He is talking to climate alarmists, not climate deniers. He is not saying climate deniers need to adopt critical thinking. He is telling climate alarmists to start thinking critically. He is correct. Climate alarmists need to start thinking critically. It will change their understanding of the world.
Since carbon dioxide increased from 3 molecules per 10,000 molecules of air to the very dangerous 4 molecules per 10,000 molecules of air, the earth has come very close to dying. If we allow the air to contain 5 or even 6 molecules of CO2 per 10,000 molecules of air, convection will cease which will stop heat transfer to space, and the air will then have 100 times the heat capacity of the oceans which will completely evaporate while simultaneously rising to cover everything except mount everest