The temperature reconstruction of the past millennium developed by Mann and colleagues Ray Bradley and Malcolm Hughes and published in 1998 and 1999 has since been extended and confirmed by multiple independent research groups.

Ironically, the only published claims that their methodology was fundamentally flawed came from Canadian economist Ross McKitrick and mining analyst Steve McIntyre, and their approach was deeply deceptive. Their R code, released after publishing a paper In Geophysical Research Letters in 2005, showed that 10,000 time series from randomized test runs were sorted by the degree of uptick at the end (M&M's 'Hockey Stick Index') and only the highest 100 (1%) used in their analysis and paper. Oops. Guess it was an honest mistake, right?

http://onlinelibrary.wiley.com/doi/10.1029/2004GL021750/full