Guy Callendar wrote a game-changing paper in 1938 documenting rising global temperatures and linking this to the increasing burning of fossil fuels since the Industrial Revolution. Since 1900 humans had added over 150,000 million tons of CO2 to the air, and this was not dissipated in any of the ways that scientists had suggested (absorbed by oceans, circulation of the atmosphere, etc). He predicted a steady increase in temperatures in the century ahead.
However as James Fleming has pointed out, “Callendar was by no means an environmental alarmist, since he wrote of the benefits of the combustion of fossil fuels, including extension of the cultivated region northward, stimulation of plant growth by CO2, and the probable indefinite delay of the ‘return of the deadly glaciers’ “. [Callendar Effect].
Callendar’s thesis was for a time weakened by the fact that there was a period of relative cooling from the 40s to the 60s. Callendar himself became more seriously alarmed about disastrous future trends in climate change as he got older, and his “Callendar Effect” would go on to become a highly influential climate change theory.