Professor Michael Mann plotted a graph in the late 1990s that showed global temperatures for the last 1,000 years. It showed a sharp rise in temperature over the last 100 years as man made carbon emissions also increased, creating the shape of a hockey stick.
The graph was used by Al Gore in his film “An Inconvenient Truth” and was cited by the United Nations body the Intergovernmental Panel on Climate Change (IPCC) as evidence of the link between fossil fuel use and global warming.
The issue became a central argument in the climate change debate and was dragged into the ‘climategate’ scandal, as the sceptics accused Prof Mann and his supporters of exaggerating the extent of global warming.
However, speaking to the BBC recently, Prof Mann, a climatologist at Pennsylvania State University, said he had always made clear there were “uncertainties” in his work.
“I always thought it was somewhat misplaced to make it a central icon of the climate change debate,” he said.