Mean State of the Present Global Energy Balance

The above discussion is summarized in Figure 14 to represent the present mean state of the energy exchange in the climate system. The global radiation at the surface, 170Wm^-2, accounts for 50% of the primary energy source from the Sun, 340 Wm^-2.

This value is smaller than previously proposed, 186Wm-2 by Sellers, 185 W m^-2 by Salby, 198Wm^-2 by Kiehl and Trenberth, and 185 W m^-2 by Wild et al. The discrepancy of 15-30 W m^-2 for global radiation is one of the most important findings of recent years. The present value can be compared with 170 Wm-² proposed earlier by Budyko et al. The new value presented here is the result of the improved network of direct observations of global radiation at the Earth’s surface.

Figure 14. The annual mean global energy balance of the Earth

The Earth’s surface albedo was also newly evaluated and found to converge at around 15%. This value is somewhat larger than 14% by Budyko, 11% by Sellers, 9% by Salby, and 13% by Wild et al. The present value of 15% coincides with those proposed by Budyko et al. and Kiehl and Trenberth.

The present value of the Earth’s surface albedo is mainly from the accumulated data on albedo measurements over a variety of surfaces by terrestrial and airborne instruments, the long-term mapping of the distributions of sea ice and snow cover obtained by satellites, and the detailed investigations of the albedo of water under variable solar elevation angle and cloud conditions.

Keen readers probably noticed that the longwave (terrestrial) incoming radiation from the atmosphere in Figure 8 is considerably larger than in previously published sources. Figure 8 is based on the simulation with ECHAM4T106 and quality controlled by Wild et al. The recent advances in IR radiometry have corrected previous underestimations.

A proper treatment of the water vapor continuum absorption and the effect of clouds are also responsible for the present improvement. The longwave outgoing radiation is calculated on the basis of the recently obtained Earth’s surface temperatures by Jones et al. and current evaluation of the surface temperature from HadCRUT4.

The difference in the net radiation at BOA and TOA is separately evaluated for solar and terrestrial radiation. The divergence of solar radiation in the atmosphere is due to atmospheric absorption; it amounts to 93 W m^-2. This component was previously grossly underestimated. The most frequently quoted value for the atmospheric absorption of solar radiation is about 60 Wm^-2 (or 17% of the TOA solar irradiance) by Sellers.

The absorption due to water vapor beyond 2.8 pm, the collective effect of a number of minor absorption lines and wing regions of absorption lines (considered to be the main cause of the continuum absorption), aerosol, and clouds, supports the present result of larger absorption. The divergence of longwave radiation in the atmosphere is equivalent to radiative cooling of 195 Wm^-2. The net all wave divergence of atmospheric radiation is cooling at 102 W m^-2. It is balanced by the convergence of vertical convective fluxes (sensible and latent).

The present state of the observation network does not allow the evaluation of the global mean turbulent heat fluxes based solely on observations. The evaluation of turbulent heat fluxes is a case for which model-based computations play an important role. The present evaluation is mainly based on the computations by ECHAM4 and ERA15.

Latent heat flux is, however, better understood than the sensible heat flux, as the evaporation is measured at more sites than the sensible heat flux. Often, evaporation is measured not only in the boundary layer but also by weighing lysimeters and the basin hydrological balance, three observing methods with different principles. The global mean evaporation can also be estimated from the global precipitation.

Summing up the above discussions, the annual mean global latent heat of evaporation is very likely to be 83 W m^-2. The sensible heat flux of 18Wm^-2 is estimated as the difference between the net radiation and latent heat flux. The change in radiation, such as the greenhouse effect, can change the surface temperature only with weight of 18%. The major consequence, 83% of radiation change, arrives upon condensation, 10-15 days after the evaporation. The evaporation happens rather gradually from the entire Earth’s surface, while the condensation takes place in clouds, in much more restricted space and time.

This is the main reason why the change in radiation is connected to extreme events in precipitation and temperature. A small amount of 5% of the total turbulent heat fluxes (sensible and latent heat fluxes) is continuously converted into available potential energy in the atmosphere and ocean, maintaining the atmospheric and oceanic circulation. The kinetic energy and the conversion rate of the available potential energy to kinetic energy are due to Kung.

Summing up the entire section, the mean energy flow through the climate system can be interpreted in the following manner: The Earth receives a quarter of TSI, 340 W m^-2, of which 102 W m^-2 leaves the planet by reflection. The remaining 238 W m^-2 is absorbed by the atmosphere and the Earth’s surface. The atmosphere absorbs solar radiation of 93 Wm^-2, leaving the remaining 145 W m^-2 to be absorbed by the surface. This amount can be considered as the difference between solar global radiation (170 W m^-2) and the reflection loss by the Earth’s surface (25 Wm^-2).

This result indicates the likely global mean Earth’s surface albedo of 15%. The longwave net radiation at the surface is -43Wm^-2. The net radiation at the Earth’s surface, 102 W m^-2, is transferred to the atmosphere primarily as latent heat of vaporization (83Wm^-2) and considerably less by sensible heat flux (18Wm^-2). Because of the ongoing climate warming, there is a small, but not insignificant, loss of heat by 0.8 W m^-2.

The items are 0.7 W m^-2 [63] for warming the ocean and 0.08 W m^-2 [78] for the mass loss of glaciers. The sum of these terms (102 Wm^-2), augmented by the atmospheric absorption of solar radiation (93Wm^-2) and net longwave radiation from the surface (43 Wm^-2), is emitted by the Planet Earth back into space at a rate of 238 W m^-2, exacdy the same amount as solar radiation originally absorbed by the Earth’s planetary system. Between the absorption of solar radiation and the ultimate emission by Earth to space, the energy flow accomplishes kinetic and biological activity. The efficiency of the Earth as a mechanical engine is 1.3% of the primary solar radiation at TOA.

The possible errors of the radiative fluxes of Figure 14 differ for the individual components. Ohmura and Raschke estimated ±5-7 W m^-2 as the possible error for the TOA net radiation. Ohmura et al. estimated the error at ±5-10Wm^-2 for the BOA net radiation, and ±12 Wm^-2 for the total atmospheric radiation flux divergence.

The scheme of the energy balance at the Earth’s surface presented in the present article represents the mean state of the last quarter century leading to the time of the publication. Any changes in an involved component will lead to alterations of the energy balance scheme and the climate changes. An example is the steady increase in longwave incoming radiation at a rate of 2.5 W m^-2/decade, as a result of the increase in the greenhouse gas concentration and climate warming monitored at the BSRN stations.