"Ross McKitrick is a Canadian environmental economist and global warming skeptic, best known for his statistical reviews of temperature record reconstructions that purport to show dramatic recent global warming relative to history."
http://en.wikipedia.org/wiki/Ross_McKitrick

His expertise is in statistics. The hockey Stick depends largely upon using statistical tools to make the Hockey Stick graph from temperature proxies. Mann did have much expertise in statistics. And if one wishes to be generous, Mann made a number of fundamental errors having to do with his famous Hockey Stick graph which depicted that the 20th Century was warmest period in last 1000 year- this graph has has been shown to be incorrect. One could say that the 20th Century was warmest in last 400 years, but this was scientifically accepted prior to the Hockey stick and has nothing to do with the famous Hockey Stick graph.