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

The atmosphere 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, 0.04% carbon dioxide and variable water vapour) and its processes support life.

Differential heating (steeper Sun angle and shorter atmospheric path at the equator) drives the tricellular model, which redistributes heat from the equator towards the poles.

• Hadley cell: air rises at the equator (ITCZ), giving heavy rain, e.g. Manaus, and sinks at about 30° N and S, giving dry conditions, e.g. the Atacama Desert.
• Ferrel cell: rises at about 60°, giving rain.
• Polar cell: sinks at the poles.

GHGs and aerosols absorb and re-emit some of the infrared (long-wave) radiation emitted by the surface. They include water vapour, carbon dioxide, methane and nitrous oxide, and black carbon (aerosol).

The greenhouse effect keeps the Earth about 33 °C warmer than it would be without an atmosphere: the broad spectrum of solar radiation warms the surface, which emits infrared radiation that GHGs trap and re-radiate.

Global warming potential (GWP) compares gases tonne for tonne with CO2 over 100 years: methane about 28, nitrous oxide about 265.

(HL) The atmosphere is a dynamic system: its layers and components are the result of continuous physical and chemical processes, for example evaporation and condensation, circulation, and the formation and destruction of ozone.

(HL) Gravity pulls gas molecules towards the surface. Because gravitational force is inversely proportional to distance, the atmosphere thins as altitude increases: pressure falls to about half at 5.5 km and about one third at the summit of Everest, while the percentage of oxygen stays about 21%.

(HL) Milankovitch cycles are regular changes in Earth's orbit that change how much solar radiation reaches the Earth and so cause climate cycles over tens to hundreds of thousands of years:

• Eccentricity: shape of the orbit, about 100 000 years.
• Obliquity: tilt of the axis, about 41 000 years.
• Precession: wobble of the axis, about 21 000 to 23 000 years.

The Quaternary period has had repeated glacial and interglacial cycles, amplified by feedbacks (ice-albedo, CO2). (HL) Global warming is moving the Earth away from the glacial-interglacial cycle towards new, hotter conditions.

(HL) The evolution of life changed the atmosphere, and the changed atmosphere influenced the evolution of life:

• Early atmosphere: almost no free oxygen.
• Cyanobacteria photosynthesised, releasing oxygen; the Great Oxidation Event (about 2.4 billion years ago) raised oxygen levels, and photosynthesis removed carbon dioxide.
• Aerobic respiration released more energy, allowing larger, more complex organisms; many anaerobes declined.
• The ozone layer formed from oxygen, absorbing UV and allowing colonisation of land.