By Ed Hawkins
In April 2014, a significant milestone was reached: the concentration of carbon dioxide in the atmosphere reached 400 parts per million (ppm). Exactly 76 years before, in April 1938, the first evidence that increasing carbon dioxide was influencing global temperatures was presented.
Although climate change is often viewed as a recent phenomenon, its roots are actually far older – the effects of human activity on the global climate have been discussed since the mid-19th century.
In 1861, the Irish physicist John Tyndall performed an experiment which changed our view of the atmosphere. Tyndall demonstrated that gases such as methane and carbon dioxide absorbed infra-red radiation, and could trap heat within the atmosphere. He immediately realised the implications and remarked that these gases “… would produce great effects on the terrestrial rays and produce corresponding changes of climate”.
However, it took an amateur meteorologist, Guy Stewart Callendar (1898-1964), to demonstrate that the planet was indeed warming. In 1938, he showed that global temperatures had risen by about 0.3 degrees Celsius over the previous 50 years (see Figure 1). Callendar did this by collecting temperature readings from weather stations scattered over the planet and averaging them together to produce an estimate of global temperatures, but doing all the calculations by hand, without the aid of a computer. His estimates turned out to be remarkably accurate.
Callendar was also the first to show that the carbon dioxide concentration in the atmosphere had increased from 290 ppm in 1900 to 320 ppm in 1958, largely due to the burning of fossil fuels (see Figure 2). Furthermore, he suggested that about half of the observed temperature increase was due to this accumulation of carbon dioxide. This 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.
By 1961, Callendar had collected many more widespread temperature readings, and was able to show increasing global temperatures from 1870 to 1950. However, doubts regarding the role of carbon dioxide remained, partly because the world did not warm further – in fact temperatures fell slightly until around 1975, before the warming resumed. This temperature plateau is very likely due to increased levels of particulates in the atmosphere reflecting solar radiation back into space. Ironically, these particulates are also the product of fossil fuel burning. 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.
In the 76 years since Callendar demonstrated the planet was warming, 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. Callendar believed that this warming would be beneficial, largely because “the return of the deadly glaciers should be delayed indefinitely”.
As we consider the milestone of reaching 400 ppm CO2, and with 2014 being the 50th anniversary of Callendar’s death, his considerable efforts in improving our understanding deserve to be more widely acknowledged.
For those interested in the history of the greenhouse effect, the Royal Meteorological Society is holding an open meeting on the topic in London on 15 October 2014.