Most scientists agree that man-made climate change, or global warming, began in earnest during the industrial revolution, which occurred between the 18th and 19th centuries. This occurred, they say, as a result of a large number of greenhouse gases released into the atmosphere. However, in 2008, researchers from the University of Wisconsin-Madison concluded that global warming began as many as 8,000 years earlier with the advent of agriculture and deforestation.
Greenhouse gases released into the atmosphere during and since the industrial revolution remain trapped, where they continue to accumulate. They also serve to retain heat, which has had a series of effects on the planet's climate.
Early in the 20th century, most scientists and industrial leaders believed it was possible to alter a local climate but not the global climate, based on local activities. Although some scientists, including Svante Arrhenius, a Swedish chemist, saw the possibility for global climate change, they were dismissed as being alarmists.
Beginning in the late 1950s, scientists observed a steady increase in carbon dioxide in the atmosphere based on precise measurements. By the 1980s, the global temperature had begun to increase to such a degree that the media heard about it and began to spread the news.