Over the 4.6 billion years of its existence, the Earth’s temperature has varied widely, between Snowball Earth conditions when the land and oceans may have been entirely frozen over, to Greenhouse Earth episodes when forests grew at the poles.
But human civilisation has evolved during a recent period of relatively stable climate (the Holocene), when the Earth’s average surface temperature has varied probably less than +/- 0.5°C from the 1950 temperature of 13.8°C.
Since 1950 the average temperature has already risen 0.6°, probably already exceeding the highest experienced during civilisation, and it is still rising fast each decade. Without urgent and strong global action, global average temperature is projected to rise between 2°C and 6°C by 2100.
The IPCC’s 1990 report is not available online, but the 2007 report concludes:
“[M]edieval warmth was heterogeneous in terms of its precise timing and regional expression. ...the evidence is not sufficient to support a conclusion that hemispheric mean temperatures were as warm, or the extent of warm regions as expansive, as those in the 20th century as a whole, during any period in medieval times.
A recent scientific paper summarised in the Skeptical Science blog supports the IPCC’s conclusion that the Mediaeval Warm Period was regional, not global. It involved warmer conditions over part of the North Atlantic, Southern Greenland, the Eurasian Arctic and parts of North America. Other regions, such as central Eurasia, north-western North America and the tropical Pacific were substantially cooler.
Interestingly the figure Ashby uses in slide 20 below shows a Mediaeval Warm Period running from 800-1000 AD, rather than 1100-1300 AD as shown here. This also supports the IPCC’s conclusion that the Mediaeval warming was not a global phenomenon, but occurred at different times in different places.