RESEARCHERS from the University of East Anglia are scouring icy marshes and dredging up bogs to plot summer weather from up to 8,000 years ago.
Preliminary findings show unexpected temperature fluctuations, and throw into question any straightforward view of the greenhouse effect.
Keith Biffa, from the UEA's climatic research unit, has been collecting data from the rings inside tree trunks, which vary in density and width according to the summer temperature. He explained: "If it is a very cold summer, there's hardly a ring. In a warm summer, a tree can grow quickly and put on a good ring."
Sub-fossilised wood preserved in ice bogs and lakes is radio-carbon dated.
Through broad collaboration projects with counterparts such as the Department of Physical Geography in Stockholm, a wide geographical and chronological picture is emerging. Dr Biffa expects eventually an answer on whether human activity in the last century started global warming.