Introduction: Ozone Layer Depletion

The distribution of ozone in the stratosphere is a function of altitude, latitude and season. It is determined by photochemical and transport processes. The ozone layer is located between 10 and 50 km above the Earth's surface and contains 90% of all stratospheric ozone. Under normal conditions, stratospheric ozone is formed by a photochemical reaction between oxygen molecules, oxygen atoms and solar radiation.

The ozone layer is essential to life on earth, as it absorbs harmful ultraviolet-B radiation from the sun. In recent years the thickness of this layer has been decreasing, leading in extreme cases to holes in the layer. Measurements carried out in the Antarctic have shown that at certain times, more than 95% of the ozone concentrations found at altitudes of between 15 and 20 km and more than 50% of total ozone are destroyed, with reductions being most pronounced during winter and in early spring. Natural phenomena, such as sun-spots and stratospheric winds, also decrease stratospheric ozone levels, but typically not by more than 1-2%.

The main cause of ozone layer depletion is the increased stratospheric concentration of chlorine from industrially produced CFCs , halons and selected solvents. Once in the stratosphere, every chlorine atom can destroy up to 100 000 ozone molecules. The amount of damage that an agent can do to the ozone layer is expressed relative to that of CFC-11 and is called the Ozone Depletion Potential (ODP), where the ODP of CFC-11 is 1.

The lifetime of some of these ozone depleting substances is very long, and they may continue to deplete the ozone layer long after their use has been phased out. In this publication the ODP values for 100-year timespan are used. Nevertheless some shorter-lived substances may have a very high chlorine loading potential and thus their effect in the short term is much larger than reflected by their ODP value.

Aircraft emissions of nitrogen oxides and water vapour add to this depletion effect by creating ice crystals that serve as a base for ozone destroying reactions.

The main potential consequences of this ozone depletion are:

increase in UV-B radiation at ground level: a one percent loss of ozone leads to a two percent increase in UV radiation. Continuous exposure to UV radiation affects humans, animals and plants, and can lead to skin problems (ageing, cancer), depression of the immune system, and corneal cataracts (an eye disease that often leads to blindness). Increased UV radiation may also lead to a massive die-off of photoplancton (a CO ₂ "sink") and therefore to increased global warming.
disturbance of the thermal structure of the atmosphere, probably resulting in changes in atmospheric circulation;
reduction of the ozone greenhouse effect: ozone is considered to be a greenhouse gas. A depleted ozone layer may partially dampen the greenhouse effect. Therefore efforts to tackle ozone depletion may result in increased global warming.
changes in the tropospheric ozone and in the oxidising capacity of the troposphere.

International targets for the reduction of ozone depleting substances have resulted in the almost complete phasing out of CFCs, halons and carbon tetrachloride in the EU. Methyl chloroform and methyl bromide will be phased out by 2005 and HCFC by 2040.

The policy fields Ozone Layer Depletion and Climate Change are different, but closely related and indicators such as CFCs and NO _x emissions appear in both chapters. However, only the potential effects on the ozone layer will be taken into account under Ozone Layer Depletion whereas Climate Change will focus on the effects on global warming.