The latest report from the United Nations climate panel - being prepared for release on Friday - will draw on a wide range of research into how temperatures have changed.
Thermometers provide the most reliable record but only for the past century or so.
To look further back, scientists use indicators from the natural world such as the patterns of growth revealed by tree rings.
One team of marine researchers, at Bangor University in north Wales, has even turned to clams to find out about the past climate.
The molluscs can live for hundreds of years, as our science editor David Shukman has been finding out.
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