A study looking at geological data from marine and lake sediment cores, ice cores and coral cores found surface temperatures are higher now than they have been since at least 700AD.

If controversial tree ring data is included, scientists say the Northern Hemisphere is now warmer than at any time since 300AD, when the Roman Empire covered large parts of Europe and the Aztec and Mayan civilisations were flourishing in central America.

The US study, published in the Proceedings of the National Academy of Sciences, said the results showed that global warming is a reality – even without the evidence of tree rings.

“Some have argued that tree-ring data is unacceptable for this type of study,” said Professor Michael Mann, director of Penn State University’s Earth System Science Centre.

“Now we can eliminate tree rings and still have enough data from other so-called ‘proxies’ to derive a long-term Northern Hemisphere temperature record.”

Tree-ring data is often contentious because of a problem known as the “segment length curse”. This is where older trees put on narrower rings, making it difficult to compare with data from younger trees.

Professor Mann added: “Ten years ago, we could not simply eliminate all the tree-ring data from our network because we did not have enough other proxy climate records to piece together a reliable global record.

“With the considerably expanded networks of data now available, we can indeed obtain a reliable long-term record without using tree rings.”

The research was conducted to update a previous study by the team, which also included scientists from the Universities of Massachusetts and Arizona, and the Roger Williams University, in Rhode Island.

The study, published in 1990, had only been able to reconstruct surface temperatures for the past 1,000 years.

However, the researchers admitted their updated study was unable to reach definitive conclusions about temperatures for the Southern Hemisphere.

The scientists said this was the result of more limited data being available in the southern hemisphere.

Kate Martin

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