Heat Budget of Earth

Notes

The Earth neither warms nor cools in the long run, because the energy received equals the energy lost: this is the heat budget (heat balance). Take the insolation at the top of the atmosphere as 100 units.

Incoming (100 units)

35 units are reflected back to space (the albedo): 6 scattered by the atmosphere, 27 reflected by clouds, 2 by snow and ice.
14 units are absorbed by the atmosphere (ozone, water vapour, dust).
51 units are absorbed by the Earth's surface (land and sea).

Outgoing

The Earth radiates the 51 units back as long-wave radiation: 17 units go directly to space, and 34 units are absorbed by the atmosphere (6 by direct absorption, 9 by convection and turbulence and 19 by the latent heat of condensation).
The atmosphere radiates 48 units (14 + 34) to space.
Total return to space = 35 + 17 + 48 = 100 units. The Earth's surface balance (51 in, 51 out) and the atmosphere's balance (48 in, 48 out) both hold.

Latitudinal heat balance

There is a surplus between about 40°N and 40°S and a deficit towards the poles. Heat is carried poleward by winds and ocean currents, which prevents the tropics from heating up and the poles from freezing further.

Greenhouse effect

Greenhouse gases (water vapour, CO₂, methane, nitrous oxide, CFCs) absorb long-wave radiation and re-emit it. This natural effect keeps the mean temperature at about 15°C instead of about -18°C.
Enhanced greenhouse effect from human emissions causes global warming.

Temperature distribution

Vertical: temperature falls with height (lapse rate); a temperature inversion (temperature rising with height) occurs on calm, clear winter nights in valleys.
Horizontal: shown by isotherms (lines joining places of equal temperature), which bend at coasts because land and sea heat unequally.

Key Points

100 units in: 35 reflected, 14 absorbed by the atmosphere, 51 absorbed by the surface.
Surface returns 17 directly and 34 via the atmosphere; the atmosphere radiates 48.
Surplus at 40°N-40°S, deficit at the poles; heat moved by winds and currents.
Greenhouse gases keep the Earth at about 15°C.
Albedo of the Earth is about 35%.

MCQs

Q1. Out of 100 units of insolation, the number of units reflected back to space is:

(a) 14
(b) 35
(c) 51
(d) 48
Ans: (b).

Q2. The Earth's surface absorbs how many of the 100 units of insolation?

(a) 35
(b) 14
(c) 51
(d) 100
Ans: (c).

Q3. The tropics have a heat surplus, and the poles a deficit; heat is transferred poleward by:

(a) Earthquakes
(b) Winds and ocean currents
(c) Volcanoes
(d) Tides only
Ans: (b).