By the end of the 19th century, the world had changed more since 1800 than in the thousand years before. From under 1 billion at the beginning of the century, the population was 1.5 billion and rising nearly 1% a year. Humankind was rumbling towards the population explosion of the 20th century. Advanced economies like the UK (35 million), Germany (60 million) and the USA (75 million) were industrialized and consuming hundreds of millions of coal per year. Railroads and coal burning ships had revolutionized transportation---the telegraph and telephone had revolutionized communications. Automobiles were just beginning to emerge, although not an environmental factor yet. Oil consumption was low, but about to explode. Humankind was on the threshold of being able to effect global environmental changes.
The dates of geological periods are always a bit arbitrary, except for the Cretaceous extinction--but sometime between 1800 and 2000 future scientists will say "Here marks the beginning of the Anthropocene period" and they will be right---although most were unaware of it at the time, and it is debated even now.
Svante August Arrhenius (1859-1927) was the first to realize that humankind could affect the global environment, and was in fact doing so. Although all the other figures I have discussed were important, it is with Arrhenius that the realization began that we are changing the global environment and the climate.
Arrhenius was a prodigy as a child, teaching himself to read at age 3 from the figures in his fathers' surveyor books and accounting figures. At age 8 he entered school at the fifth grade, and quickly became more adept at math than any of the other students in his school (which ran to high school) He sent for mathematical books from colleges and universities when his instructors were no longer able to teach him---and he was bored in high school. He wanted to attend the University of Uppsala early, but they would not admit him until he was 18--so in high school he studied his borrowed mathematics texts and waited.
In 1876 he entered the University of Uppsala and quickly became dissatisfied with the level of instruction he received--although he broadened his interests to chemistry. Due to rocky relationships with most of the faculty, whom he regarded as fools, he left the University of Uppsala in 1881 (without academic credit) after a serious dispute with Per Teodor Cleve, who found his chemistry work incomprehensible, to study at the Physical Institute of the Swedish Academy of Sciences in 1881. There Arrhenius researched under Erik Edlund about the electrical conductivity of electrolytes.
Michael Faraday had believed that salts dissolved into charged particles, which he called 'ions', a name we used for charged atoms and molecules today. This required an electric current--Faraday believed small electric currents were required to allow ionic chemical reactions to proceed. Arrhenius' insight was that ions could react with each other by themselves, without an intervening electrical current.
Arrhenius finished his doctoral thesis in 1884, a 150 page work (not exceptional for a doctoral thesis). What was exceptional was that his thesis contained 56 new ideas about physical chemistry--mostly to do with ionic chemical reactions. His thesis team was lead by Per Teodor Cleve, with whom Arrhenius had battled in the past--he came to the Physical Institute of the Swedish Academy of Sciences after Arrhenius did.
Arrhenius' thesis advisers thought his doctoral thesis mostly incomprehensible--giving it a rating in the 4th class. Upon Arrhenius' defense, they upgraded it to third class--barely acceptable. Arrhenius was incensed!
Arrhenius had his thesis translated into German (Germany being the center of physical chemistry and research in Europe) where it received wide admiration. Rudolf Clausius, Wilhelm Ostwald, and Jacobus Henricus van 't Hoff were very impressed. Ostwald even came to visit Arrhenius to persuade him to join his research team. Arrhenius wanted to very badly, but his father was very ill and in fact died later that year, so he declined.
Arrhenius's 1884 doctoral thesis won him the Nobel Prize for Chemistry in 1903.
After the heavyweights of the German chemists' establishment leaped so strongly to his defense, Arrhenius had a secure academic reputation--his old academic enemies could no longer hurt him. He became a traveling researcher for the Physical Institute of the Swedish Academy of Sciences, studying with Ostwald in Riga (then in Russia, now in Latvia), with Friedrich Kohlrausch in Würzburg, Germany, with Ludwig Boltzmann in Graz, Austria, and with van 't Hoff in Amsterdam.
In 1889, Arrhenius developed the concept of activation energy, the energy chemical reactions must absorb from their environment to proceed. Energy didn't come from nowhere, it must be present in the environment for chemical reactions to occur, and Arrhenius worked out how to determine what activation energies were needed for various chemical reactions to occur. This work resulted in the Arrhenius equation
In 1891 Arrhenius became lecturer in Physics at the Stockholm University College (now Stockholm University) and promoted to professor in 1895 (over much bitter opposition from his earlier academic battles). Arrhenius also had a bit of a personal scandal. In 1892 he began dating his student, Sofia Rudbeck--the old 'sex with the professor' thing. It was....unseemly. He married her in 1894, and it seems clear that the marriage was something he felt required to do to get the professorship. The marriage quickly curdled in a few months--she left him in 1895 right after he got the professorship. Arrhenius was devastated. And bored. He had worked hard to become a professor, but it was not what he wanted. Teaching second-rate minds in a second-rate college. Office hours. Grading papers. Quite a come-down after his achievements!
Arrhenius was bored. His reputation had gotten him a professorship, but it was not what he expected. He felt depressed, and trapped.
Arrhenius came across some works by Joseph Fourier, who had originated the idea that the atmosphere can cause a greenhouse effect. For the first time, infrared observations of the moon, by Frank Washington Very and Samuel Pierpont Langley beginning in 1890 had confirmed that the average temperature of the moon, day and night sides put together, was around 0 F. This confirmed that the calculations by Fourier and others were correct---without a greenhouse effect, the temperature of the Earth would be close to 0 F. And the only thing that could account for Earth's warmer temperature was the atmosphere. It had been known since Tyndall's time that carbon dioxide was a an infrared absorber.
Wait. Coal consumption was now a couple hundred million tons a year---carbon dioxide therefore was being emitted at almost a billion tons a year. Could humankind be changing the atmospheric composition and climate?
Arrhenius' head snapped up. It was a cloudy dull morning. February 21, 1895.