Newsweek: One of the most impressive visuals in Al Gore’s now famous slide show on global warming is a graph known as the “hockey stick.” It shows temperatures in the Northern Hemisphere rising slowly for most of the last thousand years and turning steeply upward in the last half of the 20th century. As evidence of the alarming rate of global warming, it tells a simple and compelling story. That’s one reason the U.N.’s Intergovernmental Panel on Climate Change included the graph in the summary of its …