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E. Linacre and B. Geerts |
11/'97 |
The longterm and global-mean radiation intensity reaching the Earth from the Sun, Rx, is 342 W/m2, i.e. a quarter of the solar constant. This 'extra-terrestrial radiation' can be divided into three parts (1). The first is reflected back into space, currently representing about 30% of Rx, the second part is absorbed by the atmosphere (i.e. 24%) and the rest is absorbed by the ground (46%).
This topic was considered in more detail by Kiehl and Trenberth (2). Measurements indicate that the global-mean longwave radiation flux upwelling at the top of the atmosphere into space is 265 W/m2 and 235 W/m2 for clear and cloudy skies, respectively. What is defined as the longwave ‘radiative forcing’ due to clouds (the blanketing effect) is the difference in the longwave radiation loss, i.e. 30 W/m2.
The global-mean total longwave forcing of the atmosphere (i.e. the difference between the longwave radiation emitted by the Earth surface with, and without an atmosphere) is calculated as 155 W/m2. This means that the forcing due to a clear sky is 125 W/m2 (155-30), i.e. 75 W/m2 from water vapour, 32 W/m2 from carbon dioxide, 10 W/m2 from ozone, and 8 W/m2 from methane and nitrogen oxides. Note that the component due to carbon dioxide is proportional to the logarithm of the concentration of CO2, so the incremental effect of CO2 on surface temperature declines as the concentration of CO2 rises. This is relevant to the greenhouse effect.
Global-mean surface energy balance assessments in thirteen papers (2) show a broad spread of estimates (Table 1). The scatter of values for each flux is mainly due to uncertainty about the height of cloudbase, and hence the estimate of downwards longwave radiation.
Table 1: Spread in estimates of the global mean energy balance at the surface of the Earth. The consensus is not the average but the currently most accurate estimate (2).
|
Energy component |
Amount (W/m2) |
Consensus (W/m2) |
|
net shortwave radiation received |
142-174 |
168 |
|
net longwave radiation lost |
40-72 |
66 |
|
sensible heating lost to the air |
16-27 |
24 |
|
latent heating lost to the air |
78-90 |
78 |
Recent estimates of global mean precipitation rates range between 966-1041 mm/a. The greatest uncertainties are over the oceans, which has justified the spaceborne Tropical Rainfall Measurement Mission.
There must be an equal amount of evaporation overall. A global mean evaporation rate of 1 m/a implies a latent-heat flux averaging 78 W/m2 (the consensus value in Table1). The remainder of the surface energy balance is used to heat the air (sensible heat flux). A net incoming shortwave flux of 168 W/m2, a net upwards longwave flux of 66 W/m2 and a latent-heat flux of 78 W/m2 leaves an average income of sensible heat of 24 W/m2 (ie 168 - 66 - 78).
As regards shortwave radiation, about 60 W/m2 is absorbed is passing from top to bottom of the atmosphere. 72% of that absorption is due to moisture, and 23% to ozone.
Let us compare these values with those in Fig 5.3 of the book. The shortwave radiation from ground to space is now reckoned to be almost 9% (instead of 3%) of the extraterrestrial radiation Rx, that from the atmosphere back to space 22% (instead of 27%), and, as regards longwave flux, 68% (instead of 65%) from atmosphere to space, 12% from ground to space (instead of 5%), sensible heating 7% (not 5%), terrestrial radiation 114% (not 109%). Other changes would be only 1% or less. Such changes illustrate the way that research refines our knowledge.
References
(1) Li, Zhanqing, L. Moreau and A. Arking 1997: On solar energy disposition. Bull. Amer. Meteor. Soc., 78, 53-70.
(2) Kiehl, J.T. and K.E. Trenberth 1997: Earth’s annual global mean energy budget. Bull. Amer. Meteor. Soc., 78, 197-208.