Environmental Engineering Reference
In-Depth Information
THE CREATION OF THE GREENHOUSE
EFFECT
is the most abundant, but methane, nitrous oxide,
the chlorofluorocarbons and tropospheric ozone
are potentially significant, although the impact
of the ozone is limited by its variability and short
life span. Water vapour also exhibits greenhouse
properties, but it has received less attention in
the greenhouse debate than the other gases since
the very efficient natural recycling of water
through the hydrologic cycle ensures that its
atmospheric concentration is little affected by
human activities. Any change in the volume of
the greenhouse gases will disrupt the energy flow
in the earth/atmosphere system, and this will be
reflected in changing world temperatures. This
is nothing new. Although the media sometimes
seem to suggest that the greenhouse effect is a
modern phenomenon, it is not. It has been a
characteristic of the atmosphere for millions of
years, sometimes more intense than it is now,
sometimes less.
The greenhouse effect is brought about by the
ability of the atmosphere to be selective in its
response to different types of radiation. The
atmosphere readily transmits solar radiation—
which is mainly short-wave energy from the
ultraviolet end of the energy spectrum—allowing
it to pass through unaltered to heat the earth's
surface. The energy absorbed by the earth is
reradiated into the atmosphere, but this terrestrial
radiation is long-wave infrared, and instead of
being transmitted it is absorbed, causing the
temperature of the atmosphere to rise. Some of
the energy absorbed in the atmosphere is returned
to the earth's surface, causing its temperature to
rise also (see Chapter 2). This is considered
similar to the way in which a greenhouse works—
allowing sunlight in, but trapping the resulting
heat inside—hence the use of the name
'greenhouse effect'. In reality it is the glass in the
greenhouse which allows the temperature to be
maintained, by preventing the mixing of the
warm air inside with the cold air outside. There
is no such barrier to mixing in the real
atmosphere, and some scientists have suggested
that the processes are sufficiently different to
preclude the use of the term 'greenhouse effect'.
Anthes
et al.
(1980) for example, prefer to use
'atmospheric effect'. However, the use of the term
'greenhouse effect' to describe the ability of the
atmosphere to absorb infrared energy is so well
established that any change would cause needless
confusion. The demand for change is not strong,
and 'greenhouse effect' will continue to be used
widely for descriptive purposes, although the
analogy is not perfect.
Without the greenhouse effect, global
temperatures would be much lower than they
are—perhaps averaging only -17°C compared to
the existing average of +15°C. This, then, is a
very important characteristic of the atmosphere,
yet it is made possible by a group of gases which
together make up less than 1 per cent of the total
volume of the atmosphere. There are about
twenty of these greenhouse gases. Carbon dioxide
The carbon cycle and the greenhouse effect
Three of the principal greenhouse gases—CO
2
,
methane (CH
4
) and the CFCs—contain carbon,
one of the most common elements in the
environment, and one which plays a major role
in the greenhouse effect. It is present in all organic
substances, and is a constituent of a great variety
of compounds, ranging from relatively simple
gases to very complex derivatives of petroleum
hydrocarbons. The carbon in the environment is
mobile, readily changing its affiliation with other
elements in response to biological, chemical and
physical processes. This mobility is controlled
through a natural biogeochemical cycle which
works to maintain a balance between the release
of carbon compounds from their sources and
their absorption in sinks. The natural carbon
cycle is normally considered to be self-regulating,
but with a time scale of the order of thousands
of years. Over shorter periods, the cycle appears
to be unbalanced, but that may be a reflection of
an incomplete understanding of the processes
involved or perhaps an indication of the presence
of sinks or reservoirs still to be discovered (Moore
and Bolin 1986). The carbon in the system moves
