Atmosphere, climate and climate change
Domain 8 - Atmosphere, climate and climate change. Questions combine atmospheric structure, the global energy budget and circulation with the physical basis of the enhanced greenhouse effect and regional climate variability.
Structure and composition
- Layers: troposphere (weather, lapse rate about 6.5 °C/km, temperature falls), stratosphere (ozone absorbs UV, temperature rises, stable), mesosphere, thermosphere, exosphere.
- Pressure falls exponentially; about half the atmospheric mass lies below 5.5 km. Composition: N2 78 %, O2 21 %, Ar 0.9 %, CO2 about 0.042 % and rising.
Circulation
- Three-cell model: Hadley, Ferrel and Polar cells; ITCZ, trade winds, westerlies, polar easterlies; the Coriolis effect deflects flow.
- Jet streams steer mid-latitude storms. Mediterranean climate: summer Azores anticyclone and subsidence; winter mid-latitude fronts; autumn DANA when cold air aloft overlies warm sea.
- Variability modes: ENSO, North Atlantic Oscillation.
Energy budget and the greenhouse effect
- Solar constant 1,361 W/m2, divided by 4 for the sphere gives about 340 W/m2; with albedo 0.30 the planet absorbs about 238 W/m2.
- Greenhouse gases: H2O, CO2, CH4, N2O, O3, halocarbons; they absorb outgoing longwave radiation, raising surface temperature about 33 °C above the effective radiating temperature.
- Forcing in W/m2; feedbacks: water vapour (positive), ice–albedo (positive), clouds (uncertain), carbon uptake by land and ocean.
- Orbital forcing (Milankovitch: 100 ka eccentricity, 41 ka obliquity, 23 ka precession) paced Pleistocene glaciations.
Contemporary change
- CO2 rose from about 280 ppm pre-industrial to over 420 ppm; global mean temperature is about 1.2 °C above pre-industrial; sea level rises through thermal expansion and ice loss; oceans acidify.
- Mediterranean projections: hotter, drier summers, heavier short-duration rainfall, increased fire and desertification risk.
Worked example
Absorbed flux 238 W/m2: σT^4 = 238 gives T = (238 / 5.67×10⁻⁸)^0.25 ≈ 255 K = −18 °C. The observed mean surface temperature is about 288 K, so the natural greenhouse effect adds roughly 33 °C. Doubling CO2 adds about 3.7 W/m2 of forcing.
Traps
- Confusing ozone depletion (chlorine chemistry in the stratosphere) with global warming.
- Mixing weather (short-term) with climate (30-year statistics).
- Claiming the greenhouse effect is entirely anthropogenic - it is natural; its enhancement is not.
- Forgetting the divide-by-4 factor in the planetary energy budget.