It’s clear that the world was warmer during medieval times. Marked on the map are study after study (all peer-reviewed) from all around the world with results of temperatures from the medieval time compared to today. These use ice cores, stalagmites, sediments, and isotopes. They agree with 6,144 boreholes around the world which found that temperatures were about 0.5°C warmer world wide.
What follows is a sordid tale of a graph that overthrew decades of work, conveniently fitted the climate models, and was lauded triumphantly in glossy publication after publication. But then it was crushed when an unpaid analyst stripped it bare. It had been published in the highest most prestigious journal, Nature, but no one had checked it before or after it was spread far and wide. Not Nature, not the IPCC, not any other climate researcher.
In 1995 everyone agreed the world was warmer in medieval times, but CO2 was low then and that didn’t fit with climate models. In 1998, suddenly Michael Mann ignored the other studies and produced a graph that scared the world — tree rings show the “1990’s was the hottest decade for a thousand years”. Now temperatures exactly “fit” the rise in carbon! The IPCC used the graph all over their 2001 report. Government departments copied it. The media told everyone.
But Steven McIntyre was suspicious. He wanted to verify it, yet Mann repeatedly refused to provide his data or methods — normally a basic requirement of any scientific paper. It took legal action to get the information that should have been freely available. Within days McIntyre showed that the statistics were so flawed that you could feed in random data, and still make the same hockey stick shape nine times out of ten. Mann had left out some tree rings he said he’d included. If someone did a graph like this in a stock prospectus, they would be jailed.