The subject of reconstructions of temperature variations of the past millennium has been discussed many times before on this site (see e.g. here, here, here, and here). (realclimate.org)
Year - to - year variability, however, is determined mostly by temperature variations in the upper atmosphere. (giss.nasa.gov)
The hockey stick was a term coined for a chart of temperature variation over the last 1,000 years, which suggested a recent sharp rise in temperature caused by human activities. (vimeo.com)