section_path: History/Early discoveries
section_name: Early discoveries
paragraph_text: Early discoveries
  
  Scientists in the 19th century such as Alexander von Humboldt began to foresee the effects of climate change. In the 1820s, Joseph Fourier proposed the greenhouse effect to explain why Earth's temperature was higher than the Sun's energy alone could explain. Earth's atmosphere is transparent to sunlight, so sunlight reaches the surface where it is converted to heat. However, the atmosphere is not transparent to heat radiating from the surface, and captures some of that heat, which in turn warms the planet.
  
  In 1856 Eunice Newton Foote demonstrated that the warming effect of the Sun is greater for air with water vapour than for dry air, and that the effect is even greater with carbon dioxide (CO). She concluded that "An atmosphere of that gas would give to our earth a high temperature..."
  
  Starting in 1859, John Tyndall established that nitrogen and oxygen—together totalling 99% of dry air—are transparent to radiated heat. However, water vapour and gases such as methane and carbon dioxide absorb radiated heat and re-radiate that heat into the atmosphere. Tyndall proposed that changes in the concentrations of these gases may have caused climatic changes in the past, including ice ages.
  
  Svante Arrhenius noted that water vapour in air continuously varied, but the CO concentration in air was influenced by long-term geological processes. Warming from increased CO levels would increase the amount of water vapour, amplifying warming in a positive feedback loop. In 1896, he published the first climate model of its kind, projecting that halving CO levels could have produced a drop in temperature initiating an ice age. Arrhenius calculated the temperature increase expected from doubling CO to be around 5–6 °C. Other scientists were initially sceptical and believed that the greenhouse effect was saturated so that adding more CO would make no difference, and that the climate would be self-regulating. Beginning in 1938, Guy Stewart Callendar published evidence that climate was warming and CO levels were rising, but his calculations met the same objections.