Principles of terrestrial ecosystem ecology.pdf

22 2. Earth’s Climate System
the climate. The sulfur released to the atmosphere
by the volcanic eruption of Mount
Pinatubo in the Philippines in 1991, for
example, caused a temporary atmospheric
cooling throughout the globe.
Clouds have complex effects on Earth’s radiation
budget. All clouds have a relatively high
albedo and reflect more incoming shortwave
radiation than does the darker Earth surface.
Clouds, however, are composed of water vapor,
which is a very efficient absorber of longwave
radiation. All clouds absorb and re-emit much
of the longwave radiation impinging on them
from Earth’s surface. The first process (reflecting
shortwave radiation) has a cooling effect by
reflecting incoming energy back to space. The
second effect (absorbing longwave radiation)
has a warming effect, by keeping more energy
in the Earth System from escaping to space.The
balance of these two effects depends on the
height of the cloud. The reflection of shortwave
radiation usually dominates the balance in high
clouds, causing cooling; whereas the absorption
and re-emission of longwave radiation generally
dominates in low clouds, producing a net
warming effect.
Atmospheric Structure
Atmospheric pressure and density decline with
height above Earth’s surface. The average vertical
structure of the atmosphere defines four
relatively distinct layers characterized by their
temperature profiles. The atmosphere is highly
compressible, and gravity keeps most of the
mass of the atmosphere close to Earth’s surface.
Pressure, which is determined by the mass
of the overlying atmosphere, decreases exponentially
with height. The vertical decline in air
density tends to follow closely that of pressure.
The relationships between pressure, density,
and height can be described in terms of the
hydrostatic equation
dP
dh
=-rg
(2.1)
where P is pressure, h is height, r is density, and
g is gravitational acceleration. The hydrostatic
equation states that the vertical change in pressure
is balanced by the product of density and
gravitational acceleration (a “constant” that
varies with latitude). As one moves above the
surface toward lower pressure and density,
the vertical pressure gradient also decreases.
Furthermore, because warm air is less dense
than cold air, pressure falls off with height more
slowly for warm than for cold air.
The troposphere is the lowest atmospheric
layer and contains most of the mass of the
atmosphere (Fig. 2.3). The troposphere is
heated primarily from the bottom by sensible
and latent heat fluxes and by longwave radiation
from Earth’s surface. Temperature therefore
decreases with height in the troposphere.
Above the troposphere is the stratosphere,
which, unlike the troposphere, is heated from
the top. Absorption of UV radiation by O3 in
the upper stratosphere warms the air. Ozone is
concentrated in the stratosphere because of a
balance between the availability of shortwave
UV necessary to split molecules of molecular
oxygen (O2) into atomic oxygen (O) and a high
enough density of molecules to bring about the
required collisions between atomic O and mol-
Height (km)
110
100
90
80
70
60
50
40
30
20
10
Mt. Everest
Thermosphere
Mesopause
Mesosphere
Stratopause
Stratosphere
Tropopause
Troposphere
-90 -60 -30 0 30
Temperature ( o C)
Figure 2.3. Average thermal structure of the atmosphere
showing the vertical gradients in Earth’s major
atmospheric layers. (Redrawn with permission from
Academic Press; Schlesinger 1997.)