Environmental Engineering Reference
In-Depth Information
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.
Figure 2.14
Negative feedback loop. More heating will
produce more cloud which in turn will reduce surface heating
and so offset the original extra heating.
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 2.14). 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 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 absorption (and hence reradiation) 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.
