A study published Tuesday in the Proceedings of the National Academy of Scientists reveals that the decade which ended in 2006 was the warmest time period in the Northern Hemisphere in the last 1,300 years or more.

The study was conducted by a team of researchers led by climatologist Michael Mann, the one who, in 1998, helped develop the „hockey stick”graph, which is a diagram of the temperature behavior of the past 1000 years showing that, during reccent decades, temperatures have reached unprecedented values.

The study showed that the average temperature between 1996 and 2006 was at least half a degree Fahrenheit higher than averages dating approximately 1,700 years back. Moreover, it is said that it was in fact one degree warmer than historical averages.

This is the latest of three such reports but, since it is based on much more sources-alongside tree-ring records- that provide useful data, such as coral reef skeletons, sea floor sediments, stalactites and stalagmites formed in caves, it has managed to give rise to many a heated debate on human-induced changes in worldwide climate.

In 2006, the National Academy of Sciences reported that above-ground temperatures at the end of last century were warmer than any others registered during the last 400 years.

This year’s study was based on a paper issued by the National Research Council- a nonprofit scientific institution that advises the U.S. government-that requested an analysis performed using new techniques, since there has been controversy concerning the tree-ring method, which is deemed by many as inaccurate.