Julian Lee at Planetair informed me that this statistic is from the UN Intergovernmental Panel on Climate Change's Fourth Assessment report (February 2007).
He directed me to the following link:
The reference to the warmest years is found on page 4, under the heading "DIRECT OBSERVATIONS OF RECENT CLIMATE CHANGE". It relates to
My only quibble with the statement at this point is that it does include the relevant qualifier, "since 1850".