Environmental Engineering Reference
In-Depth Information
the Ice Ages, whereas others have only been
detectable through detailed instrumental analysis.
Some have lasted for centuries, some only for a
few years. While it is relatively easy to establish
that climatic change has taken place, it is quite
another matter to identify the causes. There are
a number of elements considered likely to
contribute to climatic change, however.
Since the earth/atmosphere system receives the
bulk of its energy from the sun, any variation in
the output of solar radiation has the potential to
cause the climatic change. The links between
sunspot cycles and changes in weather and
climate have long been explored by climatologists
(see e.g. Lamb 1977), and there are researchers
who claim that solar variability has a greater
impact on global climate than the greenhouse
effect. For example, a report prepared for the
Marshall Institute—a think-tank in Washington
DC—suggested that reduced solar output in the
near future might offset current global warming
sufficiently to initiate a new Ice Age. The IPCC
assessment considered this unlikely, however,
pointing out that the estimated solar changes are
so small that they would be overwhelmed by the
enhanced greenhouse effect (Shine
et al.
1990).
Even if the solar energy output remains the same,
changes in earth/sun relationships may alter the
amount of radiation intercepted by the earth.
Variations in the shape of the earth's orbit, or
the tilt of its axis, for example, have been
implicated in the development of the Quaternary
glaciations (Pisias and Imbrie 1986).
The present intensification of the greenhouse
effect is directly linked to the anthropogenic
production of CO
2
; in the past, however CO
2
levels have increased with no human contribution
whatsoever. During the Cretaceous period,
millions of years before the Industrial Revolution,
CO
2
concentrations were much higher than they
are today (Schneider 1987). Other changes in the
composition and circulation of the atmosphere
have to be considered also. The impact of
increased atmospheric turbidity is not clear. It
may add to the general warming of the
atmosphere (Bach 1976) but it has also been used
to explain global cooling between 1940 and
1960, at a time when CO
2
levels continued to
rise (see Figure 7.13). Although this cooling is
usually acknowledged as a problem by
researchers, it has not yet been adequately
explained (Wigley
et al.
1986). More recently,
the eruption of Mount Pinatubo in mid-1991
caused cooling which appears to have been
sufficient to reverse the global warming of the
1980s and early 1990s. There is also some
evidence that the turbidity increase caused by the
eruption may have contributed to regional
warming (see Chapter 5).
Thus there are many elements in the earth/
atmosphere system capable of producing
measurable climate change. Given their past
contribution to change, it seems unlikely that they
will now remain quiescent while anthropogenic
CO
2
provides its input. Despite this, they have
been ignored by most researchers or are
considered of minor importance compared to the
potential impact of the greenhouse gases (Roberts
1989).
Research into global warming is continuing
at a high level, and it is possible that a better
understanding of its interaction with other
elements in the earth/atmosphere system will
emerge to resolve some of the existing
uncertainties. If not, society will have to deal with
the future environmental changes in much the
same way as it has done in the past—by reacting
to them after they have happened.
SUMMARY
The earth's greenhouse effect has been
intensifying since the latter part of the nineteenth
century, largely as a result of human activities.
The increased use of fossil fuels has raised the
level of CO
2
in the atmosphere, and the
destruction of natural vegetation has prevented
the environment from restoring the balance.
Levels of other greenhouse gases—including CH
4
,
N
2
O and the CFCs—have also been rising and
the net result has been a gradual global warming.
If present trends continue, a rise in mean global
temperatures of 1.5°C-4.5°C is projected by the
early part of the twenty-first century. These values
