The ozone hole and global warming
ENVIRONMENTAL SCIENCE PUBLISHED FOR EVERYBODY ROUND THE EARTH
If environmental problems are discussed, people tend to link the ozone hole and global warming together. The fact is that the ozone hole is not a direct consequence of global warming and global warming is not a direct consequence of the ozone hole. But since nearly everything in the climate system is related there are, however, links between the two effects.
The big misunderstanding
Since research into the climate system is rather new, many people never learnt about it in school. So it's not really suprising that the two major environmental challenges currently affecting the Earth, global warming and the ozone hole, are often mixed up.
The fact that more and more cars circulate on our planet and burn petrol and that more and more energy from fossil fuels is consumed by industry and households is not the reason why the ozone hole forms. Likewise, the formation of the ozone hole does not lead to global warming.
However, both result from the impact of human activity on the atmosphere and there are indirect links between the two phenonmena.
1. There is no direct link between the formation of the ozone hole and the greenhouse effect. Emissions from fossil fuel burning (CO2) lead to the greenhouse effect but not to the ozone hole.
Image by Elmar Uherek.
Where are these links?
Global warming is a phenomenon that mainly affects human life and causes the troposphere to warm. The ozone hole forms in the stratosphere. By reducing the amount of ozone at altitudes of between 15 and 40 km, the ozone hole allows more harmful ultra-violet radiation to reach the Earth.
Chlorofluorocarbons (CFC's) play a role in both global warming and ozone hole formation. In the troposphere, they act as greenhouse gases. They absorb infra-red radiation coming from the surface of the Earth and, by trapping this heat close to the Earth they contribute to global warming. In the stratosphere they are broken down by high intensity ultra-violet radiation from the Sun into chlorine radicals and these have the ability destroy ozone. Other greenhouse gases, such as carbon dioxide and methane, do not have a comparable role in ozone depletion.
2. The radiative forcing diagram shows to what extent factors favour (positive columns) or counteract (negative columns) the greenhouse effect. The two columns on the left show that halocarbons are greenhouse gases and contribute to the anthropogenic greenhouse effect.
Stratospheric ozone loss however slightly counteracts the greenhouse effect and leads to a cooling.
Source: IPCC TAR 2001.
Since ozone prevents high intensity ultra-violet radiation from reaching the surface of the Earth and causes stratospheric warming, it can be assumed that formation of the ozone hole changes the total radiation budget of the Earth. This is, indeed, the case. However, ozone depletion and the formation of the polar ozone holes doesn't lead to a further warming of the troposphere, but to a slight cooling.
The impact of the ozone hole on the Earth's radiation budget
It is not directly obvious that the ozone hole should cause so called 'negative radiative forcing', i.e. lead to a cooling of the troposphere. Instinctively we think, as the ozone layer becomes thinner, more high energy ultra-violet radiation reaches the Earth's surface. This means more energy from the Sun. This is certainly true and is the main reason for the increasing risk of skin cancer. However, there is an effect which counteracts this.
3. a) An intact ozone layer holds back most of the solar ultra-violet radiation and transfers its energy into heat radiation.
As we know, absorption of ultra-violet radiation by ozone molecules causes warming in the stratosphere. Some of this heat emitted in the stratosphere is transferred to the troposphere causing slight tropospheric warming as well. In addition, in the lower stratosphere, ozone can still act as a greenhouse gas and absorb infra-red radiation coming from the Earth's surface. So absorption of both ultra-violet and infra-red radiation by ozone leads to a warming of the upper troposphere. If ozone levels decrease, the upper troposphere will, therefore, get cooler.
3. b) The depleted ozone layer (ozone molecules in both pictures are shown in blue) absorbs only a small fraction of the ultra-violet radiation from the sun.
Less of this radiation is converted to heat. More sunlight reaches the Earth but a large fraction is scattered back into space, particularly from the white surface of the Antarctic continent. Only a small fraction is transferred into infrared radiation.
Images by Elmar Uherek.
But there is also an opposite effect. Less absorption of ultra-violet radiation by ozone means more light and, therefore, more energy from the Sun reaches the ground. If this energy is converted to heat, the troposphere should get warmer. However, a proportion of the solar radiation which does enter the troposphere is simply reflected (backscattered) into space and the energy is lost.
Backscattering of solar radiation is particularly strong over the Antarctic where the strongest ozone depletion occurs. This is because the snow and ice covered ground has a very high albedo (between 0.6 and 0.8 - which means that between 60% and 80% of the radiation that hits the ground is scattered directly back into space). Because of this high backscattering, only a small fraction of the extra ultra-violet radiation that enters the troposphere from ozone loss causes heating.
Overall, the cooling effect of ozone loss is the highest and decreases in ozone levels cause cooling not only in the stratosphere but also slight cooling in the troposphere.
The impact of global warming on the ozone hole
The next section on stratospheric cooling shows how global warming in the troposphere causes stratospheric cooling. This cooling promotes ozone depletion in the Southern Hemisphere and perhaps also in the Northern Hemisphere.
About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz, Germany
scientific reviewer: Dr. Christoph Bruhl - Max Planck Institute for Chemistry, Mainz
educational proofreading: Michael Seesing - Uni Duisburg - 2003-08-07
last published: 2003-05-11