The global average surface temperature fluctuates over time, but recently it has increased dramatically. From 1920 to the present, the Earth’s average surface temperature has increased by around 1.4 °F. The current warming trend is proceeding at a rate that is unprecedented in at least the past 1,300 years. (IPCC AR4)
The sharpest rise occurred between 1975 and 2010, when temperature rose steadily by over 1 °F. The graph below holds 1880-1920 as the baseline climate period and temperatures are expressed as the difference from that era.