.

What are the effects on marine and aquatic life?

Experiments have shown that increased UV-B harms phytoplankton, zooplankton, juvenile fish and larval crabs and shrimps. Harming these small organisms could threaten the productivity of fisheries. More than 30 per cent of animal protein consumed by humans comes from the sea, and in many developing countries the share is higher. In Antarctic seas, plankton production has already been reduced under the annual ozone hole.

Marine life also plays an important role in global climate because phyto-plankton absorb vast quantities of carbon dioxide, the main greenhouse gas. A decrease in phytoplankton production could leave more carbon dioxide in the atmosphere, contributing to global warming.