Principles of terrestrial ecosystem ecology.pdf

Energy (W m -2 )
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Absorptivity 0
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Solar incoming radiation
Terrestrial outgoing radiation
0 5 10 15 20 25
Visible region
CH4
N2O
O2 and O3
CO2
H2O
Atmosphere
5 10 15 20 25
Wavelength (µm)
Figure 2.1. The spectral distribution of solar and
terrestrial radiation and the absorption spectra of
the major radiatively active gases and of the total
atmosphere.These spectra show that the atmosphere
absorbs terrestrial radiation more effectively than
solar radiation, explaining why the atmosphere is
heated from below. (Sturman and Tapper 1996, Barry
and Chorley 1970.)
49% reaches Earth’s surface as direct or diffuse
radiation and is absorbed.
Over time scales of a year or more, Earth is
in a state of radiative equilibrium, meaning that
it releases as much energy as it absorbs. On
average, Earth emits 79% of the absorbed
energy as low-energy longwave radiation (3000
to 30,000nm), due to its relatively low surface
temperature (288K). The remaining energy is
transferred from Earth’s surface to the atmosphere
by the evaporation of water (latent heat
flux) (16% of terrestrial energy loss) or by the
Earth’s Energy Budget 19
transfer of heat to the air from the warm
surface to the cooler overlying atmosphere
(sensible heat flux) (5% of terrestrial energy
loss) (Fig. 2.2). Heat absorbed from the surface
when water evaporates is subsequently
released to the atmosphere when water vapor
condenses, resulting in formation of clouds and
precipitation.
Although the atmosphere transmits about
half of the incoming shortwave radiation to
Earth’s surface, it absorbs 90% of the longwave
(infrared) radiation emitted by the surface
(Fig. 2.2). Water vapor, carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), and
industrial products like chlorofluorocarbons
(CFCs) effectively absorb longwave radiation
(Fig. 2.1). The energy absorbed by these
radiatively active gases is reradiated in all
directions as longwave radiation (Fig. 2.2).
The portion that is directed back toward the
surface contributes to the warming of the
planet, a phenomenon know as the greenhouse
effect. Without a longwave-absorbing atmosphere,
the mean temperature at Earth’s surface
would be about 33°C lower than it is
today and would probably not support life.
Radiation absorbed by clouds and radiatively
active gases is also emitted back to space,
balancing the incoming shortwave radiation
(Fig. 2.2).
Long-term records of atmospheric gases,
obtained from atmospheric measurements
made since the 1950s and from air bubbles
trapped in glacial ice, demonstrate large increases
in the major radiatively active gases
(CO2,CH4,N2O, and CFCs) since the beginning
of the Industrial Revolution 150 years ago
(see Fig. 15.3). Human activities such as fossil
fuel burning, industrial activities, animal husbandry,
and fertilized and irrigated agriculture
contribute to these increases.As concentrations
of these gases rise, more of the longwave radiation
emitted by Earth is trapped by the atmosphere,
enhancing the greenhouse effect and
causing the surface temperature of Earth to
increase.
The globally averaged energy budget outlined
above gives us a sense of the critical
factors controlling the global climate system.
Regional climates, however, reflect spatial