BackBackNextNext
Ozone
Ozone (O3) There has been some confusion on the ozone.
For some time especially mass media mistakenly promoted the view that the depletion of the ozone layer in the stratosphere led to the greenhouse effect.
In the science of ecology everything is related. This is also the case with ozone and the greenhouse effect.
However, the ozone depletion and the greenhouse effect are two clearly separate issues, and must be treated that way.
  1. Ozone (O3)
    The ozone layer acts like a giant sunshade, protecting plants and animals from much of the sun's harmful ultraviolet radiation.
    Ozone forms a layer in the stratosphere, 15-40 km above earth surface.
    If the ozone in the atmosphere from ground level to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 mm thick.

    Global stratospheric ozone levels have declined, which means that the ozone layer is changing.
    Stratospheric ozone has large natural variations, up to 30 percent variation may be regarded as normal.
    However, there has been a significant thinning of the ozone layer during spring and summer the last decades.
    This is observed in both the northern and the southern hemispheres at middle and high latitudes.

    A depletion of the ozone layer will increase the UV-radiation at ground level, in the troposphere.
    Increasing doses of UV-B may cause skin cancer, eye cataracts, damage to the immune system in animals as well as human beings, and have an adverse impact on plant growth.

    The graph depicts an illustration of an ozone molecule in the stratosphere
    (Illustration: "Action on Ozone". UNEP 1993)
  1. Ozone depleting substances - CFCs and halon
    Ozone forms a layer in the stratosphere that absorbs dangerous solar ultraviolet radiation.
    Increasing amounts of man-made chemicals are accelerating the speed at which ozone molecules are destroyed.
    These ozone-destroying chemicals come mainly from chlorofluorocarbons (CFCs) used in a range of products from refrigerators and air conditioners to soft foam and cleaning solvents; and from halons, used for fire-fighting.
    CFCs and halons remain in the atmosphere for decades - in some cases for more than a hundred years.

    Illustration:
    Ozone is depleted by chlorine (From: "Action on Ozone". UNEP 1993)

  1. Tropospheric vs stratospheric ozone
    Ozone close to the Earth's surface is also called tropospheric ozone, and is a strong greenhouse gas.
    This corroding gas may arise through photochemical reaction in polluted air (smog).
    This gas is chemically reactive and is damaging to health, vegetation and other material.
    Tropospheric ozone is as a rule a result of human activity.

    Ozone at high altitude - stratospheric ozone - reflects sunlight back into space, and has a negative forcing effect.
    Ozone in the stratosphere acts as "sunglasses", filtering away the dangerous ultraviolet light rays from the sun.
    Ozone has this peculiar quality of having negative as well as positive radiative forcing effect depending on where in the atmosphere it is located

    Biomass burning, pollutants from vehicles and industry in combination with sunlight in photochemical processes produce smog, of which O3 is one substance.

    Pre-industrial tropospheric global mean ozone levels are estimated to 25 Dobson Units (DU). The total amount of O3 in the troposphere is estimated to have increased to 34 DU or 36% since 1750, due primarily to anthropogenic emissions.
    An ozone-hole is defined as an area in the stratosphere with less than 150 Dobson units

    Note!
    A common misunderstanding is to confuse the issue of the Greenhouse effect with the issue of the Ozone hole.
    These issues must not be confused.
    The Ozone hole issue is treated under a completely different international convention called the Vienna Convention and its Montreal Protocol.

    When the layer of stratospheric ozone is thinning, more ultraviolet light of the types UVB and UVC may pass through the ozone filter.
    Ultraviolet light may harm eyes, result in more skin cancer and stifle plant growth, but have little direct implications on global warming.

    It seems that the stratospheric ozone layer has stabilised, and that it may start 'healing' from around 2015.
    The Montreal Protocol is therefore often used as a successful example of international cooperation on an environmental issue.

  1. Impacts of ozone depletion
    Increasing amounts of UV radiation will have an impact on plankton and other tiny organisms at the base of the marine food web.
    These organisms provide the original food source for all other living organisms in the oceans.
    Plankton- phytoplankton as well as zooplankton are highly sensitive to UV radiation, as they lack the protective UV-B-absorbing layers that higher forms of plants and animals have. (Phyto = plant. Zoo = animal).

    More UV-B radiation reduces the amount of food phytoplankton create through photosynthesis.
    Zooplankton, feeding off the phytoplankton, are also affected.
    UV-B also damages small fish, shrimp and crab larvae.
    It has been estimated that on shallow coastal shelves, a 16 percent reduction of the ozone layer would kill more than 50 percent of e.g. anchovy larvae, and cause a 5 percent drop in plankton numbers and a 6-9 percent drop in fish yield.

    Global warming
    Ozone reflects light in the upper part of the stratosphere, and thereby has a cooling effect.
    However, ozone in the troposphere acts as a greenhouse gas, and has a direct warming effect.
    In addition, ozone-layer depletion seems likely to increase the rate of greenhouse warming to some extent, by reducing the effectiveness of the carbon dioxide sink in the oceans: Phytoplankton in the oceans assimilate large amounts of atmospheric carbon dioxide.

    Increased UV radiation will reduce phytoplankton activity.
    Phytoplankton is an important sink of carbondioxide, and may be stunted or even destroyed by more UV rays.
    However, the main production of plankton takes place in the temperate and polar zones at spring time.
    In those regions and at that time, the sun is fairly low on the horizon, and the solar rays will not penetrate deep at that angle.

    The ocean sink is crucial for the balance of greenhouse gases in the atmosphere. Theoretically, a 10 percent decrease in carbon dioxide uptake by the oceans would leave about the same amount of carbon dioxide in the atmosphere as is produced by fossil fuel burning.

    Note: The oceans absorb much of the carbondioxide directly from the air, through e.g. the thermo-haline circulation.
    Several species of calcium-encrusting zoo-plankton and corals also absorb carbondioxide.
    The phytoplankton is just one factor of many in the marine ecosystems.

    Sources: UNEP/GEMS Library series no 7: The Impact of Ozone-Layer Depletion.

    To distinguish between the concepts of global warming and ozone layer depletion, you can say that ozone layer depletion allows more UV-A and UV-B rays to penetrate the atmosphere and increase the amount of UV rays hitting ground level.
    The anthropogenic global warming issue increases the number of greenhouse gas molecules in the atmosphere, preventing infrared rays from escaping from ground level out through the atmosphere to the universe. In other words, opposite direction and different rays.

    Ozone hole 'changes Southern Hemisphere weather'

    Climate Change and Your Health: Rising Temperatures, Worsening Ozone Pollution (Union of concerned scientists)


  1. Impact on land plants
    A high increase in UV radiation may disrupt many ecosystems on land.
    Rice production may be drastically reduced by the effects of UV-B on the nitrogen assimilating activities of micro-organisms.
    With a diminishing ozone layer, it is likely that the supply of natural nitrogen to ecosystems, such as tropical rice paddies, will be significantly reduced.
    Most plants (and trees) grow more slowly and become smaller and more stunted as adult plants when exposed to large amounts of UV-B. Increased UV-B inhibits pollen germination.
    Stunted growth on land plants may also have an indirect forcing effect on the climate.

    The figure illustrates how exposure of some plants to continuous daylight and UV-B radiation can inhibit flowering, resulting in the similar stunted growth shown by those exposed only to limited light. From: "Action on Ozone". UNEP 1993)

  1. Increased air pollution
    UV-B stimulates the formation of reactive radicals - molecules that react rapidly with other chemicals, forming new substances.
    The hydroxyl radicals, for example, stimulate the creation of tropospheric ozone and other harmful pollutants.
    Smog formation creates other oxidized organic chemicals, such as formaldehydes.
    These molecules can also produce reactive hydrogen radicals when they absorb UV-B.
    In urban areas, a 10 percent reduction of the ozone layer is likely to result in a 10-25 percent increase in tropospheric ozone.
    More UV-B radiation seems likely to cause global increases in atmospheric hydrogen peroxide.
    This is the principal chemical that oxidizes sulfur dioxide to form sulfuric acid in cloud water, making it an important part of acid rain formation.
BackBackNextNext