Global Energy Balance

At the planetary scale, the energy balance is driven by the absorption of sunlight and the emission of radiation to space. Planetary properties and the global energy balance give a first impression of the relevant processes that shape the environmental conditions at the surface and how habitable these are to life.

Planetary Energy Balance

The planetary energy balance is driven by the absorption of about 240 W m~ of solar radiation, which is then re-emitted into space as long-wave radiation. The planetary energy balance is approximately at a steady state when the amount of absorbed radiation is balanced by the emission of radiation. In this case, the planetary energy balance is

where the amount of absorbed solar radiation is expressed by the mean incident amount of solar radiation at the Earth's orbit /0,mean — 342 W m~ and the Earth's planetary albedo (or reflectivity, see below) aP — 0.30. The amount of emitted radiation is expressed by the Stefan-Boltzmann radiation law, with a — 5.67 x 10~8Wm~2KT4 and TR being the radiative temperature. These numbers yield a value of TR — 255 K for present-day Earth. The cooling of the Earth's core adds less than 0.1 Wm~ , which is very small in comparison to the amount of absorbed solar radiation, and can therefore be neglected for Earth's energy balance consideration.

The observed global mean surface temperature of Ts — 288 K is notably higher by 33 K than the radiative temperature. This additional warming of the surface is due to the atmospheric greenhouse effect. It results from the absorption of long-wave radiation by greenhouse gases in the atmosphere that were emitted from the surface (Figure 1). The absorbed radiation is re-emitted to space, but also back to the surface, thereby providing additional heating to the surface. The comparison of Earth's planetary characteristics to those of the planetary neighbors shows the importance of a well-balanced greenhouse effect in providing a habitable environment (Table 1).

Surface Energy Balance

At the surface, the energy balance is written as

(1 - as ) - Qlw .net' - Qsh - Qh - Qtrans where Qg is the ground heat flux (positive adds heat to the ground), Qjw,down is the downwelling flux of solar radiation at the surface, as is the surface albedo, Qlw,net is the net emission of terrestrial radiation from the surface, Qsh and Qh are the turbulent fluxes of sensible and latent heat, respectively, and Qtrans the horizontal transport of heat (relevant for oceans, but not for land).

The surface energy balance directly links the change in surface temperature Ts with the heating and cooling terms at the surface. Important factors that determine surface temperature change are:

1. the ground heat flux Qg: it depends on the conductivity and specific heat capacity of the ground surface. The conductivity describes how well heat is conducted by the material, while the specific heat capacity measures by how much the temperature changes with a certain change in heating. For instance, water surfaces have a much higher heat capacity; therefore, they show a much reduced temperature response than dry soil or air for the same amount of heating or cooling (Table 2).

2. the amount of heating by absorption of solar radiation: it combines the effects of incoming solar radiation and the reflectivity of the surface.

3. the partitioning of net radiative heating (solar and terrestrial) into sensible and latent heat (evaporation).

4. the amount of heat transport at the surface: through the oceanic circulation, warm water is removed from the tropics and transported toward the poles. The heat transport by the oceanic circulation averages to zero at the global scale.

Global Energy Balance Components

In the global climatic mean, dTs/dt = 0 and ^ Qtrans = 0. The estimates of each of the flux components are shown in Figure 1. Of the incoming 342 W m~ of solar radiation

Sun emits solar

Sun emits solar

Figure 1 Earth's global energy balance. The dominant energy fluxes and their brief description within the Earth's climate system, expressed as percentage of the average amount of incoming solar radiation of 342 W m~2.
Table 1 The Earth in comparison to its planetary neighbors. Orbital charateristics, atmospheric composition, albedo, absorbed radiation, radiative temperature, surface temperature, and the strength of the atmospheric greenhouse effect for selected inner planets and the Earth's moon

Earth

Venus

Mars

Moon

Orbital characteristics

Distance to Sun

150x106km

108 x 106 km

228 x 106 km

b

Obliquity

23.45c

<3°a

25.2°

6.7°

Eccentricity

0.017

0.007

0.094

0.055

Length of day

24 h

2802 h

24.7 h

708.7 h

Length of year

365.2 days

224.7 days

687 days

27.3 days

Atmospheric compositor)

3 x 10~15 bar

Surface pressure

1 bar

92 bar

6.4mbar

Carbon dioxide (CO2)

360 ppm

96.5%

95%

Nitrogen (N2)

78%

3.5%

2.7%

Oxygen (O2)

21%

0

0.13%

Climatic properties

Planetary albedo

0.30

0.71

0.16

0.11

Absorbed solar radiation

239 Wm~2

190 Wm~2

124Wm~2

304 W m ~2

Radiative temperature

255 K

233 K

210K

275 K

Surface temperature

288 K

737 K

210K

100-400K

Greenhouse effect

+33 K

+504 K

+0 K

aVenus rotates in the opposite sense than Earth. bDistance Moon-Earth is 0.378 x 106km.

aVenus rotates in the opposite sense than Earth. bDistance Moon-Earth is 0.378 x 106km.

Table 2 Specific heat capacity of selected substances. Substances change their temperature by differing amounts for a given amount of heat, depending on their specific heat capacity and their density. The last column, the product of the former two quantities, describes the amount of heat that is necessary to raise the temperature of 1 m3 of a given substance by 1 K

Table 2 Specific heat capacity of selected substances. Substances change their temperature by differing amounts for a given amount of heat, depending on their specific heat capacity and their density. The last column, the product of the former two quantities, describes the amount of heat that is necessary to raise the temperature of 1 m3 of a given substance by 1 K

Specific heat

Density

Heat capacity

(J kg " K |

(kgm3)

(J m 3 K")

Water®

4182

1000

4.18 x 106

Sandy soil, saturated

1480

2000

2.96 x 106

Sandy soil, dry

800

1600

1.28 x 106

Soil, inorganic

733

2600

1.91 x 106

Soil, organic

1921

1300

2.50 x 106

Peat soil, saturated

3650

1100

4.02 x 106

Peat soil, dry

1920

300

0.58 x 106

Snow, fresh

2090

100

0.21 x 106

Snow, old

2090

480

1.00 x 106

Ice

2100

920

1.93 x 106

Air®

1004

1.2

0.001 x 106

aDensity depends on temperature. Values given are for 293 K.

aDensity depends on temperature. Values given are for 293 K.

(/0,mean) at the top of the atmosphere, 22% is reflected by clouds and aerosols in the atmosphere and another 8% by the surface. These two numbers add up to the planetary albedo of about aP = 0.30. The remaining radiation is absorbed in the atmosphere (20% of /0,mean, by ozone in the stratosphere and by clouds) and at the surface (50% of /0,mean). Additional surface heating is provided by the atmospheric greenhouse effect (gray arrows in Figure 1), which adds almost twice as much energy to the surface than solar radiation. These heating terms are balanced by cooling through emission of terrestrial radiation and turbulent fluxes (the sum of sensible and latent heat flux). The atmosphere is heated by the absorption of solar radiation (20% of /0,mean), absorption of terrestrial radiation emitted by the surface (115% of /0,mean), turbulent fluxes (30% of /0,mean), and cooled by the emission of terrestrial radiation to space (70% of /0,mean) and to the surface (95% of /0,mean, the greenhouse effect).

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