The claim that the globe is warming depends on knowing earlier temperatures. Such information can only be obtained indirectly. Climate scientists depend on tree rings, bore holes, ice cores, the skeletons of marine organisms. The graph that was most effective in persuading policy-makers became known as the hockey stick. The temperature line is mostly horizontal, perhaps declining slightly for 900 years, then abruptly heading up into a warmer range over the last 100 years. The 900 years are the handle, the last hundred are the blade.
Read more at http://spectator.org/articles/55208/false-alert-global-warming