Ozone (O3) 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 temporal and spatial variations, up to 30 percent variation may be regarded as normal.
However, we now have evidence of a significant thinning of the ozone layer during spring and summer. This is observed in both the northern and the southern hemispheres at middle and high latitudes.
From 1979 to 1995 the total amount of ozone over Oslo has decreased by 4 parts per thousand yearly. (NILU 1996a) The most significant reductions have been observed during springtime, with an average yearly decrease of 7 parts per thousand.
A depletion of the ozone layer will increase the UV-radiation at ground level. 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.