By Richard Black
Environment correspondent, BBC News website
Ice cores indicated that temperatures now are unusually high
A new study by climate scientists behind the controversial 1998 "hockey stick" graph suggests their earlier analysis was broadly correct.
Michael Mann's team analysed data for the last 2,000 years, and concluded that Northern Hemisphere temperatures now are "anomalously warm".
Different analytical methods give the same result, they report in Proceedings of the National Academy of Sciences.
The 1998 hockey stick was a totem of debates over man-made global warming.
The graph - indicating that Northern Hemisphere temperatures had been roughly constant for 1,000 years (the "shaft" of the stick) before turning abruptly upwards in the industrial age - featured prominently in the Intergovernmental Panel on Climate Change's (IPCC) 2001 assessment.
But some academics questioned its methodology and conclusions, and increasingly strident condemnations reverberated around the blogosphere.
One US politician demanded to see financial and research records from the scientists involved.
However, a 2006 report from the National Research Council (NRC), commissioned by the US Congress, broadly endorsed its conclusion that Northern Hemisphere temperatures in the late 20th Century were probably warmer than at any time in the previous 400 years, and perhaps at any time during the previous 1,000 years.
Since then, a number of research groups have produced new "proxy records" of temperatures from the centuries before thermometers were widely deployed.
Such proxies include the growth patterns of trees and coral, the contents of ice cores and sediments, and temperature fluctuations in boreholes.
In their latest study, Dr Mann's group collated more than 1,200 proxy records - the majority from the Northern Hemisphere - and used different statistical methods to analyse their cumulative message.
"We used two different methods that are quite complementary in the assumptions they make about data, so that provides a test of the sensitivity of data to the methods used," he told BBC News.
"We also made use of a far wider network of proxy data than previously available.
"Ten years ago, the availability of data became quite sparse by the time you got back to 1,000 AD, and what we had then was weighted towards tree-ring data; but now you can go back 1,300 years without using tree-ring data at all and still get a verifiable conclusion."
Both analytical methods produced graphs similar to the original hockey stick, though starting further back in time. The "shaft" now extends back to about 700 AD.
The same basic pattern emerged when tree-ring data - whose reliability has been questioned - was excluded from the analysis.
"I think that having this extra data and using more methods to analyse it makes the conclusions more robust," commented Gabi Hegerl from the University of Edinburgh, UK, who was not involved in the research.
Critics of the idea of man-made climate change argue that conditions 1,000 years ago were as warm as, if not warmer than, they are today.
The new paper adds to the evidence against that notion. One of the analytical methods used suggests that temperatures in the Mediaeval Warm Period could have been no higher than they were in about 1980; the other suggests they were no higher than those seen 100 years ago.
The growth patterns of corals are a valuable proxy record of climate
In any case, said Dr Hegerl: "The whole line of argument [about whether temperatures have been as high in the past as they are now] is not very relevant."
The climate has always responded to factors such as changes in solar activity or volcanic eruptions, and always will, she said; the issue now is how it is responding to greenhouse gas emissions.
"In any case, the paper still comes to the firm conclusion that the most recent decades are unusual."
Ten years on from the study that provoked all the ire, Michael Mann's conclusion is that far from being broken, "the hockey stick is alive and well".
But, the Penn State University researcher added: "If we want to understand things like El Nino and how it relates to climate change, it's not enough to know just how anomalously warm the climate is today.
"We need to learn from the palaeoclimatic record. The science is not all done, there's still a lot of work to do; but what we are seeing now is definitely unusual in the context of the past."
A particular desire of scientists in the field is to increase the amount of data from the Southern Hemisphere. The majority of proxy records come from land rather than sea, and from land not covered in ice at that, which is in relatively short supply south of the equator.