Geoscience Reference
In-Depth Information
at a wavelength of 2,898/290K or 2,898/250K, which is
9·99 μm or 11·59 μm; and overall emission is entirely
within the infra-red range ( Figure 2.2 ).
In this form the energy is susceptible to absorption by
the atmosphere, so very little escapes directly to space;
most is repeatedly absorbed and emitted before it is able
to leave the system. The ability of the globe to modify
energy flows in this way helps to keep the temperature of
Earth and its atmosphere higher than it would otherwise
be. In other words, it promotes energy storage within the
system.
At a global scale these processes lead to energy outputs
of which about 36 per cent are in the short wavelengths
derived from reflected insolation, and about 64 per cent
in the long wavelengths, largely from emission by the
atmosphere. Taken together, the difference between the
incoming radiation and the outgoing radiation is Earth's
net radiation .
Heating at surface
Convection
Surface
heating
Convective
cloud
Figure 2.15 Negative feedback loop. More heating will
produce more cloud, which in turn will reduce surface heating
and so offset the original extra heating.
of these changes are due to movement of the continental
plates but some may be related to alterations in energy
inputs and, if so, it is clear that outputs, too, must have
changed. As the snow cover was extended during the
Ice Ages reflection must have increased, while absorp-
tion (and hence re-radiation) must have been reduced.
Ultimately, however, a new equilibrium seems to be
established as energy outputs decline to match the new,
lower levels of input.
It is an intriguing question, also, whether changes in
global conditions could arise owing to adjustments in the
outputs independently of change in energy inputs. Any
event that significantly alters the reflectivity of Earth's
surface might trigger such changes. An increase in the
extent of the oceans relative to land due, perhaps, to major
earth movements; increased snow cover as a result of
mountain building; changes in vegetation cover due to
these events (or even human activity); or changes in the
atmosphere brought about by massive volcanic eruptions
- all could lead to significant changes in the global climate
and hence in energy outputs. The implications for the
world's climate are very important.
What is certain is that marked variations in global
energy outputs do occur in the long term. Many of these
variations are probably cyclical, related to changes in solar
inputs such as those resulting from differences in the tilt
and orbit of Earth. It is also apparent that such variations
in output are critical if Earth is to adjust to alterations
in the energy inputs that are known to occur, and thereby
maintain steady-state equilibrium. An unanswered ques-
tion is: to what extent can humans change these outputs
and upset the equilibrium?
Spatial and temporal variations in
outputs
Radiation outputs from the globe vary considerably over
time and across the global surface. Spatial fluctuations
depend upon a number of factors, including the character
of the atmosphere (e.g. its temperature and the degree of
cloudiness) and the nature of Earth's surface (e.g.
vegetation cover and topography). From the polar regions
an output of about 140 W m -2 compares with 250 W m -2
from equatorial areas - a ratio of about 2 : 1 - whereas
the ratio for short-wavelength input is about 6 : 1. These
aspects will be covered in Chapter 3.
Over the long term the fluctuations in global energy
outputs possibly relate to outside influences; a change in
input may lead to an adjustment in the output. The ways
in which these adjustments take place are complex, and
involve interactions called feedback mechanisms ( Figure
2.15 ). Vegetation cover, atmosphere conditions (including
moisture content and cloud cover), the extent of polar and
mountain snow cover, the area of the sea surface and even
soil cover and roughness may change in response to
alterations in energy inputs. Through such changes Earth
is able to adjust its energy outputs in the event of any long-
term variation in inputs by altering the balance between
the absorption, retention, emission and reflection of
energy.
The question, however, is whether long-term variations
of this kind occur. Certainly over geological time quite
marked fluctuations in climate have taken place, as is
attested by the evidence of Ice Ages and tropical conditions
contained in the rocks of many parts of the world. Some
 
 
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