According to Wikipedia …
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 degree Celsius (1.1 ± 0.4 degree Fahrenheit) in the 20th century . A widespread scientific opinion on climate change is that “most of the warming observed over the last 50 years is attributable to human activities”.
The increased amounts of carbon dioxide (CO2) and other greenhouse gases (GHGs) are the primary causes of the human-induced component of warming. 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. Read more here.