It was a lot longer back than you would imagine.
Svante August Arrhenius (February 19, 1859 – October 2, 1927) was a Swedish chemist and one of the founders of the science of physical chemistry. The Arrhenius equation and the lunar crater Arrhenius are named after him.
In 1908 Arrhenius predicted that significant global warming would take ~3000 years to develop. This is now recognised as a substantial underestimate due in part to his failure to foresee the rapid increases in fossil fuel use during the twentieth century.
The PDF link contains "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground" by Svante Arrhenius. This paper, published in 1896, is the first to quantify the impact of carbon dioxide on the Earth's greenhouse effect and to suggest that its variations have been an important influence on previous long-term changes in climate. His crude estimate that a doubling of carbon dioxide would result in a ~5 °C warming is larger but not greatly different from the 1.5-4.5 °C now estimated for such a doubling (IPCC 2001).
Combining these calculations with existing work suggesting that the burning of fossil fuels could significantly alter the concentration of carbon dioxide in the atmosphere (Högbom 1894), Arrhenius later became the first person to predict the possibility of man-made global warming.