6.1 Introduction to the atmosphere

The atmosphere is a gaseous boundary coupled to Earth’s surface, not a solid lid; it supplies gases, pressure, energy regulation and radiation filtering.

Focus on exchanges with biosphere, hydrosphere and lithosphere: gases, particles, water and energy cross the boundary. Most weather and water vapour are in lower layers.

Evaporation moves water from ocean to atmosphere; precipitation returns it to the hydrosphere.

The atmosphere is coupled through ongoing flows and has no sharp solid edge.

Do not list functions without naming the exchange that supports life.

Unequal solar heating creates pressure and temperature contrasts; rising, sinking and rotating air redistribute energy through idealized Hadley, Ferrel and Polar cells.

Warm air expands and rises, cool air sinks, and pressure gradients move air. The cells help explain wind and rainfall belts, but they do not erase temperature differences.

More direct equatorial sunlight warms air that rises and transports energy poleward aloft before descending elsewhere.

To explain broad circulation and energy transport, not predict every local wind or storm.

The three-cell model is idealized; mountains, seasons and oceans modify real circulation.

Greenhouse gases absorb outgoing infrared radiation; aerosols are suspended particles that scatter or absorb radiation.

Water vapour, CO2, methane and nitrous oxide absorb and re-emit outgoing infrared radiation. CO2 and water vapour are abundant greenhouse gases, while methane also has a strong warming effect. Many aerosols scatter sunlight; black carbon absorbs radiation and can darken snow or ice.

CO₂ is a greenhouse gas; soot on snow is black carbon aerosol that lowers albedo. Both affect radiation, but through different classes.

Classify by physical form and radiation pathway. Water vapour is a greenhouse gas but its atmospheric abundance responds dynamically to temperature, so it is usually treated as a feedback rather than a direct mitigation target.

‘Warms’ does not identify the category; classify by physical form and mechanism.

The natural greenhouse effect makes Earth habitable; human increases in long-lived greenhouse gases enhance it and warm the climate.

Short-wave sunlight enters; the surface emits long-wave infrared; gases absorb and re-emit some energy. Extra GHGs require a warmer surface–troposphere system to restore balance. Global warming is temperature trend; climate change includes wider responses.

Adding CO₂ does not add sunlight; it slows outgoing infrared, so equilibrium is restored at a higher mean temperature.

Specify natural versus enhanced effect and the short-wave/long-wave energy pathway.

The atmosphere does not warm by trapping all heat forever; energy still leaves, but the balance shifts.

A stable concentration can result from opposing processes continuing at similar rates; it is not a static layer.

Air movement and radiative balance redistribute energy physically. UV splits O₂ and oxygen atoms combine with O₂ to form ozone; ozone is also destroyed, creating a dynamic equilibrium.

Ozone concentration may stay roughly stable while formation and destruction both continue.

A sustained change in one rate, such as extra chemical loss, without an equal compensating process.

Stable amount does not mean no reactions or no movement.

Gravity pulls atmospheric molecules toward Earth's surface. With increasing altitude there is less air above, so pressure and density decrease; oxygen remains a similar percentage of air, but its partial pressure falls.

Worked example: for a 500 m climb, ΔT ≈ -(500 m / 100 m) × 1°C = -5°C. If the starting temperature is 20°C, the standard-lapse-rate estimate is about 15°C.

The lapse rate is an approximate tropospheric temperature relationship, not a formula for gas pressure; actual temperature profiles vary with weather and atmospheric layer.

Lower oxygen partial pressure at altitude does not mean the oxygen percentage has fallen by the same amount.

Eccentricity changes orbit shape (~100,000 years), obliquity changes axial tilt (~41,000 years), and precession changes axis direction (~26,000 years).

Together they redistribute insolation by latitude and season. Ice-albedo and carbon-cycle feedbacks can amplify effects, influencing glacial–interglacial pacing.

A cooler high-latitude summer can let snow survive, increasing albedo and reinforcing cooling.

No; their timescales and current forcing do not match the recent human-driven rate.

Milankovitch cycles redistribute sunlight; they do not directly add modern greenhouse gases.

The Quaternary period began about 2.5 million years ago and contains repeated glacial–interglacial cycles; current global warming is moving Earth away from that recent pattern toward hotter conditions.

Climate has changed naturally over geological time, but current anthropogenic change is unusually rapid. Human greenhouse-gas emissions add positive forcing that the current orbital pattern does not explain; the syllabus places this human-dominated change in the Anthropocene.

Evidence of past natural cycles shows that climate can change without people, but it does not explain the rapid modern rise in greenhouse gases and temperature.

Attribution compares timescale, forcing and observed rate: long natural variability is a baseline, not an alternative explanation automatically.

‘Natural’ and ‘human’ are not mutually exclusive causes; quantify which forcing explains the observed change.

Oxygenic photosynthesis lowered CO₂ and raised O₂, enabling ozone formation, aerobic respiration and new atmospheric conditions for life.

O₂ oxidized reduced minerals such as iron and some became stratospheric ozone, filtering harmful UV. The atmosphere changed life’s opportunities, while life changed atmospheric composition.

Photosynthetic microbes released O₂; oxidized mineral bands record the change, and later ozone reduced surface UV exposure.

O₂ supports high-yield aerobic respiration; ozone separately filters UV. Keep the two mechanisms distinct.

Photosynthesis does not directly make ozone; atmospheric chemistry converts some O₂ into O₃.