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Temperature record of the past 1000 years
People measure modern temperature records using instruments. Records only cover the last 150 years or so. The temperature record of the past 1,000 years or more is found by using data from what are called "climate proxy" records.
Proxies can be anything which relates to climate. Short term autobiographies often talk about the weather in past eras, and we do have some autobiographies from 2000 years ago. There are also tree rings and other methods which go back about 20,000 years. Further back still, there is evidence from geology, the record of the rocks. However, this page is just about the last 1000 years.
We are fairly sure there was a warm period about 1,000 years ago, and a cold period about the 17th century. Even in the 19th century people skated on the Thames in wintertime.
Although people today talk about climate change as if it were entirely man-made, this is not the complete picture. It is quite certain that climate has always been changing on Earth. Man-made changes are on top of changes which occur naturally. The Milankovich cycle is an important factor in these changes, and there are also changes in the heat put out by the Sun.
- Jones P.D; Briffa K.R; Barnett T.P. & Tett S.F.B. 1998, High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures, The Holocene, 8 (4): 455–471. 
- Mann M.E; Bradley R.S. & Hughes M.K. 1999. Northern hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Geophysical Research Letters 26 (6): 759–762. 
- Powell, Alvin 2003. Sun's warming is global: CfA lecture links solar activity and climate change. Harvard University Gazette.