In the past 7,500 years, it has never been as warm in the Arctic as it has recently been. This is the result of an analysis of annual rings of ancient trees that dates back to 5618 BC, as researchers report in the journal "Nature Communications".

Reconstructing the climate of past millennia was possible because erosion on the Yamal Peninsula in north-western Siberia had uncovered ancient tree trunks. Summer temperatures, which influence growth, can be read from the annual rings.

"The warming today is reaching temperatures that were unprecedented in the last 7,500 years," said Patrick Fonti of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) on Thursday. In total, more than 3,500 subfossil tree trunks had been recovered from the peninsula over a period of more than 40 years. 1425 were used for the annual ring chronology.

WSL collaborated with scientists from the Russian Academy of Sciences, as well as experts from the University of Geneva and East Anglia in the UK.