Notes on 6.1 Introduction to the Atmosphere

6.1 Introduction to the Atmosphere

  • The atmosphere forms the boundary between Earth and space. It is the outer limit of the biosphere and its composition and processes support life on Earth.

6.1.1 The atmosphere forms the boundary between Earth and space

  • It is the outer limit of the biosphere and its composition and processes support life on Earth.

6.1.2 Composition of the Atmosphere

  • The atmosphere contains water vapour and aerosols such as dust, ash and soot.
  • Human activities impact atmospheric composition through altering inputs and outputs of the system.
  • Changes in the concentrations of ozone, carbon dioxide and water vapour have significant effects on ecosystems.
  • Atmospheric gas composition (major constituents and trace gases):
    • Argon: 0.93\%
    • Nitrogen: 78.08\%
    • Oxygen: 20.95\%
    • Xenon: 0.000009\%
    • Neon: 0.0018\%
    • Hydrogen: 0.00005\%
    • Helium: 0.0005\%
    • Krypton: 0.0001\%
    • Carbon dioxide: 0.038\%
  • The atmosphere also contains water vapour and aerosols in varying amounts that influence climate and air quality.
  • Source: layers of atmosphere reference (link provided in transcript).

6.1.3 GHGs and aerosols in the atmosphere

  • Greenhouse gases (GHGs) and aerosols absorb and re-emit some of the infrared (long-wave) radiation emitted from the Earth’s surface, preventing it from being radiated out into space.
  • GHGs include: water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxides (N₂O).
  • Aerosols include black carbon (soot) among others.

6.1.4 The greenhouse effect

  • The greenhouse effect keeps the Earth warmer than it otherwise would be due to the broad spectrum of the Sun’s radiation reaching the Earth’s surface and infrared radiation emitted by the warmed surface then being trapped and re-radiated by GHGs.
  • Diagrammatic/process description (as described in transcript):
    1. Sunlight passes through the atmosphere and warms the surface of the earth.
    2. Infrared radiation is given off by the Earth.
    3. Most escapes to outer Space and cools the Earth.
    4. But some infrared radiation is trapped by gases in the air, thus reducing the cooling.

6.1.5 The atmosphere is a dynamic system

  • The atmosphere is a dynamic system whose components and layers arise from continuous physical and chemical processes.
  • Physical processes include:
    • Greenhouse effect
    • Enhanced greenhouse effect (global warming)
    • Air movements and atmospheric circulation
  • Chemical processes include:
    • Ozone production
    • Other chemical reactions that affect atmospheric composition (e.g., ozone depletion/formation chemistry, oxidation processes)

6.1.6 Gravity and atmospheric thinning with altitude

  • Molecules in the atmosphere are pulled towards the Earth’s surface by gravity.
  • Since gravitational force is inversely proportional to distance, the atmosphere thins as altitude increases.
  • The gravitational force keeps the atmosphere from escaping into open space.
  • At higher altitudes, the gravitational force decreases and the air molecules are only feebly attracted towards the Earth, leading to thinning of the atmosphere with height.
  • Quantitative note (conceptual): F \propto \frac{1}{r^{2}}\quad\text{and}\quad g(h) = \frac{G ME}{(RE + h)^2} where:
    • F is gravitational force,
    • r is distance from Earth's center,
    • G is the gravitational constant,
    • M_E is Earth's mass,
    • R_E is Earth's radius,
    • h is altitude above Earth's surface.

6.1.7 Milankovitch cycles and long-term climate variation

  • Milankovitch cycles affect how much solar radiation reaches the Earth and lead to cycles in the Earth’s climate over tens to hundreds of thousands of years.
  • The cycles are driven by three key factors that alter Earth’s position relative to the Sun:
    • Eccentricity (shape of Earth’s orbit)
    • Obliquity (angle of tilt)
    • Precession (wobble of Earth's axis)
  • These factors cause significant climate variations over long timescales.
  • The timescales are typically on the order of tens to hundreds of thousands to millions of years.

6.1.8 Global warming and the Quaternary climate context

  • Global warming is moving the Earth away from the glacial–interglacial cycle that has characterized the Quaternary period, toward new, hotter climatic conditions.
  • Earth’s climate has naturally fluctuated over geological time, with significant shifts between glacial (cold) and interglacial (warm) periods.
  • The current geological era, the Quaternary, began approximately 2.5\times 10^{6} years ago and has been marked by these regular cycles.
  • However, human activities are driving global warming, pushing Earth away from natural glacial–interglacial cycles toward a warmer climate state.

6.1.9 Evolution of life and atmospheric composition

  • The evolution of life on Earth changed the composition of the atmosphere, which in turn influences the evolution of life on Earth.

6.1.10 References / Work cited

  • Britto, Virginia, et al. Environmental Systems and Societies for the IB Diploma. Hodder Education, 2024.
  • “ESS Subtopic 6.1: Introduction to the Atmosphere.” AMAZING WORLD OF SCIENCE WITH MR. GREEN, www.mrgscience.com/ess-subtopic-61-introduction- to-the-atmosphere.html.
  • Williams, Gillian, and Jill Rutherford. Oxford Resources for IB DP Environmental Systems and Societies: Course eBook. Oxford UP - Children, 2024.

SL Topics end here