The thought experiment that made him one:

I think any physical scientist should be extremely skeptical that a long-term stable system is dominated by positive feedback. Systems dominated by positive feedback — and we are talking about incredibly high implied feedback percentages to get to these catastrophic forecasts — don’t tend to be very stable, but it is Michael Mann himself who has argued over and over with his hockey stick chart that past temperatures have only varied in very narrow ranges for thousands of years. Not the behavior one would expect of a system dominated by strong positive feedbacks.

To me, this thought experiment demonstrated that it was more likely that net climate feedbacks were zero or even negative (if only half of past warming was due to man, and half due to nature, it would imply a sensitivity around 0.7C). In either case, the resultant warming would be far from catastrophic. To believe the IPCC forecasts, one would have to believe there were either really long time delays, or natural and manmade cooling factors off-setting the warming. These have all been debated and I won’t go into them today, but I didn’t find the higher forecasts of 5-10C to be at all credible.

I don’t, either.