During the
last 20-30 years, world temperature has fallen, irregularly at first but
more sharply over the last decade.
The cause of the cooling trend is not known with certainty. But
there is increasing concern that man himself may be implicated, not
only in the recent cooling trend but also in the warming temperatures
over the last century. According to this view, activities of the
expanding human population — especially those involved with the
burning of fossil fuels — raised the carbon dioxide content of the
atmosphere, which acts as a "greenhouse" for retaining the heat
radiated from the Earth's surface. This, it is believed, may have
produced the warming temperatures after the mid-19th century. But
simultaneously, according to this view, growing industrialization and
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the spread of agriculture introduced increasing quantities of dust into
the atmosphere which reduced the amount of solar radiation reaching
the earth. By the middle of this century, the cooling effect of the dust
particles more than compensated for the warming effect of the carbon
dioxide, and world temperature began to fall.
The colder temperatures have been accompanied by marked
changes in the circulation patterns of the atmosphere, which are prime
determiners of weather. Several consequences of these recent climatic
changes have been observed: midsummer frosts and record cold
autumns in the midwest of the United States, shortening of the crop
season in Great Britain, and the southward intrusion of sea-ice on the
shores of Iceland. Possibly linked to these changes in temperature and
circulation is the occurrence of an unusually large number of severe
storms in many parts of the world, and the development of a
calamitous drought belt extending around the world, passing through
the sub-Sahara, Middle East, India, China's Yangtze Valley, and
Central America.
The state of knowledge regarding climate and its changes is too
limited to predict reliably whether the present, unanticipated cooling
trend will continue, or to forecast probable changes in precipitation if
the trend persists. The practical consequences of an extended cooling
period — the effects on food production, energy consumption, and the
location of human settlements — make it important to monitor climatic
changes closely and widely, to determine their cause, particularly the
role of human activities, and to seek countermeasures.
The atmospheric sciences have advanced considerably in the last
20 years, in part because of access to sophisticated devices and facilities
developed for national defense and space purposes (e.g., high
resolution and doppler radar, high altitude aircraft, and rocket and
satellite observation platforms). One small indication of the progress
is the current ability to make 48-hour weather forecasts that are
comparable in quality to earlier 24-hour forecasts. While segments of
the total weather and climate system are yielding to understanding,
only in the most recent years has it been possible to begin studying the
system as a whole. Even now, only the broadest limits can be placed on
the magnitude of natural and man-made influences on weather and
climate. There is probably less agreement now, for example, on the
likely effects of carbon dioxide than there was a decade ago, when the
complexity of the overall system was not yet appreciated. There is also
lack of agreement as to whether the particulate content of the
atmosphere is primarily the product of human activity in agriculture
and industry or of natural causes such as volcanic dust.
Before such questions as these can be resolved, major advances
must be made in understanding the chemistry and physics of the
atmosphere and oceans, and in measuring and tracing particulates
through the system. Comprehensive models which integrate the
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many interacting components of the system must be developed and
tested. Advances in technology are needed for measuring and
monitoring the system, as well as for ameliorating the deleterious
effects of man and nature. Finally, greater understanding of the
economic, legal, and social implications associated with changes in
weather and climate are needed.