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Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades.
The Earth's average near-surface atmospheric temperature rose 0.6 ± 0.2 °Celsius (1.1 ± 0.4 °Fahrenheit) in the 20th century [1]. A widespread scientific opinion on climate change is that "most of the warming observed over the last 50 years is attributable to human activities"[2].
The increased amounts of carbon dioxide (CO2) and other greenhouse gases (GHGs) are the primary causes of the human-induced component of warming[3]. They are released by the burning of fossil fuels, land clearing and agriculture, etc. and lead to an increase in the greenhouse effect. The first speculation that a greenhouse effect might occur was by the Swedish chemist Svante Arrhenius in 1897, although it did not become a topic of popular debate until some 90 years later.
