The atmosphere is the envelope of gases surrounding Earth, held in place by gravity. It forms the boundary between Earth and space, supports life, and is structured into layers distinguished mainly by how temperature changes with altitude.
This is IB ESS subtopic 6.1 Introduction to the atmosphere. The atmosphere's role in supporting life is required at both SL and HL, while its dynamic structure and layers are HL-only content.
Gravity pulls atmospheric molecules toward Earth's surface. Because gravitational force weakens with distance, air pressure and density generally decrease with altitude; therefore, most atmospheric mass is concentrated near the surface.
| Layer | Structure and significance |
|---|---|
| Troposphere | Lowest layer, containing most atmospheric mass and almost all weather. Temperature generally decreases with altitude. Human emissions of greenhouse gases and air pollutants mainly enter here. |
| Stratosphere | Temperature rises with altitude because stratospheric ozone absorbs ultraviolet radiation. The ozone layer protects organisms from harmful UV radiation. |
| Mesosphere | Temperature decreases with altitude. Many meteors burn up here because of collisions with atmospheric particles. |
| Thermosphere | Temperature rises sharply as sparse gas particles absorb high-energy solar radiation. |
The atmosphere is a dynamic system: matter and energy move continually within it and between the atmosphere, biosphere, hydrosphere, cryosphere, geosphere and anthroposphere. For example, anthropogenic greenhouse gas emissions alter atmospheric composition and enhance the greenhouse effect, while the Montreal Protocol has reduced emissions of ozone-depleting substances.
A common misconception is that the ozone layer causes global warming. This is incorrect: stratospheric ozone depletion increases UV exposure, whereas the enhanced greenhouse effect results from increased absorption and re-emission of infrared radiation by greenhouse gases.
Exam technique: For an SL response, define the atmosphere and explain how it supports life. At HL, also describe the layered structure and explain why density decreases with altitude. Distinguish clearly between stratospheric ozone depletion and climate change; conflating them can lose marks.