The observed and projected increases in the average temperature of Earth's atmosphere and oceans. The Earth's average temperature rose about 0.6° Celsius (1.1° Fahrenheit) in the 20th century, see temperature graphs below.
Fig. 1: Definition for global warming: Temp. increase in the last 1'000 years (graph from http://www.globalwarmingart.com/images/b/bb/1000_Year_Temperature_Comparison.png)
Fig. 2: Definition for global warming: Temp. increase in the last 150 years (graph from http://www.globalwarmingart.com/wiki/Image:Instrumental_Temperature_Record_png)
Fig 3: Definition for global warming: Temp. increase in the last 25 years (graph from http://www.globalwarmingart.com/images/a/af/Short_Instrumental_Temperature_Record.png)
Prediction for future temperature increase (global warming predictions)
According to different assumption about the future behaviour of mankind, a projection of current trends as represented by a number of different scenarios gives temperature increases of about 3° to 5° C (5° to 9° Fahrenheit) by the year 2100 or soon afterwards. A 3°C or 5° Fahrenheit rise would likely raise sea levels by about 25 meters (about 82 feet).
Fig 4: Definition for global warming: Temp. increase until the year 2100 (graph from http://www.globalwarmingart.com/wiki/Image:Global_Warming_Predictions_png.
