In the first half of the twentieth century, the carbon dioxide theory of climate change had fallen out of favour with climatologists. Beginning in 1938, Guy Stewart Callendar (1898-1964), a noted steam engineer and amateur meteorologist, revived this theory by arguing that rising global temperatures and increased coal burning were closely linked. Working from his home in West Sussex, England, Callendar collected weather data from frontier stations around the world, formulated a coherent theory of infrared absorption by trace gases, and demonstrated that the carbon dioxide content of the atmosphere, like the temperature, was indeed rising.
Noting an upward trend in temperatures for the first four decades of the twentieth century, Callendar combined these results with studies of the retreat of glaciers, measurements of rising concentrations of atmospheric carbon dioxide since pre-industrial times, and information newly available concerning the infrared absorption bands of atmospheric constituents. He concluded that the trend toward higher temperatures was significant, especially north of the forty-fifth parallel; that increased use of fossil fuels had caused a rise of the concentration of carbon dioxide in the atmosphere of about ten percent from nineteenth century levels; and that increased sky radiation from the extra carbon dioxide was linked to the rising temperature trend.
Although he was an amateur meteorologist, Callendar worked on a truly global scale, compiling a reliable world data set of surface temperatures from earliest times and insisting - long before it became fashionable to do so - that climatology must deal with physics and atmospheric dynamics. Even in the depths of World War II, Callendar remained active in climate research, publishing two papers while working on technical problems (including infrared absorption) with the Ministry of Supply. In 1944 climatologist Gordon Manley noted Callendar's valuable contributions to the study of climatic change. A decade later, Gilbert Plass and Charles Keeling consulted with Callendar as they began their research programs. Just before the beginning of the International Geophysical Year in 1957, Hans Seuss and Roger Revelle referred to the 'Callendar effect' - defined as climatic change brought about by anthropogenic increases in the concentration of atmospheric carbon dioxide, primarily through the processes of combustion.