Global warming/hockey stick/deception
Arguments
...the data used for temperatures further back than about 100 years represent long term averages (100-1000 years) and are based on things like Deuterium concentrations, organism distributions and other factors that change slowly. Temperatures from about 100 to 30 years ago are sparsely sampled surface temps and grossly homogenized while temperatures up to 30 years ago tend to be either sparse, highly homogenized data (Mann's tree rings, Hansen's GISS temp, etc.) or satellite data, both of which are representative of shorter term averages. It should be clear to anyone who understands statistical analysis that a short term averages change more rapidly and have more p-p variability than long term averages. If you applied the same multi century averaging to all of the data, the hockey stick would completely disappear. Mixing long term averages with short term averages is a common trick used in these hockey stick plots which as far as I can tell is done for no other reason than to confuse you into believing fiction.