How do we know how warm or cold it was in the past?

A frequently asked question is about changes in temperatures in the past.  This short article in The Guardian provides a quick introduction to the science  and the  sources of evidence that are used to piece together past climates.  The record from weather stations goes back 200 years at best.  Before that there are a range of proxies that can be used to assemble a picture of climate change in historical and geological time. 

Follow this link for the full article:

http://www.guardian.co.uk/environment/2012/mar/07/past-climate-temperature-proxies