For all but the most recent 150 years scientists relied on "proxy" evidence from tree rings, glaciers and ice cores, cave deposits, corals, ocean and lake sediments, boreholes and other sources. ( For precise instrumental values from NASA see 1880-2006 )

Based on the evidence available, the projected change of 1.5 - 5.8 degrees Celsius (ICPP) over the 21st century would be unprecedented in comparison with the best available records from the last several thousand years. Note that the difference between the Optimum (the Viking age) and the little Ice Age was only about - 0.6 degrees Celsius.

But recent measurements around the world indicate that the latest ICPP report - far from being alarmist - has underestimated the warming trend.
For example:
In its June 2007 issue, the Swiss Revue, an official magazine from a rather conservative Swiss government, writes "that the mean temperature in Switzerland for the winter 2006/2007 was 3.1 degrees higher than the average from 1961 to 1990. Swiss glaciers have lost over 50 per cent of their volume since 1850"

These actual measurements show that temperature increases at present are already twice as high than the minimum the ICPP estimated for the entire 21st century