Climate-change modeling: A brief history of the theory and recent twenty-first-century ensemble simulations

Introduction There is a long scientific history about the effects of increasing CO₂ on the climate system. The first scientist to provide a scientific basis and concern was the Swedish chemist Svante Arrhenius (Figure 2.1), who published a paper in 1896. He was the first researcher to explain in a qualitative way the possible climatic effects of increased concentrations of greenhouse gases. The Industrial Revolution had just started in the 1890s and in the newly developed countries of Europe and the United States there was an increase in the use of fossil fuels, at that time mostly coal. As we now know this was the start in the wide-spread use of fossil fuels as an energy source that is a major factor in the observed increases of CO₂ concentrations in the atmosphere. Arrhenius concluded that atmospheric CO₂ was important to Earth’s heat balance and that increases in this gas would lead to increased atmospheric temperature. Interestingly, Arrhenius estimated that an increase of 2.5 to 3 times the CO₂ concentration would result in a globally averaged temperature increase of 8-9 °C, a climatic global warming effect not too different from that estimated by the present generation of complex and comprehensive computer climate models. In Arrhenius’s simple scientific argument only some elements of the complex interactions of the climate system were known and included in his estimate. The latter does not give a detailed analysis of the changes in the climate system. Such an analysis can only be derived from advanced computer climate models.