Guy Stewart Callendar (1898 - 1964) was an English steam engineer and inventor. His main contribution to knowledge was developing the theory that linked rising carbon dioxide concentrations in the atmosphere to global temperature.
Guy Callendar was the first to demonstrate that the Earth’s land temperature had increased over the previous 50 years in 1938. This theory has been called the Callendar effect but his interpretation of it would not accord with today's views. Callendar thought this warming would be beneficial, delaying a "return of the deadly glaciers."
In 1938, Callendar compiled measurements of temperatures from the 19th century on, and correlated these measurements with old measurements of atmospheric CO2 concentrations. This was a painstaking, meticulous task. He concluded that over the previous fifty years the global land temperatures had increased, and proposed that this increase could be explained as an effect of the increase in carbon dioxide.
These estimates have now been shown to be remarkably accurate, especially as they were performed without the aid of a computer.
Not unusually for new ideas, his findings were met with scepticism at the time; for example, Sir George Simpson, then director of the British Meteorological Society thought his results must be taken as a coincidence. However, his calculations did influence the scientific discourse of the time, which had been generally sceptical about the influence of changes in CO2 levels on global temperatures in the previous decades after debate over the idea in the early 20th century.
His papers throughout the 1940s and 50s slowly convinced some other scientists of the need to conduct an organised research programme on CO2 concentrations in the atmosphere, leading eventually to Charles Keeling's Mauna Loa Observatory measurements from 1958, which proved pivotal to advancing the theory of anthropogenic global warming.
Callendar remained convinced of the accuracy of his theory until his death in 1964 despite continued mainstream scepticism.