1824 Physicist Joseph Fourier figures out that Earth's atmosphere keeps the planet comfortable by holding in some heat from the sun.
1859 On the other hand, John Tyndall discovers that certain gases, such as carbon dioxide (CO2) and water vapor, absorb heat and trap it in the atmosphere, making the planet too warm.
1896 Svante August Arrhenius calls this kind of heating the "greenhouse effect." He believes that burning of fossil fuels by factories will increase the amount of carbon dioxide in the atmosphere.
1938 Studying weather as a hobby, Guy S. Callendar examines historic weather records and finds that Earth's temperature is rising. His work supports the theory that industrial activity is increasing carbon dioxide in the atmosphere, raising the temperature and changing the climate.
1954 Biologist G. E. Hutchinson links forest destruction with Earth's rising temperatures. Trees use CO2 to make food, so when they die they release it into the atmosphere.
1958 Astronomers studying Venus realize that a greenhouse effect there has raised the planet's temperature and made life impossible.
1958 On a volcano in Hawaii, Charles Keeling starts using precise equipment to measure the amount of CO2 in the atmosphere. Annual measurements prove that it is on the rise.
1971 Mariner 9 spacecraft finds evidence that the atmosphere of Mars, now very hot, was quite different in the past.
1975 Scientists suspect that human-made chemicals threaten the ozone layer of the atmosphere. The ozone layer protects life on Earth from the sun's ultraviolet rays.
1985 An expert on ocean currents, Wallace Broecker believes that changes in water temperature and movement caused by global warming could speed up climate change even more.
1997 Countries come together to sign an agreement called the Kyoto Protocol to reduce human-made greenhouse gases. The plan takes effect in 2005, and by late 2007, over 170 nations have agreed to participate.
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