Charles C. Mann in Wired:
None of this is why Guy Callendar’s name will be boldfaced in tomorrow’s textbooks. Instead it will be because he was willing to delve into fields he knew nothing about, atmospheric science among them. Nobody knows why he got so interested in the air. Callendar himself attributed it to ordinary curiosity: “As man is now changing the composition of the atmosphere at a rate which must be very exceptional on the geological time-scale, it is natural to seek for the probable effects of such a change.”
In the early 1930s Callendar began collecting measurements of the properties of gases, the structure of the atmosphere, the sunlight at different latitudes, the use of fossil fuels, the action of ocean currents, the temperature and rainfall in weather stations across the world, and a host of other factors. It was a hobby, but a remarkably ambitious one: He was producing the first rough draft of the huge climate models familiar today.