Arrhenius developed a theory to explain the ice ages, and first speculated that changes in the levels of carbon dioxide in the atmosphere could substantially alter the surface temperature through the greenhouse effect.[3] He was influenced by the work of others, including Joseph Fourier. Arrhenius used the infrared observations of the moon by Frank Washington Very and Samuel Pierpont Langley at the Allegheny Observatory in Pittsburgh to calculate the absorption of CO2 and water vapour. Using 'Stefan's law' (better known as the Stefan Boltzmann law), he formulated his greenhouse law. In its original form, Arrhenius' greenhouse law reads as ***ows:
if the quantity of carbonic acid increases in geometric progression, the augmentation of the temperature will increase nearly in arithmetic progression.
This simplified expression is still used today:
ΔF = α ln(C/C0)
Arrhenius' high absorption values for CO2, however, met criticism by Knut Ångström in 1900, who published the first modern infrared spectrum of CO2 with two absorption bands. Arrhenius replied strongly in 1901 (Annalen der Physik), dismissing the critique altogether. He touched the subject briefly in a technical book titled Lehrbuch der kosmischen Physik (1903). He later wrote Världarnas utveckling (1906), German tras*lation: Das Werden der Welten (1907), English tras*lation: Worlds in the Making (1908) directed at a general audience, where he suggested that the human emission of CO2 would be strong enough to prevent the world from entering a new ice age, and that a warmer earth would be needed to feed the rapidly increasing population. He was the first person to predict that emissions of carbon dioxide from the burning of fossil fuels and other combustion processes would cause global warming. Arrhenius clearly believed that a warmer world would be a positive change. From that, the hot-house theory gained more attention. Nevertheless, until about 1960, most scientists dismissed the hot-house / greenhouse effect as implausible for the cause of ice ages as Milutin Milankovitch had presented a mechanism using orbital changes of the earth (Milankovitch cycles). Nowadays, the accepted explanation is that orbital forcing sets the timing for ice ages with CO2 acting as an essential amplifying feedback.
Arrhenius estimated that halving of CO2 would decrease temperatures by 4 - 5 °C (Celsius) and a doubling of CO2 would cause a temperature rise of 5 - 6 °C[4]. In his 1906 publication, Arrhenius adjusted the value downwards to 1.6 °C (including water vapour feedback: 2.1 °C). Recent (2007) estimates from IPCC say this value (the Climate sensitivity) is likely to be between 2 and 4.5 °C. Arrhenius expected CO2 levels to rise at a rate given by emissions in his time. Since then, industrial carbon dioxide levels have risen at a much faster rate: Arrhenius expected CO2 doubling to take about 3000 years; it is now estimated in most scenarios to take about a century.
