Ozone is a natural sunblock

Go outside on a fine day and feel the sun warm your face. What happens when a cloud passes over? You’ll notice that the cloud takes away some of the heat and light coming from the sun. In much the same way that a cloud blocks the heat on a hot day, the ozone layer in the stratosphere blocks out the sun’s deadly ultraviolet rays. It acts as our planet’s natural sunblock.

The sun doesn’t just produce heat and light. It throws out all sorts of other types of electromagnetic radiation, including ultraviolet radiation. Because ultraviolet radiation can damage DNA it is potentially harmful to most living things, including plants.

• The ultraviolet family and sunburnt plants

  Meet the ultraviolet family

  Electromagnetic radiation is divided into different types according to its wavelength. Visible light is just a small part of the whole spectrum. Ultraviolet (UV) radiation, as you can tell from its name, lies beyond the violet end of visible light and has shorter wavelengths.

  Ultraviolet is normally used to describe radiation with wavelengths between about 100 and 400 nanometres. (A nanometre is one-millionth of a millimetre.) As with all electromagnetic radiation, the shorter the wavelength the greater the energy carried.

  The ultraviolet family can be divided into three parts:

  • UV-A (315–400 nanometres) has the longest wavelengths of the family and is the least damaging. However, it does cause sunburn and has been implicated in causing sun-induced premature ageing of skin and some cancers. This is the form of ultraviolet produced in most solariums.
  • UV-B (280–315 nanometres) can cause skin cancer and eye damage. It also causes sunburn. Radiation with a wavelength close to 280 nanometres is strongly absorbed by proteins, altering and often damaging their function. In this way, UV-B can reduce the immune response and it also interferes with photosynthesis in some crop plants. A very small amount of exposure to UV-B is necessary to produce vitamin D in human skin.
  • UV-C (100–280 nanometres) is the most dangerous member of the family. Wavelengths around 260 nanometres are absorbed by DNA and so nearly all life forms are irreparably damaged by this radiation.

  The good news is that the stratospheric ozone layer absorbs all UV-C, the most deadly form, and even a thinned ozone layer is unlikely to let much through.

  The intact ozone layer does, however, let through some UV-A, especially when the sun is high in the sky, and a very small amount of UV-B. The proportion of both of these reaching ground level will increase with ozone loss.

  Many species have some natural protection against UV-A. For example, we can produce melanin, a dark pigment, in the outer layer of our skin. However, pale-skinned people can't produce enough melanin to protect against the amount of ultraviolet radiation that occurs across all of Australia for most of the year (even on cloudy days). Even dark-skinned people, who naturally have high melanin concentrations in their skin, can suffer sunburn after long periods of exposure.

  Can plants get sunburn?

  Plants are always exposed to UV-A and have mechanisms for coping with UV-induced damage. But high levels of UV-B—far higher than are occurring anywhere at the moment—have been shown to cause great damage. The main effect is on the photosynthetic apparatus—the pigments and enzymes that absorb light and use its energy to process carbon dioxide into sugar.

  Tests have shown that plants vary in their sensitivity to UV-B. Most species tested so far have been crops. In experiments subjecting rice plants to a 33 per cent increase in UV, the plants were visibly damaged and the yield of rice-grain fell by 20 per cent. A 33 per cent increase in UV-B at mid-latitudes is not considered likely to occur. However, a 20 per cent increase is a possibility.

  Some rice varieties are more resistant to UV than others. As might be expected, these have been grown in high-altitude areas for generations—where the thinner atmosphere ensures naturally higher UV levels. It is quite possible that non-domesticated, wild relatives of food crops may contain valuable genes coding for UV resistance or for repair mechanisms for UV-induced damage. Also, many plants can produce UV-absorbing compounds; and increased exposure to UV stimulates greater production, up to a point. Just as different races of humans can produce different quantities of protective melanin, so plants differ.

  In the sea, phytoplankton are also at risk. Phytoplankton are important because they remove carbon dioxide from the air. They are also at the base of many marine food chains.

Unfortunately our bodies can't detect ultraviolet radiation directly. We can be unaware of the harm it is doing until it is too late—for example, at the end of a day in the sun without adequate protection.

When there is less ozone in the stratosphere, more ultraviolet radiation hits us.

Even a 1 per cent reduction in the amount of ozone in the upper atmosphere causes a measurable increase in the ultraviolet radiation that reaches the Earth's surface. If there was no ozone at all, the amount of ultraviolet radiation reaching us would be catastrophically high. All living things would suffer radiation burns, unless they were underground, in protective suits, or in the sea.

What exactly is ozone?

Ozone is a form of oxygen. Each ozone molecule is made of three oxygen atoms, so its chemical formula is O₃. But unlike oxygen, ozone is a poisonous gas, and an increase in its concentration at ground level is not something that we want. But in the stratosphere, where ozone exists naturally, it blocks out the sun's ultraviolet rays and is a life-saver.

Ozone-depleting substances usually contain chlorine or bromine.

The synthetic chemicals called chlorofluorocarbons (CFCs) are now well-known as environmental ‘baddies’, even though they are useful and completely non-toxic substances. They get their bad name because they are ozone-eaters (properly called ozone-depleting substances). CFCs are not the only ozone-depleting substances, but they are the most abundant. Some ozone-depleting substances are naturally occurring compounds.

Ozone-depleting substances are generally very long-lived, and it takes several years for them to to drift up into the stratosphere. When they arrive, they are broken apart by exposure to ultraviolet radiation and that releases the chlorine atoms. These are the real ozone-killers. The chlorine atoms react with ozone, to form oxygen and chlorine monoxide.

Ozone depletion

The ozone-destroying reactions take place most rapidly only under certain conditions in the stratosphere. These conditions—extreme cold, darkness and isolation, followed by exposure to light—occur over the polar regions after the long polar winter has finished and the first spring sun appears.

Antarctica is the worst affected area, probably because the air above it is most isolated from the rest of the atmosphere. Scientists often refer to the part of the atmosphere where ozone is most depleted as the ‘ozone hole’, but it is not really a hole—just a vast region of the upper atmosphere where there is less ozone than elsewhere. At its most extreme, the 'ozone hole' contains 60 per cent less ozone than normal.

• How ozone is lost

  Ozone loss is most severe in the world’s coldest regions because of clouds in the stratosphere known as polar stratospheric clouds. They provide a surface on which the chemical reactions that result in the destruction of ozone can take place.

  The final link in the destruction of ozone requires light. During the winters in the polar regions, the sun never rises. Polar stratospheric clouds form in the cold conditions, and chemical reactions that are necessary for ozone destruction occur. But the final link in the chain does not start until the sun returns in spring. Then, the ozone is destroyed rapidly. Fortunately, the polar stratospheric clouds also disappear as the stratospheric temperature warms up. Net ozone loss then ceases, and the layer will gradually be replenished, but not quite to its former level. Then, the following spring, further depletion will occur.

  Arctic ozone loss does occur, and is worsening, but it is not as severe as that measured over Antarctica. The reason is that strong winds develop around Antarctica in the winter, isolating the atmosphere there from the rest of the world. As a result of being shut off from any warmer air from elsewhere, temperatures in the stratosphere fall so low that the formation of polar stratospheric clouds is greater than over the Arctic. (And there is no ‘fresh’ air coming in to dilute the build-up of reactants in the chain of destruction.) In September, ozone loss occurs at its greatest rate over Antarctica as the sun rises. In November, warm air from the rest of the world starts to break through and ozone-poor air moves away thus reducing the average concentration of ozone in the southern hemisphere.

  Recently, scientists have become concerned about ozone loss over other regions of the world. Although these losses are nowhere near as great at that recorded over Antarctica, they are worrying because of their possible effects on humans and our crop plants. Over North America, for example, ozone levels fell by about 0.5 per cent per year from 1978 to 1990. The details of the mechanisms causing these mid-latitude losses remain obscure. Polar stratospheric clouds don't seem to be involved. The only region where stratospheric ozone has not diminished is the tropics—and that’s just as well because levels of ultraviolet radiation are at their highest there.

  Over Australia, the average amount of stratospheric ozone has also declined, with the losses being greatest in the more southern latitudes. As a result, ultraviolet radiation at ground level has increased. Hobart, for example, has recorded increases of UV-B of about 4 to 6 per cent since 1980.

Ozone-poor air can spread out from the polar regions and move above other areas. In addition, direct ozone loss elsewhere is slowly increasing, although it is not occurring at the same rate as over the poles.

Scientists around the world regularly monitor ozone-depleting substances and the amount of ozone in the stratosphere. In Australia, the Australian Bureau of Meteorology and the CSIRO Division of Atmospheric Research jointly manage the Cape Grim Baseline Air Pollution Station. The Cape Grim station is located in remote north-western Tasmania, in the path of strong westerly winds that carry air thousands of kilometres across the Southern Ocean. Air at Cape Grim is regularly sampled in order to monitor atmospheric composition. (Another reason to monitor ozone-depleting substances is because most are also powerful greenhouse gases.)

Reversing the depletion

Banning and controlling ozone-depleting substances

Many substances other than chlorofluorocarbons are also ozone-depleting. Examples are carbon tetrachloride (used in dry cleaning), and methyl bromide (used as an insecticide for soil fumigation). An Australian scientist (Jonathan Banks) has been internationally recognised for his work in finding a replacement for methyl bromide.

• Australia finds a replacement for methyl bromide

  For years, horticulturalists have been using methyl bromide to sterilise soil. But with the knowledge that it is an ozone-depleting substance, it is becoming imperative to find alternatives.

  Unlike chlorofluorocarbons (CFCs), methyl bromide gets into the atmosphere naturally—particularly in volcanic eruptions—but human activity is probably responsible for releasing much more. Molecule for molecule, it is actually a more efficient ozone-killer than the CFCs, but it breaks down faster than they do.

  The Montreal Protocol set out controls for the use of methyl bromide, and in 1997, it was agreed that developed countries would discontue their use of methyl bromide by 2005, and developing countries to cut their production by 20 per cent of their 1995-1998 usage by 2005, with complete phase out of methyl bromide by 2015. By 2009, the global production of methyl bromide controlled under the Montreal Protocol had decreased to 13 per cent of the 1991 amount.

  Dr Jonathan Banks, a researcher in CSIRO’s Stored Grain Research Laboratory at the Division of Entomology, has been at the forefront of finding ways to avoid using methyl bromide. For example, covering an area of ground with plastic and then waiting for the sun to heat it up is quite an effective way to sterilise soil. If the temperature is carefully monitored, there’s even a bonus: it is possible to kill the harmful bugs without affecting many of the good ones!

  Fumigation of produce can also be carried out using other chemicals—for example, carbon dioxide—and a whole suite of different methods can be used to manage pest infestations.

  In recognition of his work, Dr Banks received the 1996 Stratospheric Ozone Protection Award from the US Environmental Protection Agency.

CFCs, previously used as refrigerants, foam-blowing agents and propellants in spray cans, are now banned in Australia (and many other countries). Their temporary replacements, the hydrochlorofluorocarbons, are still slightly ozone-depleting, though not to the same extent. HCFCs are also being phased out.

An international agreement to address the problem of ozone depletion was devised in 1987. Called the Montreal Protocol, this agreement limits the production and use of ozone-depleting substances. By 2009, the Montreal Protocol had been agreed to by all United Nation (UN) member states, making it the most widely ratified treaty in UN history. It was agreed that all CFC production world wide would be stopped by 1 January 2010. By September 2011, nearly 100 ozone depleting substances (amounting to 97 per cent of the substances controlled by the Montreal Protocol) had been phased out.

A slowing down in the rate of ozone loss has been measured, and the concentration of CFCs in the atmosphere is slowly levelling off. But because of the long-lived nature of the ozone-depleting substances, they will hang around and continue doing their nasty work for a long time after their actual production has stopped. It is estimated that it will take decades to reverse the problem, though scientists are hopeful that the biggest holes in the ozone layer will recover by around 2040.