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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