Introduction
OUR SOURCES
The greenhouse effect is a sustained, natural phenomenon: gases which are radiatively active have ’warmed’ the surface of the Earth since the planet and its atmosphere were formed about four and a half billion years ago. Mankind’s activities, particularly those associated with industrialization, large-scale land clearance and chemically improved agriculture, are adding increasingly large amounts of these radiatively active gases to the atmosphere.
At present there seems little likelihood that this global scale atmospheric pollution will cease much less that removal of the pollutants will be initiated. This planet therefore seems to be committed to a global scale warming, an intensification of the hydrological cycle (the coupled processes of evaporation, cloud formation, precipitation and runoff) and many associated climatic shifts. It is thus essential that policies be developed which recognize the reality of the predicted changes. The greenhouse effect is an unambiguous theory which is well understood by atmospheric and climatic scientists and which successfully predicts temperatures on the Earth and on other planets. There are, however, a number of controversies 2 associated with the greenhouse effect which are beloved by the media
.
These include:
1) The warming observed so far this century cannot be stated unambiguously to be the result of additional greenhouse warming caused by mankind’s activities. Moreover, it is unlikely that such an unambiguous statement will be possible in less than 10 to 15 years.
2) Precise predictions of the increase in the Earth’s surface temperature resulting from a specified increase in one or more trace greenhouse gases differs somewhat from model to model as a function of the number of feedbacks incorporated and the realism of the physical parameterizations.
3) Reconstructions of the Earth’s surface temperature (e.g. Figure 1) for the past century may be contaminated by the urban heat island effect.
4) The term ’greenhouse’ is a poor one and probably not generally applicable to unheated horticultural glasshouses.
Nonetheless, the scientific basis for the predictions of global greenhouseinduced climatic change are very firmly founded. In 1985, a large (and representative) group of atmospheric scientists met in Villach, Austria - the result of this meeting was the ’Villach Statement’ (see foreword in Bolin et al. , 1986) which has come to be recognized as a turning point in awareness of greenhouse issues.This statement opens by saying: ’as a result of the increasing concentrations of greenhouse gases, it is now believed that in the first half of the next century a rise of global mean temperature could occur which is greater than any in Man’s history’, and continues:
"Many important economic and social decisions are being made today on long-term projects - major water resource management activities such as irrigation and hydropower, drought relief, agricultural land use, structural designs and coastal engineering projects, and energy planning - based on the assumption that past climatic data without modification are a reliable guide to the future. This is no longer a good assumption ... It is a matter of urgency to refine estimates of future climatic conditions to improve these decisions."
THE GREENHOUSE THEORY
The greenhouse theory is based upon the fact (readily demonstrated by
experiment) that whilst gases in the Earth’s atmosphere are transparent to
incoming solar radiation, some of them absorb outgoing thermal (or heat)
radiation emitted from the Earth’s surface. This radiative interaction between
selected gases in the atmosphere (termed the greenhouse gases) and the outgoing
heat radiation causes those gases to warm, and consequently they themselves
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| GREENHOUSE EFFECT THEORY |
In the Earth’s atmosphere, only trace gases (less than a few per cent of the total atmosphere) contribute to this greenhouse warming. These trace gases are water vapour, carbon dioxide (C02), methane (CH4), nitrous oxide (N20), tropospheric ozone (03) and the chlorofluorocarbons (CFCs) although other gases such as carbon monoxide and sulphur dioxide also make very small radiative contributions to the surface warming.
Greenhouse and sea-level change
Mean sea levels (MSL) have been rising since the last glacial maxima. Variations
in sea levels on glacial time scales may be seen as a response of the oceans and
polar ice sheets to periods of glaciation and deglaciation. At the height of the last
glaciation (circa 18 000 years BP) for example, sea levels were over 110 metres
lower than the present day. There are in fact only two periods in the recent
geological past that have superseded the present MSL; between 120 000-125 000
years BP (+6 metres above MSL) and 135 000 years BP (+2 metres above MSL).
Over the past century, the rise has been estimated at 15.1 cm ± 1.5 cm per
century (Barnett, 1983) and 17 cm per century (Revelle, 1983), based on eustatic
sea level changes and isostatic readjustment.
A further consideration is the
background warming of the atmosphere which is very slowly altering the ratio
between the oceans and ice sheets (the melting of the Greenlandic ice cap is at
present more than adequately compensated by the growth of Antarctic ice). With
the inadvertent warming of the atmosphere by humans, these processes are
accelerated and as the atmosphere gradually warms, the oceans will follow suite.
The lag time is estimated at about 25 years by Revelle (1983) for the upper mixed
ocean layer (approximately the top 100 metres of the ocean). The net result will
be a thermal expansion of the oceans as temperatures increase and the density
of the oceanic waters decreases, so occupying a greater volume (assuming the
areal extent does alter appreciably).
Predicting greenhouse/induced climatic change
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| climate change |
There are only two ways presently available to estimate how climate will change
in the era of the greenhouse effect: the use of computer-based global climate
models and analogies drawn between historical and palxoclimatological records
of periods when the Earth was warmer and the future.
The historical analogue approach has been used by a number of authors to
construct warmer climate scenarios for Europe, America and Australia: there are
two basic methods of using the instrumental record to create an analogy for the
future: (1) comparison between climates of a pre selected number of years, usually
10 to 20, which are the hottest on record and their coldest year counterparts; and
(2) selection of a warm period and for comparison a cold period. For example,
in one study for Australia, the cold period 1913-45 has been compared with the more recent warm period of 1946-78. Comparison is made between rainfall, and
sometimes even cloudiness, from the warm years as compared with the cold years.
The assumption is that the same incremental change added to today’s conditions
will give us a picture of how the climate will be in the next century.
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