Ozone layer

The creature in the sky Got sucked in a hole, now there's a hole in the sky And the ground's not cold And if the ground's not cold, everything is gonna burn We'll all take turns, I'll get mine too The ozone layer is a chunk of the atmosphere located anywhere between 15 km and 35 km above the surface of the earth. The name is a slight misnomer as the percentage of ozone (an allotropic form of oxygen) in the layer is only a few parts per million; however, over ninety percent of the free natural ozone in the atmosphere is found within this layer. It varies in thickness throughout the year and across the globe.

The most notable effect of the ozone layer is to absorb most of the high frequency ultraviolet light from the sun. This light would be very harmful to most terrestrial life if it reached the earth's surface.

Annual fluctuations occur naturally in the position, thickness and density of the layer. However, it has been affected by man-made chemicals transported into the stratosphere by air currents. Chlorofluorocarbons, which are inert and harmless in the lower atmosphere, and were used heavily for refrigeration and as propellants in aerosols, are long lasting pollutants which form radicals that catalyze the breakdown of ozone.

Ozone depletion and the "ozone hole"
In the second half of the 20th century, an ozone "hole" has developed above Antarctica each southern spring (August-December), getting larger and lasting longer each year. Technically, it's not a "hole" as such, but a dramatic thinning of the ozone layer. This was caused by chlorofluorocarbons (CFCs), compounds used in air propellants and refrigerants. As a result of this, CFCs have been largely phased out of use. While the hole continued to get larger, the rate at which it did so decreased before beginning to "heal" in 2016, largely thanks to the CFC ban - a great success for the environmental movement.

Can't do any better than quote NASA:

Global warming and the ozone layer
Because of global warming, the troposphere has been warming while the stratosphere has been cooling due to the greenhouse effect (i.e., the heat is trapped in the lower atmosphere due to increased carbon dioxide levels). This change in temperature affects the movement of air, speeding it up in some areas. Due to this, less ozone may be produced in the stratosphere, leading to a larger ozone hole.

Ozone depletion denialism
Although various hypotheses about ozone depletion already existed at the time, the first piece of research suggesting a specific link between CFCs and ozone depletion was published by Mario Molina and Sherry Rowland in 1974. Molina, Rowland, and Paul Crutzen were awarded the 1995 Nobel Prize in chemistry for their work on ozone depletion.

However, it was in the 1980s, when the ozone hole over the Antarctic was discovered, that oil and other companies responsible for aerosol emissions mounted a denialist campaign. Perennial experts for hire S. Fred Singer and the crew at the George C. Marshall Institute provided "expert" testimony before Congress, penned op-eds decrying the ozone "alarmists," and cooked up their own "research" in think tanks.

Common talking points
Ozone depletion deniers still rely mainly on these debunked myths:
 * CFCs can't reach the ozone layer. While CFCs are heavier than air, they can reach the ozone layer from the force of tropospheric winds.
 * It's the volcanoes! Volcanic eruptions cannot account for all ozone depletion.
 * Ozone depletion only occurs in Antarctica, and so cannot be anthropogenic. It's most dramatic in the Antarctic, but ozone depletion occurs all over the world.
 * Ozone depletion doesn't increase UV levels. Ozone absorbs UVB (this can be confirmed with a simple lab experiment) and numerous studies have shown a link to increased UVB levels.
 * It's caused by chlorine evaporating from salt water bodies, not CFCs. This comes from The Holes in the Ozone Scare, a 1992 book by Rogelio Maduro and Ralf Schauerhammer published by the Lyndon LaRouche organization. Chlorine is highly reactive and reacts with water in the troposphere, being "rained out" long before it reaches the ozone layer.
 * The hole was always there, so it's natural! This is a quote mine from G.M.B. Dobson's reports on his expeditions in the 1950s where he notes the discrepancy between Arctic and Antarctic ozone levels. No such "hole" was reported then.
 * But we use chlorine every day in our water and chlorine also destroys ozone and no one worries about that. This is a claim by Prof. Ian Plimer. This ignores two facts: 1) Chlorine is highly reactive and, if released at ground level, will react with water in the troposphere and be "rained out" long before it reaches the ozone layer. CFCs are unreactive, so they have no such impediment. 2) "CFCs" don't destroy ozone, it is the chlorine CFCs break down into under the extreme ultraviolet of those altitudes that destroys ozone. Plimer's use of the word "also" suggests he doesn't understand this.