Could varying concentrations of atmospheric carbon dioxide cause the planet to warm and cool? This was a key question facing scientists from the mid-1800s onwards – not because of a concern over man-made emissions of CO2, but because of a desire to understand the causes of the ice ages (identified by Louis Agassiz in 1837).
Then, exactly 75 years ago, in April 1938, a little appreciated scientist, Guy Stewart Callendar, presented the first evidence that the planet had recently warmed. Callendar also suggested that changes in atmospheric CO2 had caused a large part of this observed warming. A new paper reanalyses Callendar’s work to mark the 75th anniversary of his landmark study.
Guy Callendar (1898-1964) was, like his father, a internationally renowned steam engineer by day. But, in his spare time, he was also an avid amateur climatologist. In 1938, he published the first evidence that the planet had recently warmed – by about 0.3°C over the previous 50 years. He did this by collecting temperature readings from 147 weather stations scattered over the planet, and averaging them together to produce an estimate of global temperatures. Callendar had little or no data from the Arctic, Antarctica or over the oceans, so his estimates actually represented the temperature of land regions between 60°S-60°N. But, what is most remarkable is that he did all the calculations by hand, without the aid of a computer.
By 1961 he had collected many more widespread temperature readings, and was able to produce an extended estimate for near-global land temperatures stretching over the period 1870-1950. Fortunately, Callendar tabulated his temperature estimates in his published papers, and these have now been compared with more recent analyses which use data from many more weather stations. The agreement between Callendar’s and the modern estimates (shown in the Figure) is perhaps surprising – Callendar had managed to measure the temperature change of the planet, from his desk, to within the modern estimates of the uncertainties that exist.
John Tyndall had been the first to suggest a role for CO2 in regulating the climate in his pioneering laboratory experiments in the 1860s. These experiments demonstrated that CO2 (and other gases) could effectively absorb infra-red radiation and therefore alter the planet’s temperature. However, many scientific disagreements remained over the role of changes in atmospheric CO2, largely because of uncertainty in many of the measurements (particularly the atmospheric absorption spectrum).
Callendar suggested that about half of the observed temperature increase in the early 1900s was due to the accumulation of CO2 in the atmosphere. Although his estimate used a rather simplistic representation of the atmosphere, it renewed the scientific debate over whether carbon dioxide could regulate the climate and spurred many other scientists to make improved measurements of how infra-red radiation was absorbed in the atmosphere.
However, doubts in the role of CO2 remained, partly because the world did not warm further – in fact land temperatures fell slightly until around 1975, before the warming resumed. This temperature plateau is very likely due to increased levels of particulates (or ‘aerosols’) in the atmosphere reflecting solar radiation back into space. Ironically, these aerosols are also the product of fossil fuel burning and strict regulations were imposed in the developed world on their emissions in the 1960s and 1970s which allowed the warming from carbon dioxide to emerge again. Aerosol emissions from the developing world may also have played a role in the temperature plateau since around 2000.
In the last 75 years, much has been learnt about the atmosphere and the climate system. Although there is still some uncertainty in the exact amount of warming produced by increasing carbon dioxide, the fact that it does warm the planet is now without question, at least partly due to the work of Guy Stewart Callendar. His considerable efforts in performing the tedious calculations required to measure the temperature of the planet should be more loudly acknowledged.
Ed Hawkins & Phil. D. Jones (2013). On increasing global temperatures: 75 years after Callendar Quarterly Journal of the Royal Meteorological Society, preprint pdf
Callendar, G. (1938). The artificial production of carbon dioxide and its influence on temperature Quarterly Journal of the Royal Meteorological Society, 64 (275), 223-240 DOI: 10.1002/qj.49706427503
Callendar, G. (1961). Temperature fluctuations and trends over the earth Quarterly Journal of the Royal Meteorological Society, 87 (371), 1-12 DOI: 10.1002/qj.49708737102
Thanks to Doug McNeall & Gavin Schmidt for useful comments.