During Callendar’s later years (c.1950-1964) his CO2 climate warming theory was questioned by other climate scientists, who pointed to the undoubted complexity of global climate science and called for more research into factors such as the timescale of ocean-atmosphere exchange, deep ocean conditions and apparent discrepancies in the uptake of fossil fuel carbon. He continued his deep research (in a private capacity) and by the time of his death opinion was swinging strongly in his direction. As James Fleming concludes about this remarkable man:

“[His research judgment]] led him to conclude that the trend towards higher temperatures was significant, especially north of the 45th parallel: that the increased use of fossil fuels (and, to a lesser extent, deforestation and other practices) had caused a rise of the concentration of CO2 in the atmosphere that would only continue and would most likely accelerate; and that increased sky radiation from the extra CO2 was directly linked to the rising temperature trend. The world’s glaciers were shrinking, and he thought it unlikely that the oceans would absorb the excess carbon” [Callendar Effect, p.87].

Fleming goes on to say that most scientists now agree that Callendar was basically right, and that we now live in a world that has reached levels of atmospheric CO2 unknown to Callendar.