Nobody believed the Earth was flat
Not being an expert on the subject, I cannot comment on Michael O'Leary's claims about climate change, but I can tell you that his claim regarding scientists of Galileo's time believing the Earth was flat is incorrect.
Since the 3rd century BC, the overwhelming majority of educated people in the West have believed the Earth to be spherical: and that includes both scientists and priests.
The American Scientific Affiliation has published a paper which traces the origin of the flat-earth myth to a novel by Irving Washington, and a paper written by Antoine-Jean Letronne, who attempted to discredit the Church by spreading the same falsehood.
James Ingram, London SE1