Contrary to what the skeptics and deniers will tell you, the Hockey Stick graph, similar to the one I featured in "An Inconvenient Truth", is proving to be completely true.
Basically the graph shows the temperatures remain consistent for 1,000 years, and then, when the industrial revolution took place, temperatures spiked. A number of research groups have produced records of temperatures of centuries before thermometers were in use using the growth patters of corals and trees, temperature changes in boreholes, ice cores and sediments.
According to researcher Mike Mann in an interview with the BBC, "Ten years ago, the availability of data became quite sparse by the time you got back to 1,000 AD, and what we had then was weighted towards tree-ring data; but now you can go back 1,300 years without using tree-ring data at all and still get a verifiable conclusion." The results were reported in the Proceedings of the National Academy of Sciences.