Ran across this in the 1964 (!) ENCYCLOPEDIA BRITANNICA this morning in the article on AIR:

Carbon dioxide in the air reduces the loss of heat by infrared radiation from the earth. The widespread burning of fossil fuels (e.g., coal and oil) increased the carbon dioxide content of the air by about 12% during the first half of the 20th century. This, in turn, caused an increase in the average temperature of the earth of 1.1° C during the same period. It is estimated that all of the fossil fuels will be consumed in about 1,000 years, if the present rate of consumption is maintained. It has been further estimated that this will cause the temperature of the earth to rise 7° to 14° C. (13° to 25° F.), giving most of the earth a tropical and semitropical climate by the end of the period.