Climate Desk: Back in 1999, Penn State climate scientist Michael Mann released the climate change movements most potent symbol: The hockey stick, a line graph of global temperature over the last 1,500 years that shows an unmistakable, massive uptick in the 20th century when humans began to dump large amounts of greenhouse gases into the atmosphere. Its among the most compelling bits of proof out there that human beings are behind global warming, and as such has become a target on Manns back for climate denialists...