It seems John and I posted at the same time in reply to yours. Sorry for repeating John's post.

Originally Posted By: ImranCan
Guys - thanks for enlightening me. My mental model had always been that temperatures HAVE been varying through time. Hence the mideaevel warm period and the mini ice age.


Obviously you're right, temperatures have been varying, significantly, through time. Although if you listen to Mann (infamous hockey stick), the medieval warm period and the mini ice age were non-events. See here http://upload.wikimedia.org/wikipedia/en/d/dc/Hockey_stick_chart_ipcc.jpg.
Almost a completely flat line (slight trend downwards) from 1000 to 1850 AD, with the maximum variation in the 0.2-0.3 degree range. Do you see any medieval warm period or little ice age in there?