The study measured changes in tree rings, fossil shells and ice cores from 14 sites in the Northern Hemisphere to assess temperature fluctuation since the year 800, researchers wrote in the latest issue of Science magazine.

Reinforcing other evidence of global warming, the research found the 20th century stands out as having unusually warm temperatures, wrote Timothy Osborn and Keith Briffa of the University of East Anglia in Britain.

The present warm period that began in the late 20th century is the most widespread and longest temperature anomaly of any kind since the ninth century, the study said.

To determine if rising temperatures reflected a natural climate cycle, the scientists compared the relative magnitude of warming in the Northern Hemisphere with other cold and warm intervals over the past 1,200 years.

The study, setting specific thresholds to define unusually warm and cold periods, also used diaries from the Middle Ages and eyewitness accounts of extraordinary weather such as the freezing over of canals in Belgium and the Netherlands over the centuries.

The researchers looked at ring patterns in evergreen trees growing in Scandinavia, Siberia and North America. Wider rings indicated warmer temperatures.

Ice cores drilled from Greenland ice sheets showed which years were warmer than others by the chemical composition of the ice.

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