The greenhouse effect appendix B: Ozone

2022 August 15
the greenhouse effect
  1. Part 1: What is the greenhouse effect? An accessible, scientific introduction
  2. Appendix A: What is the atmosphere?
  3. Appendix B: Ozone
  4. Part 2: Physics of light and temperature
  5. Part 3: Temperature of the Earth without an atmosphere
  6. Part 4: A model of the greenhouse effect
  7. Part 5: Differences between model and reality

Ozone is a molecule with three atoms of oxygen, written O3. Ozone is a highly reactive chemical that is very damaging to the lungs in both brief and prolonged exposure: however ozone is mostly located in the ozone layer, in the stratosphere, and only rarely found near the surface.

Ozone located at the surface is mostly, though not entirely, due to man-made activity. It is formed when other dangerous pollutants such as NO and NO2 react with sunlight, and is typically associated with smog. Ozone pollution is frequently caused by car emissions and, in China and India, coal burning, but has been dramatically reduced in the West due to strict car emission standards.

The ozone in the ozone layer is both created and destroyed by reactions with sunlight. The intense ultraviolet radiation is strong enough to destroy the bond in O2, releasing free oxygen atoms, which quickly react with other O2 to form O3. Ozone is even better than O2 at absorbing ultraviolet radiation because its bonds are weaker, although it can be destroyed by the radiation in the process.

Because ozone absorbs ultraviolet radiation, the ozone layer greatly decreases the amount of ultraviolet light that reaches the surface. Therefore the ozone layer is beneficial to human health, and is speculated by some to even be necessary for complex life on land.

The location of the ozone layer is caused by a balance between intensity of sunlight and the amount of oxygen in the atmosphere. The ozone layer doesn’t go higher because the atmosphere becomes so thin that there isn’t enough oxygen for the reactions that make ozone to proceed quickly. The ozone layer doesn’t go lower because the ultraviolet light necessary to make ozone is all absorbed by the time sunlight gets that far.

The ozone hole

The ozone hole is an unnatural seasonal decrease in the amount of ozone in the ozone layer above Antarctica during spring each year. The ozone over Antarctica began to diminish around 1980, reaching one third of the natural amount by 1990; ozone levels in Antarctica have remained low since 1990, although there is some weak indication that ozone levels may have recently been increasing slowly.1The reason why the last bit of the Antarctic ozone layer was not destroyed is because the upper-most part of the Antarctic stratosphere does not have the right conditions for destroying ozone, and the stratosphere does not mix well. Ozone levels in the rest of the world have decreased by a lesser amount of around 5%.

Concern over the loss of ozone began in 1974 with the discovery that certain chemicals, called CFCs, could efficiently destroy ozone when exposed to ultraviolet radiation. CFCs are stable, non-reactive chemicals with low toxicity and a boiling point near room temperature; these properties made them well suited for use in aerosol cans, refrigerators, and fire extinguishers. Before the development of CFCs by Thomas Midgley Jr., highly toxic or explosive chemicals were often used for those purposes. Besides his research in CFCs, Midgley is best known for the development of leaded gasoline, which caused the worst man-made environmental crisis of American history (and arguably world history) by poisoning tens of millions of American children, and many more worldwide.2While the worldwide banning of leaded gasoline and the banning of leaded paint in the US and EU has greatly decreased the amount of lead in the environment, exposure to environmental lead continues to kill 140 000 people every year and contribute to 600 000 new cases of intellectual disability in children annually. However, the great stability of CFCs also meant that they could remain in the atmosphere for hundreds of years.

In 1984, on-the-ground observations in Antarctica revealed astonishingly low amounts of ozone in the stratosphere. NASA independently reported satellite measurements confirming the decline starting around 1980, although the satellite measurements were so low that they were initially believed to be a measurement error.

The announcement of the sudden destruction of the ozone layer in Antarctica brought immediate alarm, though the cause of the ozone hole remained unclear at first. It was soon confirmed that CFCs were responsible for the ozone hole through a previously unknown series of chemical reactions that were specific to the atmospheric conditions in Antarctica. In 1986 the Montreal Protocol was passed, banning CFCs globally, with the phase-out beginning in 1991. The Montreal Protocol is one of very few treaties to be ratified by every UN member and is directly responsible for averting disaster.

Today, the ozone hole poses a minor health hazard to people in and near Antarctica, and possibly a lesser contributor to skin cancer worldwide. There is not much reliable scientific data on the effects of a missing ozone layer on human health, but a study has found that a region in southern Chile experiences 50% more skin cancer than normal. (The exact shape and size of the ozone hole varies from year to year, and it can sometimes reach South America.) The ozone hole has a significant effect on the climate in Antarctica. Recall that the stratosphere is warmed by the ozone layer; in the absence of ozone, the stratosphere is cooler than usual, so wind circulation around Antarctica has strengthened significantly in response to the colder temperatures. These winds drive major ocean circulation patterns, so the stronger winds have changed the ocean circulation, leading to changes in sea ice formation, temperature, and precipitation in Antarctica. For this reason, the climate of Antarctica has been behaving anomalously for the last several decades compared to the rest of the world.

Computer models suggest that the ozone layer may return to normal levels around 2060. Without regulation of CFCs it is predicted that more than half of ozone worldwide would have been destroyed by then. The EPA estimates that the Montreal Protocol will have prevented 300 million cases of skin cancer and 2 million skin cancer deaths among Americans born before 2100.

While CFCs are greenhouse gases, their concentration is too low to contribute significantly to the greenhouse effect. Carbon dioxide does not have a significant effect on the ozone hole either directly or indirectly through climate change, and conversely the ozone hole does not contribute significantly to global climate change.

Ozone is itself a greenhouse gas, although because of its small concentration in the Earth’s atmosphere it only contributes about 5% to the greenhouse effect. Also, as ozone absorbs ultraviolet radiation it contributes an anti-greenhouse effect. The balance of these effects depends on the concentration and distribution of ozone.

Follow RSS/Atom feed for updates.