From BBC website:
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.
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.