6.1 Introduction to the atmosphereIB Environmental Systems and Societies SL: Revision notes

The atmosphere is the layer of gases held around the Earth by gravity. It forms the boundary between Earth and space and is the outer limit of the biosphere.

Its composition (about 78% nitrogen, 21% oxygen, 0.9% argon and about 0.04% carbon dioxide, plus variable water vapour) and its processes support life: oxygen for respiration, carbon dioxide for photosynthesis, a temperature range suitable for liquid water and, in the stratosphere, protection from ultraviolet radiation.

The equator is heated more than the poles because the Sun's rays hit it at a steeper angle (energy is concentrated over a smaller area) and pass through less atmosphere. This differential heating drives the tricellular model, which redistributes heat from the equator towards the poles.

• Hadley cell (0° to 30°): warm air rises at the equator (the ITCZ), cools and gives heavy rain, moves away at height, then sinks at about 30° N and S, warming and drying, which produces many of the world's deserts.
• Ferrel cell (30° to 60°): air moves at the surface towards the poles and rises at about 60°, giving rain.
• Polar cell (60° to 90°): cold air sinks at the poles and flows towards 60° at the surface.

Rising air means low pressure and rain; sinking air means high pressure and dry conditions.

Greenhouse gases (GHGs) absorb and re-emit some of the infrared (long-wave) radiation emitted from the Earth's surface. They include water vapour, carbon dioxide, methane and nitrous oxide (N2O).

Black carbon is an aerosol, made of tiny solid particles of soot from incomplete burning of wood, diesel and other fuels. It absorbs sunlight and infrared radiation in the air and, when deposited on snow and ice, lowers albedo so more energy is absorbed and melting speeds up.

The Sun emits a broad spectrum of radiation, mainly visible light with some ultraviolet and infrared. The atmosphere is largely transparent to it, so it reaches and warms the surface. The warm surface emits infrared radiation. GHGs absorb part of it and re-radiate it in all directions, including back to the surface.

This is the natural greenhouse effect. It keeps the Earth's mean surface temperature at about 15 °C, compared with about -18 °C with no atmosphere, a difference of about 33 °C. It is essential for life.

Venus (about 96% CO2, about 465 °C) shows what a very dense greenhouse atmosphere does; Mars (thin atmosphere, about -63 °C) shows the opposite.