Guy Stewart Callendar

Guy Stewart Callendar (February 1897 - October 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. This theory, earlier proposed by Svante Arrhenius, has been called the Callendar effect. Callendar thought this warming would be beneficial, delaying a "return of the deadly glaciers."

Callendar was the son of Hugh Longbourne Callendar, an English physicist who studied thermodynamics.

Callendar's professional work focused on steam and pressure under the patronage of the British Electrical and Allied Industries Research Association, which represented turbine manufacturers. He later focused on research into batteries and fuel cells. Although Callendar was an amateur climatologist, he expanded on the work of several 19th century scientists, including Arrhenius and Nils Gustaf Ekholm as a hobby. Callendar published 10 major scientific articles, and 25 shorter ones, between 1938 and 1964 on global warming, infra-red radiation and anthropogenic carbon dioxide. Other scientists, notably Gilbert Plass and Charles Keeling, expanded upon Callendar's work in the 1950s and 1960s.

In 1938, Callendar compiled measurements of temperatures from the 19th century on, and correlated these measurements with old measurements of atmospheric CO₂ concentrations. 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. Callendar assessed the climate sensitivity value at 2 °C, which is on the low end of the IPCC range. 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 ideas 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 Keeling's Maunu Loa measurements which proved pivotal to advancing the theory of anthropogenic global warming. He remained convinced of the accuracy of his theory until his death in 1964 despite continued mainstream scepticism.