The Ozone Layer: The stratosphere contains a thin layer of ozone molecules (with three oxygen atoms) which forms a protective layer shielding life on Earth from the Sun’s harmful ultraviolet radiation. But this ozone layer is being depleted, and is getting thinner over Europe, Asia, North American and Antarctica. "Holes" are appearing in the ozone layer.
The Tropopause: At the very top of the troposphere
The Greenhouse Effect: Heat from the Sun warms the Earth's surface but most of it is radiated and sent back into space.