The greenhouse effect, the cause of global warming, is an unprecedented, and possibly irreversible, environmental condition in which damaging human-produced gases build up and trap heat within the Earth's protective atmospheric shield, called the ozone layer.

The threat of global warming was first recognized in 1896 by Swedish chemist Svante August Arrhenius, who suggested that the burning of fossil fuels might have a serious impact on the Earth's temperature. But scientists at that time could not have predicted that it would become one of the world's most pressing environmental issue less than one hundred years later.

Arrhenius's early warning of the dangers of carbon dioxide was not taken seriously until 1938 when G. S. Callendar, an English physicist, pointed to meteorological records showing the gradual warming since 1880 of North America and Northern Europe. Callender was the first scientist to gather data from several sources on the danger of...