This essay does not deny the 0.6-0.8°C increase in global temperature that took place during the 20th century. It tries to understand how and why this simple fact has been transformed into a militant social movement. A clue is found in Hanna Arendt analyses of ideologies (based on the nazi and soviet totalitarianisms). All the key features of an ideology are present in “warmism”: a monocausal explanation of everything and anything; a reference/reverence to “science”; a massive intervention of the State; a total war on a terrible evil, CO2, designated as enemy n°1 (although terrorism is now becoming a serious competitor), and on all lukewarmers; and finally, a popular success. The book argues that the equation IPCC = Science is highly questionable, and that the consequences drawn and the projections/predictions made (for year 2,100) are even more fragile. It shows that the dominant warmist catechism recited by politicians and media goes much beyond this questionable scientific basis. The medicines proposed and imposed under the pretext of “saving the planet” are discussed, and shown to be generally unrealistic and regressive. In particular, contrary to what is claimed by our modern prophets, they would prevent countries of Africa and South-East Asia, which are, at last, beginning to get themselves out of misery, to stop doing so. In short, warmism is the ideology of the day, and it is dangerous like all ideologies.