## Energy Balances Of Large Scale

Figure 5.3 represents the annual average vertical fluxes of energy within the whole world. The three boxes represent different zones and the arrows stand for fluxes of energy between them. The energy-balance equation applies to each zone, with the total input exactly equal to the total output. Let us consider some details.

The top of the Earth's atmosphere receives an average of 342 W/m2 from the Sun (Note 2.F) and we represent this as 100 units of energy in Figure 5.3. On average, the Earth returns about 30 units of shortwave radiation (i.e. 3+27) to space by reflection, which implies that the 'global albedo' is 30 per cent (Section 2.5), as seen from a satellite.

The surface receives about half the incoming extra-terrestrial radiation, with 24 units as direct radiation, and the other 26 units in the form of scattered radiation. The scattering is due to clouds (77 per cent, Section 2.3), gases in the atmosphere (18 per cent), and either dust particles or aerosols (5 per cent). There is absorption by the atmosphere of 23 units (10050-27), due mainly to the gases (58 per cent), such as water vapour (Figure 2.2) and ozone (Section 1.4), and rather less to aerosols (28 per cent) and clouds (14 per cent).

It is evident from the numbers in Figure 5.3 that the atmosphere is a natural greenhouse (Note 2.L), because only 4 per cent of the longwave radiation emitted by the Earth's surface reaches space directly (i.e. 5/[109+5]). The 5 units emitted directly from the ground into space are mostly at a wavelength within the longwave-radiation window (Section 2.3, Note 2.H). The other 65 units of longwave radiation into space come from the atmosphere. The total of 70 units of longwave radiation plus the 30

units of reflected sunshine balance the input of 100 units, so that there is no overall warming or cooling. Likewise for the other zones in Figure 5.3.

We are particularly interested in the global energy balance at the Earth's surface, indicated by the box labelled 'ground'. The global-average surface albedo is only 6 per cent {3/(24 +26)}, which is much less than the planetary albedo discussed above. The chief reason is that oceans cover so much of the Earth (Note 1.A) and have a particularly low albedo (Section 2.5).

Net radiation to the ground (Section 2.8) amounts to 29 units (i.e. 24+26+96-3 -109-5), whilst the shortwave radiation from the sky is (24+26) or 50 units, so the radiation efficiency (Section 2.8) is 58 per cent (i.e. 29/50). The net radiation is offset by a latent-heat flux of 24 units (mainly from the oceans) and 5 units of sensible-heat flux.

Figure 5.3 shows that, on the whole, the radiation received by the ground contains almost twice as much longwave sky radiation (96 units) as shortwave solar radiation (50 units), which may seem surprising. Observe also that the terrestrial radiation (109 units) exceeds the sky radiation, since the ground is warmer (Section 2.7).

### Latitude

The distribution of vertical energy fluxes in various latitudinal belts of the Earth's surface is shown in Figure 5.4. (The evaporation figures reflect the variation shown in Table 4.2.) The greatest inputs are at latitudes around 30°, where there is least cloud (Chapters 8 and 12). Sensible and latent-heat fluxes are about equal from the world's lands, on average, but sensible heat is dominant in the dry subtropics (at 20°- 30° latitudes) and evaporative-energy transfer dominant in the humid tropics and at sea. The energy-balance equation for a latitude belt of ocean includes a term for advection, which is

Figure 5.4 Components of the energy balance at the ground in each latitudinal belt. Each bar in total represents the magnitude of the input of net radiation, divided according to the outputs of latent heat L.E and sensible heat H, and advection A in the case of oceans.

negative if there is a net input of heat from the currents (Note 5.F).

Table 5.1 shows the long-term average net radiation onto several continents and an ocean and its use in evaporation, sensible heat and advection. Net radiation is upwards at the poles (Figure 2.17) and heat convection downwards. Warming of the air by the ground is particularly strong in Australia.

Values in Table 5.1 enable a comparison of the climates of Australia and South America, which receive similar amounts of net radiation. However, the much greater rainfall in South America (Chapter 10) leads to wetter conditions and hence much more evaporation than in Australia, leaving less energy to heat the atmosphere.