IB Geography 2.1.1 the Atmospheric System and Energy Balance Notes

The Atmosphere as a System: Energy in Must Match Energy Out

1. The atmosphere behaves as a system, which means it can be described by its inputs, stores, transfers and outputs.
   1. The only significant input is solar radiation.
   2. The stores are heat held in the air, the oceans and the ground.
   3. The output is radiation lost to space.
2. The system is in dynamic equilibrium when the energy leaving the top of the atmosphere equals the energy arriving there.
   1. Change either side of that equation and the planet's temperature shifts until the two match again.
   2. A larger imbalance means a larger shift before the new balance is reached.

Layers of the Atmosphere: The Greenhouse Effect Operates in the Lowest 8 to 16 km

1. Troposphere
   1. Extends from the surface to about 8 km over the poles and 16 km over the equator.
   2. Holds around three quarters of the mass of the atmosphere and nearly all of its water vapour.
   3. Temperature falls with height at an average environmental lapse rate of 6.5°C per kilometre.
2. Stratosphere
   1. Runs from the tropopause to roughly 50 km.
   2. Temperature rises with height here because ozone between about 15 km and 35 km absorbs incoming ultraviolet radiation.
3. Mesosphere
   1. Reaches about 85 km, where temperature falls again to the coldest values found anywhere in the atmosphere.
4. Thermosphere
   1. Extends above 85 km, where the few remaining molecules absorb high-energy solar radiation.

Incoming Shortwave Radiation: 340 Watts per Square Metre, Delivered Unevenly

1. The Sun's visible surface is about 5,500°C.
   1. An object that hot radiates most strongly at a wavelength near 0.5 micrometres.
   2. This is shortwave radiation, covering ultraviolet, visible light and the near infrared.
2. A surface at the top of the atmosphere held at right angles to the Sun receives about 1,361 W/m², a value called the solar constant.
3. Earth intercepts that beam as a disc but radiates from a sphere.
   1. The sphere has four times the area of the disc, so the global average input is about 340 W/m².
4. Insolation falls away towards the poles because the same beam spreads across a larger area of ground and passes through more atmosphere on the way in.
   1. At the equator the beam arrives close to vertical, while at 60°N in December it arrives at a low angle.
   2. A low angle also means more of the beam is reflected off the surface.

The Fate of Incoming Radiation: 107 W/m² Is Reflected and 235 W/m² Is Absorbed

1. Reflected to space, 107 W/m²
   1. The atmosphere and clouds reflect 77 W/m².
   2. The surface reflects a further 30 W/m², with bright ice, snow and desert sand returning a larger share than dark surfaces.
   3. The share of incoming radiation a surface returns is its albedo.
   4. Fresh snow has an albedo near 0.9, while open ocean is below 0.1.
2. Absorbed within the atmosphere, 67 W/m²
   1. Ozone, water vapour, clouds and aerosols absorb part of the incoming shortwave radiation.
3. Absorbed at the surface, 168 W/m²
   1. Land and ocean absorb 168 W/m², which powers surface heating, evaporation and atmospheric circulation.

Every value is an energy flux in W/m², which measures power passing through each square metre.

Outgoing Longwave Radiation: The Same Energy Leaving at a Different Wavelength

1. Every object radiates energy at a wavelength set by its temperature.
   1. The hotter the object, the shorter the wavelength at which it radiates most strongly.
2. Earth's surface averages about 15°C, so it emits in the thermal infrared with a peak near 10 micrometres.
   1. That wavelength gap is what makes the greenhouse effect possible, because gases that let shortwave radiation pass through do not let longwave radiation pass through as readily.
3. The surface also loses energy without radiating it, as latent heat when water evaporates and as sensible heat when air warmed by contact with hot ground rises.
   1. Evaporation carries roughly 80 W/m² upward and sensible heat about 17 W/m².
   2. Condensation releases the latent heat higher in the troposphere.
   3. Rising air carries the sensible heat to the same place, where both can be radiated to space more easily.
4. For the planet to stay in balance, the 240 W/m² absorbed from the Sun has to leave the top of the atmosphere as longwave radiation.

The Natural Greenhouse Effect: Worth About 33°C

1. Greenhouse gases absorb outgoing longwave radiation and re-emit it in all directions, so part of it returns to the ground as back radiation.
   1. The gases do not hold on to the energy, because they re-radiate it almost at once.
   2. What they change is the rate at which the surface loses energy.
   3. A slower loss means a warmer surface at equilibrium.
2. Water vapour, carbon dioxide, methane and nitrous oxide all absorb in the thermal infrared, while nitrogen and oxygen, which together make up 99 percent of the atmosphere, do not.
   1. Schmidt and others (2010) attribute about half of the present greenhouse effect to water vapour, a quarter to clouds and a fifth to carbon dioxide.
3. Earth radiates to space at an effective temperature of -18°C, the value set by the 240 W/m² leaving the top of the atmosphere.
   1. Without greenhouse gases the surface itself would sit near that value, against an observed global mean of about 15°C.
   2. The natural greenhouse effect is therefore worth roughly 33°C.
   3. That figure assumes today's planetary albedo of 0.3, most of which comes from the same clouds that supply a quarter of the greenhouse effect.
4. The name is a poor description of the physics, because a garden greenhouse warms mainly by stopping warm air escaping rather than by absorbing longwave radiation.

The Global Energy Budget: Balanced at the Top, Unbalanced by Latitude

1. Low latitudes take in far more insolation than they radiate away.
   1. This gives an energy surplus reaching to about 35 to 40 degrees in each hemisphere.
2. Beyond that band every latitude radiates more than it receives.
   1. The resulting energy deficit deepens towards each pole.
   2. If nothing carried energy between the two zones, the tropics would keep warming and the poles would keep cooling until each reached its own separate balance.
3. Atmospheric circulation moves surplus energy poleward through the trade winds, the mid-latitude westerlies and the depressions that travel along the polar front.
4. Ocean currents carry the rest of the poleward transfer.
   1. Trenberth and Caron (2001) found ocean transport dominant only between the equator and about 17°N, with the atmosphere carrying most of the flux at higher latitudes.
5. Two places at almost the same latitude show what that transfer is worth.
   1. Bergen in Norway sits at 60°N and has a January mean of 2.6°C on the 1991 to 2020 normals.
   2. Churchill in Manitoba sits at 59°N and averages about -25°C in January on the same normals.
6. Warm southwesterly air, together with the heat the ocean releases each winter from the store it built up in summer, keeps Bergen mild.
   1. Seager and others (2002) found that heat carried by the North Atlantic Drift itself accounts for only a few degrees of a contrast this size.
   2. Latitude alone does not fix a place's temperature, and neither does the ocean current on its own.

Measuring the Budget: Satellites Weigh Radiation in Against Radiation Out

1. The CERES instruments aboard NASA's Terra and Aqua satellites have measured reflected shortwave and emitted longwave radiation at the top of the atmosphere since 2000.
   1. Separate detectors read the two wavebands, so both sides of the budget are measured directly rather than inferred from surface temperature.
2. The measurements show outgoing energy running slightly below incoming energy, so the system is not sitting at exact balance.