Ozone Layer
The ozone layer is a region of the stratosphere with a high concentration of ozone that absorbs most of the Sun's harmful ultraviolet radiation.
Definition
The ozone layer is the part of the stratosphere, roughly $15$ to $35\ \text{km}$ up, where ozone ($O_3$) reaches its highest concentration, still only a few parts per million, and it works like sunscreen for the whole planet by absorbing about $97$ to $99\%$ of the Sun's medium-frequency ultraviolet light. It forms when UV light splits oxygen molecules into atoms that recombine with other oxygen molecules, and it is destroyed through catalytic cycles in which a single chlorine or bromine atom, once released from chemicals such as CFCs, can break down thousands of ozone molecules. Column ozone is measured in Dobson units, with a global average near $300$ DU, and the Antarctic ozone hole, defined as column ozone below $220$ DU, forms each September and October when polar stratospheric clouds convert stored chlorine compounds into forms that sunlight can activate.
Example
Without the ozone layer, a few minutes outdoors would cause a bad sunburn and most land life could not survive, yet if all the ozone in the layer were compressed to sea-level pressure it would form a layer only about $3$ millimeters thick. The ozone hole reached a record area of about $29$ million $\text{km}^2$ in 2006, and because CFCs persist in the atmosphere for $50$ to $100$ years, the layer's recovery lags emissions cuts by decades even under the 1987 Montreal Protocol.
Key Insight
The ozone hole and global warming are different problems, since ozone loss lets in more ultraviolet light while greenhouse gases trap heat, so solving one does not solve the other. The ozone layer is also younger than life itself: it could only form once photosynthesis had filled the atmosphere with oxygen, and its appearance around $600$ million years ago is one reason life was eventually able to move from the sea onto land.