Ten thousand years ago, most of Canada was covered with ice. This would have reflected most of the summer sunlight received there back into space, and would have had a huge cooling effect on the planet. The 1990 IPCC report correctly showed this, with temperatures about 4C cooler than the Holocene Maximum.
But the new hockey stick shows temperatures 10,000 years ago within 0.1C of the Holocene Maximum. This is nonsensical and impossible. That huge mass of ice would have kept the planet very cool.
But it is worse than it seems. The latent heat of melting ice also would have had a huge cooling effect. Between 10,000 and 8,000 years ago, most of the ice melted. This would have dominated the energy balance and kept air temperatures very cool.
Additionally, Paul Homewood points out that the melting ice would have flowed into the oceans and cooled sea surface temperatures, compounding the cooling effect.
There is a zero percent chance that the new hockey stick is correct – as they have run afoul of several fundamental principles of physics. Temperatures 10,000 years ago could not have been close to the Holocene maximum.
h/t to michaelstrickland92 and northernont