In its Fourth Assessment Report, the IPCC lists the following observations (among others) in support of rising average global temperatures:
Global mean surface temperatures have risen by approximately 0.74°C as estimated by a linear trend over the last 100 years (1906-2005).
Temperature extremes consistent with a warming climate are increasingly frequent. The record-breaking heat wave over western and central Europe in the summer of 2003 is an example of a recent temperature extreme.
There have been substantial increases in heavy precipitation events on many land regions. Warmer air is capable of holding greater amounts of water vapor. These observations are therefore consistent with a warming climate, as they suggest increasing amounts of water vapor in the atmosphere.
The rate of sea level rise due to ice melt increased over the period from 1993 to 2003, primarily from increasing losses from mountain glaciers and ice caps, increasing surface melt on the Greenland Ice Sheet, and faster flow of parts of the Greenland and Antarctic Ice Sheets.
These two photos of Muir Glacier in Glacier Bay National Monument, Alaska, illustrate the effect of climate change on glaciers. Much of the glacier in the first photo, taken in 1950, is nonexistent in the second photo, taken in 2004.