Put up your hand if you’ve been a follower of news about climate change in recent years and haven’t heard of the “hockey stick” graph.
Nobody? No, didn’t think so.
These graphs get their name because of their shape.
They are reconstructions of the temperatures on Earth over several centuries to several millennia and they all have a repetitive tendency to turn sharply skyward showing the recent rapid warming of the Earth.
The most famous and first “hockey stick” came from research in the journal Nature in 1998 led by Professor Michael Mann, then of the University of Massachusetts Amherst.
Mann used historical data from tree rings and ice cores – known as “proxy records” - to determine what temperatures were like over the Northern Hemisphere over the 600 years or so before we had a reasonably well-dispersed network of thermometers.
When plotted on a chart… well, you know the rest. It looked like a hockey stick.
Mann followed up that work in 1999, refining the research for a study in Geophysical Research Letters to give a full 1000-year history of the planet’s temperatures.
His work appeared in the 2001 United Nations Intergovernmental Panel on Climate Change report. This is what it looked liked in that report (notice the red and blue colors - we'll come to that in a bit.)