[1] His main contribution to human knowledge was developing the theory that linked rising carbon dioxide concentrations in the atmosphere to global temperature.

In 1913 he enrolled at St Paul's School, but left two years later following the start of World War I. Unfit for war service because of his blindness, he instead went to work in his father's laboratory at Imperial College as an assistant to the X-ray Committee of the Air Ministry, where he was involved in testing a variety of apparatus, including aircraft engines at the Royal Aircraft Factory (later Establishment) in Farnborough.

Callendar's professional work on steam and pressure was conducted under the patronage of the British Electrical and Allied Industries Research Association, which represented turbine manufacturers.

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.

[7] 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.